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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch General Description
The AAT1171 SwitchReg, a member of AnalogicTech's Total Power Management ICTM (TPMICTM) product family, has been specifically designed to dynamically control the operating voltage of a WCDMA or CDMA power amplifier inside single lithium-ion battery-powered systems. The AAT1171 outputs a voltage between 0.6V and 3.6V, thereby optimizing the amplifier efficiency at both low and high transmit levels. The AAT1171 output voltage is controlled via an analog signal from the baseband processor. It can deliver 600mA of continuous load current while maintaining a low 45A of no load quiescent current. The 2MHz switching frequency minimizes the size of external components while keeping switching losses low. A low resistance MOSFET, typically 230m, provides a low dropout voltage as the battery input voltage approaches the programmed output voltage and the converter runs at 100% duty cycle. To further improve system efficiency, an 85m bypass MOSFET transistor is also included to allow the PA to be powered directly from the battery. The AAT1171 feedback and control method gives excellent load regulation and transient response while maintaining small external components. The output voltage responds in less than 30s. The converter can be synchronized to an external system clock, forced to operate in Light Load (LL) mode for highest efficiency at light loads, or in Pulse Width Modulation (PWM) mode for low noise operation. The AAT1171 is available in a Pb-free, space-saving TDFN33-12 package and is rated over the -40C to +85C temperature range.
AAT1171
Features
* * * * * * * * * * * * * * *
SwitchRegTM
VIN Range: 2.7V to 5.5V Variable Output Voltage: 0.6V to 3.6V 600mA Output Current DAC Input: 0.2V to 1.2V High Output Accuracy: 3% 45A No Load Quiescent Current Internal Soft Start Limits Startup Current and Output Voltage Overshoot Synchronizable to External 19.8MHz System Clock Over-Temperature and Current Limit Protection Integrated 85m Bypass MOSFET 2MHz Operation PWM/LL Control with Override Fast 150s Start-Up 3x3mm 12-Pin TDFN Package Temperature Range: -40C to +85C
Applications
* * * WCDMA or CDMA PA in Cellular Phones, Smartphones, Feature Phones, etc. Express Card PCMCIA Data Cards
Typical Application
VIN
2.2H
0.6V - 3.6V
10F
VCC
AAT1171
BYPASS
LX
-
MODE/SYNC EN DAC GNDx2
VOUT
4.7F VCC2 VCONT VREF VCC2 PA
DAC Baseband Processor
TX RX
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Pin Descriptions
Pin #
1 2, 3
AAT1171
Symbol
N/C VOUT
Function
Not connected. Feedback input pin. This pin is connected to the converter output. It is used to complete the control loop, regulating the output voltage to the desired value. When in bypass mode, a low resistance MOSFET is connected between this pin and VIN. Bias supply. Supply power for the internal circuitry. Connect to input power via low pass filter with decoupling to AGND. Analog ground. Connect the return of all small signal components to this pin. Control voltage input from a DAC. Input voltage between 0.2V and 1.2V to control output voltage of the converter. Force pin to 1.3V for bypass switch enable. Enable DC/DC converter, active high. Enable control to bypass the DC/DC converter when PA transmitting at full power from low battery voltage. Active high. This pin is used to program the device between PWM and LL mode: HIGH - PWM Mode Only LOW - LL Mode: PWM operation for loads above 100mA and variable switching frequency for loads below 100mA Connecting the SYNC pin to the system clock (19.8MHz) will override the internal clock and force the switching frequency to the external clock frequency divided by 10. Input supply voltage for the converter. Must be closely decoupled. Main power ground. Connect to the output and input capacitor return. Switching node. Connect the inductor to this pin. It is connected internally to the drain of both low- and high-side MOSFETs. Exposed paddle (bottom). Connect to ground directly beneath the package.
4 5 6 7 8 9
VCC AGND DAC EN BYPASS MODE/SYNC
10 11 12 EP
VIN PGND LX
Pin Configuration
TDFN33-12 (Top View)
N/C VOUT VOUT VCC AGND DAC
1 2 3 4 5 6
12 11 10 9 8 7
LX PGND VIN MODE/SYNC BYPASS EN
2
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Absolute Maximum Ratings1
Symbol
VCC, VIN VLX VOUT VN TJ TLEAD
AAT1171
Description
Input Voltage and Bias Power to GND LX to GND VOUT to GND EN, DAC, BYPASS, MODE/SYNC to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -0.3 to 6.0 -40 to 150 300
Units
V V V V C C
Thermal Information2
Symbol
PD JA
Description
Maximum Power Dissipation, TA = 25C Thermal Resistance, TA = 25C
Value
2.3 50
Units
W C/W
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on an FR4 board. 1171.2006.06.1.0
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Electrical Characteristics1
TA = -40C to +85C, unless otherwise noted. VIN = VCC = 3.6V; typical values are TA = 25C. Symbol
VIN VUVLO VOUT VDACIN IQ ISHDN ILIM RDS(ON)H RDS(ON)L RDS(ON)BP ILXLEAK VOUT/VOUT VOUT/ VOUT*VIN ROUT VOUT FOSC TSD THYS ILL tVOUTS
AAT1171
Description
Input Voltage UVLO Threshold UVLO Hysteresis VOUT Programmable Range Input Voltage Range from DAC Quiescent Current Shutdown Current P-Channel Current Limit High Side Switch On Resistance Low Side Switch On Resistance Bypass Switch Resistance LX Leakage Current Load Regulation Line Regulation Feedback Impedance Output Voltage Accuracy Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis Light Load Load Current Threshold Output Voltage Settling Time
Conditions
VIN Rising
Min
2.7
Typ
2.6 200
Max
5.5
Units
V V mV V V A A A m m m A % %/V k V MHz C C mA
0.6 0.2 No Load, Light Load No Load, PWM, VCC Bias Current EN = AGND = PGND TA = 25C 45 420 1.2 1.6 230 230 85
3.6 1.2 70 1.0
VDAC = 1.3V or BYPASS = VIN VCC = 5.5V, VLX = 0 to VCC ILOAD = 0 to 500mA
1 0.5 0.2
VDAC = 0.6V, ILOAD = 0
1.746
170 1.8 2.0 140 15 100
1.854
VOUT = 0.6V to VOUT(MAX), MODE/SYNC = VIN
30
s
1. The AAT1171 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and is assured by design, characterization, and correlation with statistical process controls.
4
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Electrical Characteristics1
TA = -40C to +85C, unless otherwise noted. VIN = VCC = 3.6V; typical values are TA = 25C. Symbol Description Conditions Min Typ Max
0.6 VCC = 5.5V EN = Low to High, MODE/SYNC = High, VDAC = 1.2V Sync to 19.8MHz2 1.6 VSYNC = GND or VCC -1.0 3 0.6 1.0 1.4 -1.0 150 19.8 1.0
AAT1171
Units
V V A s MHz V A V/V
PWM/Light Load/EN VEN(L) Enable Threshold Low VEN(H) Enable Threshold High IEN Input Low Current tEN SYNC FSYNC VSYNC(H) VSYNC(L) ISYNC DAC Input Gain Turn-On Enable Response Time Synchronization Frequency SYNC High Level Threshold SYNC Low Level Threshold SYNC Low Current Output Voltage/DAC Voltage3
1. The AAT1171 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and is assured by design, characterization, and correlation with statistical process controls. 2. Please contact Sales for other synchronization frequencies. 2. Please contact Sales for other output voltage/DAC voltage gains. 1171.2006.06.1.0
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Typical Characteristics
Efficiency vs. Output Current
(LL Mode; VOUT = 3.3V)
100 90 1.0
AAT1171
Load Regulation
(LL Mode; VOUT = 3.3V)
Output Voltage Error (%)
VIN = 3.9V
Efficiency (%)
0.5
80 70 60 50 40 0.1
VIN = 5.0V VIN = 3.6V
VIN = 4.2V VIN = 5.0V
0.0
-0.5
VIN = 4.2V
1
10
100
1000
-1.0 0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PWM Mode; VOUT = 3.3V)
90 80
Load Regulation
(PWM Mode; VOUT = 3.3V)
1.0
Output Voltage Error (%)
100
Efficiency (%)
70 60 50 40 30 20 10 0 0.1 1 10
0.5
VIN = 5.0V
VIN = 3.6V
VIN = 4.2V VIN = 5.0V
0.0
VIN = 3.6V
-0.5
VIN = 4.2V
1 10 100 1000
100
1000
-1.0 0.1
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(LL Mode; VOUT = 2.5V) VIN = 3.0V
Output Voltage Error (%)
100 90 1.0
Load Regulation
(LL Mode; VOUT = 2.5V)
Efficiency (%)
0.5
80 70 60 50 40 0.1 1 10
VIN = 5.0V VIN = 4.2V
VIN = 4.2V VIN = 5.0V
0.0
-0.5
VIN = 3.0V
100
1000
-1.0 0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
6
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Typical Characteristics
Efficiency vs. Output Current
(PWM Mode; VOUT = 2.5V)
90 80
AAT1171
Load Regulation
(PWM Mode; VOUT = 2.5V)
1.0
Output Voltage Error (%)
100
Efficiency (%)
70 60 50 40 30 20 10 0 0.1
VIN = 3.0V VIN = 4.2V VIN = 5.0V
0.5
VIN = 5.0V VIN = 3.0V
0.0
VIN = 4.2V
-0.5
1
10
100
1000
-1.0 0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(LL Mode; VOUT = 1.8V) VIN = 2.7V VIN = 4.2V VIN = 3.6V
Output Voltage Error (%)
100 90 1.0
Load Regulation
(LL Mode; VOUT = 1.8V)
Efficiency (%)
80 70 60 50 40 30 0.1 1 10 100 1000
0.5
VIN = 3.6V VIN = 4.2V
0.0
-0.5
VIN = 2.7V
-1.0 0
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PWM Mode; VOUT = 1.8V)
100 90 80
Load Regulation
(PWM Mode; VOUT = 1.8V)
Output Voltage Error (%)
1.0
VIN = 2.7V VIN = 3.6V VIN = 4.2V
Efficiency (%)
70 60 50 40 30 20 10 0 0.1 1 10 100 1000 .
0.5
VIN = 3.6V VIN = 4.2V
0.0
VIN = 2.7V
-0.5
-1.0 0.1 1 10 100 1000
Output Current (mA)
Output Current (mA)
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Typical Characteristics
Output Voltage vs. Supply Voltage
(LL Mode; VOUT = 1.5V)
1.514 1.514
AAT1171
Output Voltage vs. Supply Voltage
(PWM Mode; VOUT = 1.5V)
Output Voltage (V)
1.510 1.506 1.502 1.498
Output Voltage (V)
IOUT = 50mA IOUT = 300mA
1.510 1.506 1.502 1.498
IOUT = 50mA IOUT = 300mA IOUT = 600mA
IOUT = 600mA
1.494 2.7 2.9 3.1 3.3 3..5 3.7 3..9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
1.494 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
Supply Voltage (V)
Supply Voltage (V)
Output Voltage vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; VDAC = 0.6V; RL = 10)
Output Voltage Error (%)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -40 0.05
Bypass Mode Dropout Voltage vs. Load Current
Dropout Voltage (V)
0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 0.1 1 10 100 1000
-15
10
35
60
85
Temperature (C)
Load Current (mA)
Supply Current vs. Supply Voltage
(No Load; LL Mode)
70 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 2.7
Supply Current vs. Supply Voltage
(No Load; PWM Mode)
Supply Current (mA)
Supply Current (A)
65 60 55 50 45 40 35 30
VOUT = 1.8V
VOUT = 1.8V
VOUT = 0.6V
VOUT = 0.6V
3.1 3.5 3.9 4.3 4.7 5.1 5.5
Supply Voltage (V)
Supply Voltage (V)
8
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Typical Characteristics
P-Channel RDS(ON) vs. Input Voltage
400 350 140
AAT1171
Bypass RDS(ON) vs. Input Voltage
TJ = 120C TJ = 85C
TJ = 120C
TJ = 85C
120
RDS(ON) (m)
250 200 150 100 50 0 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
RDS(ON) (m)
300
100 80 60 40 20 0 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
TJ = 25C
TJ = 25C
Input Voltage (V)
Input Voltage (V)
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; RL = 10)
Output Voltage vs. DAC Voltage
(VIN = 4.2V; LL Mode)
4.5 4.0
Switching Frequency (MHz)
2.06
Output Voltage (V)
2.04 2.02 2.00 1.98 1.96 1.94 1.92 1.90 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.2 0.4 0.6 0.8
25C 85C -40C
PWM LL
1.0
1.2
1.4
Temperature (C)
DAC Voltage (V)
Heavy Load Switching Waveform
(VIN = 3.6V; VOUT = 1.8V; RL = 3; COUT = 4.7F; L = 2.2H) VOUT (AC coupled) 20mV/div IL 200mA/div
0
VLX 2V/div
0
Time (200ns/div)
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Typical Characteristics
Light Load Switching Waveform
(PWM Mode; VIN = 4.2V; VOUT = 0.6V; RL = 10; COUT = 4.7F; L = 2.2H) VOUT (AC coupled) 20mV/div IL 100mA/div
0
AAT1171
Light Load Switching Waveform
(LL Mode; VIN = 4.2V; VOUT = 0.6V; RL = 10; COUT = 4.7F; L = 2.2H) VOUT (AC coupled) 20mV/div IL 200mA/div
0
VLX 2V/div
0
VLX 2V/div
0
Time (200ns/div)
Time (1s/div)
DAC Transient Response in PWM Mode
(VIN = 3.6V; RL = 10; COUT = 4.7F; L = 2.2H) VOUT 1V/div
3.3V
DAC Transient Response in LL Mode
(VIN = 3.6V; RL = 10; COUT = 4.7F; L = 2.2H) VOUT 1V/div
3.3V
0.6V 0 0 1.2V 1.2V
0.6V
VDAC 0.5V/div
0
0.2V
VDAC 0.5V/div
0
0.2V
Time (25s/div)
Time (25s/div)
Bypass Transient Response
(PWM Mode; VIN = 3.6V; RL = 10; COUT = 4.7F; L = 2.2H)
3.5V
Bypass Transient Response
(LL Mode; VIN = 3.6V; RL = 10; COUT = 4.7F; L = 2.2H)
3.5V
VOUT 1V/div
0.6V 0
VOUT 1V/div
0.6V 0
VBYP 1V/div
0
VBYP 1V/div
0
Time (25s/div)
Time (25s/div)
10
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Typical Characteristics
DAC to Bypass Transient Response
(LL Mode; VIN = 4.2V; RL = 10; COUT = 4.7F; L = 2.2H)
AAT1171
Enable Soft Start
(VIN = 3.6V; VOUT = 1.8V; RL = 4.5; COUT = 4.7F; L = 2.2H) VOUT 1V/div
0 1.8V
4.2V
VOUT 1V/div
0
0.6V 1.3V
Enable 2V/div
0
VDAC 0.5V/div
0
0.2V
IIN 200mA/div
0
Time (25s/div)
Time (50s/div)
Load Transient Response
(VIN = 4.2V; VOUT = 3.3V; COUT = 4.7F; L = 2.2H) VOUT (AC coupled) 20mV/div
3.51V
Load Transient Response
(VIN = 3.6V; VOUT = 1.8V; COUT = 4.7F; L = 2.2H) VOUT (AC coupled) 20mV/div
1.914V
3.26V 500mA 525mA
1.798V
IOUT 200mA/div
250mA
IOUT 100mA/div
200mA
Time (20s/div)
Time (20s/div)
(VOUT = 1.5V; RL = 10; COUT = 4.7F; L = 2.2H) VIN 0.5V/div
3.6V 3.0V
Line Transient Response
1.56V
VOUT (AC coupled) 50mV/div
1.44V
Time (50s/div)
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Functional Block Diagram
VOUT VCC VIN
AAT1171
DAC EN
Error Amp
Comp
DH
Logic
LX
DL
BYPASS
MODE/SYNC
MODE/SYNC Interface
AGND
PGND
Functional Description
The AAT1171 is a 600mA 2MHz peak current mode synchronous step-down (buck) converter designed to operate from a single-cell lithium-ion battery with a 2.7V to 4.2V input range. The output voltage is dynamically programmed by the DAC input voltage. To maximize converter efficiency over all load conditions, the converter automatically transitions to a variable frequency light load (LL) mode when the load is less than 100mA. When combined with the very low quiescent current, the LL mode maintains a high efficiency over the complete load range. For noise sensitive applications, the converter can be forced into a fixed frequency PWM mode. Provisions are also made for synchronization of the converter to an external system clock. The synchronous buck converter power output devices are sized at 230m for a 600mA full load output current. In addition to the converter output, an additional low resistance bypass MOSFET (85m) can be connected between the battery input and the converter output (VIN to VOUT), 12
bypassing the converter and output inductor to improve headroom and extend the WCDMA PA full power range. This reduces the battery voltage necessary for a WCDMA RF power amplifier to meet linearity requirements, thus extending operating time. In dual mode systems, the bypass mode may also be used when the WCDMA RF power amplifier is in GSM mode. Bypass mode is activated by setting the bypass input high or by forcing the baseband DAC output voltage to 1.3V. The AAT1171 requires only three external components for operation (CIN, COUT, LX). The high 2MHz switching frequency reduces the inductor size required to 2.2H. This reduces the DC resistance and improves the converter efficiency while minimizing the inductor footprint and height. The output voltage of the converter is regulated to within 0.5% and will settle in less than 30s (according to WCDMA specifications) in response to any step change in the DAC input. Under-voltage lockout, internal compensation, softstart, over-current, and over-temperature protection are also included.
1171.2006.06.1.0
600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
DAC Output Voltage Control
The output voltage is programmed by way of the DAC input voltage. The DAC to output gain for the AAT1171 is 3.
AAT1171
LL/PWM Control
Two control modes are available with the AAT1171: LL mode and PWM mode. PWM mode maintains a fixed switching frequency regardless of load. The fixed switching frequency gives the advantage of lower output ripple and simplified output and input noise filtering. PWM mode also provides a faster output voltage response to changes in the DAC voltage. In LL mode, the converter transitions to a variable switching frequency as the load decreases below 100mA. Above 100mA, where switching losses no longer dominate, the switching frequency is fixed. The LL mode's effect on the DAC to output voltage response time is most notable when transitioning from a high output voltage to a low voltage. When the converter is in PWM mode, the inductor current can be reversed and the output voltage actively discharged by the synchronous MOSFET. While in LL mode, the output voltage is discharged by the load only, resulting in a slower response to a DAC transition from a high to a low voltage. For PWM mode, apply a logic level high to the MODE/SYNC pin; for LL mode, apply a logic level low to the MODE/SYNC pin.
VOUT = 3 * VDAC
The DAC input voltage range is 0.2V to 1.2V, which corresponds to an output voltage range of 0.6V to 3.6V (see Figure 1). For a 1.3V DAC level, the bypass switch is activated and the output voltage level is equivalent to the input voltage minus the bypass MOSFET (RDS(ON)(bp)) drop.
Bypass Mode
In bypass mode, the AAT1171 bypasses the output inductor, connecting the input directly to the output through a low RDS(ON) 85m MOSFET. Bypass mode is initiated by applying 1.3V to the DAC input or by applying a logic high to the bypass input. When not activated, a logic level low must be applied to the bypass input pin. The bypass MOSFET current is limited to 600mA.
V IN 4V 3.6V
Output to PA
3V
BYPASS MODE
2V
1V 0.6V
0.2V
1V
1.2V 1.3V
DAC Output
Figure 1: VOUT vs. VDAC.
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
Soft Start/Enable
The AAT1171 soft-start control prevents output voltage overshoot and limits inrush current when either the input power or the enable input is applied. When pulled low, the enable input forces the converter into a low-power, non-switching state with less than 1A bias current.
AAT1171
Current Limit and Short-Circuit Protection
The high-side P-channel MOSFET current limit comparator limits the peak inductor current to 1.6A. In PWM mode, the synchronous MOSFET current limit comparator limits the peak negative inductor current, and output capacitor discharge current is limited to 1A. In bypass mode, the bypass MOSFET current is limited to 600mA. In the event of an overload or short-circuit condition, the current limit protects the load and the AAT1171 power devices. Upon removal of the short-circuit or fault condition, the AAT1171 output automatically recovers to the regulated level.
Low Dropout Operation
For conditions where the input voltage drops to the output voltage level, the converter duty cycle increases to 100%. As 100% duty cycle is approached, the minimum off-time initially forces the high-side on-time to exceed the 2MHz clock period, reducing the converter switching frequency. Once the input drops to the level where the output can no longer be regulated, the high-side P-channel MOSFET is enabled continuously for 100% duty cycle. The output voltage then tracks the input voltage minus the IR drop of the high side P-channel MOSFET RDS(ON).
Thermal Overload Protection
The maximum junction temperature is limited by the AAT1171 over-temperature shutdown protection circuitry. Both the step-down converter and the bypass MOSFET are disabled when the junction temperature reaches 140C. Normal operation resumes once the junction temperature drops to 125C.
UVLO Shutdown
Under-voltage lockout (UVLO) circuitry monitors the input voltage and disables the converter when the input voltage drops to 2.4V, guaranteeing sufficient operating input voltage to maintain output voltage regulation and control. For a rising input voltage, the UVLO circuitry enables the converter 200mV above the shutdown level at 2.6V.
External Synchronization
The AAT1171 switching frequency can be synchronized to an external square wave clock via the MODE/SYNC input. The external clock frequency range and logic levels for which the AAT1171 will remain synchronized are listed in the Electrical Characteristics table of this datasheet.
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Applications Information
Inductor Selection
The step-down converter uses peak current mode control with slope compensation to maintain stability for duty cycles greater than 50%. Because the required slope compensation varies with output voltage, the AAT1171 varies the slope compensation to match the output voltage. This allows the use of a single inductor value for all output voltage levels. For the AAT1171, this value is 2.2H. Manufacturer's specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. The inductor ripple current varies with both the input voltage and the output voltage and peaks at the maximum input voltage with the output at one half of the input voltage. For the typical AAT1171, this corresponds to a 4.2V input voltage and a 2.1V output voltage. With the suggested 2.2H inductor, this corresponds to 239mA peak-to-peak ripple current. For a 600mA DC load current, the peak inductor current would be 718mA. In order to prevent saturation under normal load conditions, the peak inductor current should be less than the inductor saturation current.
AAT1171
PL = IO2 DCR = 0.6A2 0.14 = 50mW L = PO 3.4 0.6A = = 97% PO + PL 3.4V 0.6A + 50mW
The 2.2H inductor selected for the AAT1171 evaluation board has a 140m DCR and a 0.91A DC current rating. At 600mA load current, the inductor loss is 50mW which gives 2.4% loss in efficiency for a 600mA 3.4V output voltage with an inductor that measures 3.2x3.2x1.2mm.
Output Capacitor Selection
The AAT1171 is designed for use with a 4.7F 10V X5R ceramic output capacitor. Although a larger output capacitor provides improved response to large load transients, it also limits the output voltage rise and fall time in response to the DAC input. For stable operation, with sufficient phase and gain margin, the internal voltage loop compensation limits the minimum output capacitor value to 4.7F. Increased output capacitance will reduce the crossover frequency with greater phase margin. The output voltage droop due to load transients is dominated by the output capacitor. During a step increase in load current, the output capacitor supplies the load current while the control loop responds. Within two or three switching cycles, the inductor current increases to match the load current demand. The relationship of the output voltage droop during the three switching cycles to the output capacitance can be estimated by:
3 * ILOAD VDROOP * FS
IPK(MAX) = IO +
VIN(MAX) 8 L FS 4.2V 8 2.2H 2MHz
= 0.6A +
= 0.6A + 0.12A = 0.72A
Some inductors may meet peak and average current requirements yet result in excessive losses due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor. The inductor losses can be estimated by using the full load output current. The output inductor losses can then be calculated to estimate their effect on overall device efficiency.
COUT =
Once the average inductor current increases to the DC load level, the output voltage recovers. The above equation establishes a limit on the minimum output capacitor value necessary to meet a given output voltage droop requirement (VDROOP) for a given load transient.
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
The maximum output capacitor RMS ripple current is:
VOUT * (VIN(MAX) - VOUT) L * FS * VIN(MAX) 2* 3 * 1
AAT1171
The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load current.
IRMS(MAX) =
Dissipation due to the RMS current in the ceramic output capacitor ESR is typically minimal, resulting in less than a few degrees rise in hot-spot temperature.
VO V * 1- O = VIN VIN
for VIN = 2 * VO
D * (1 - D) =
0.52 =
1 2
Input Capacitor Selection
A 10V X5R or X7R ceramic capacitor is suggested for the input capacitor with typical values ranging from 4.7F to 10F. To estimate the required input capacitance size, determine the acceptable input ripple level (VPP) and solve for C, as shown below. The calculated value varies with input voltage and is a maximum when VIN is double the output voltage. Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value. For example, due to the voltage coefficient of a 10F 6.3V X5R ceramic capacitor, with an applied voltage of 5V DC the capacitance decreases to 6F.
IRMS(MAX) =
VO
V * 1- O
IO 2
The term VIN VIN appears in both the input voltage ripple and input capacitor RMS current equations and is a maximum when VIN is twice Vo; therefore, the input voltage ripple and the input capacitor RMS current ripple are a maximum at 50% duty cycle. The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT1171. Low ESR/ESL X7R and X5R ceramic capacitors are ideal for this function. To minimize stray inductance, the capacitor should be placed as closely as possible to the IC. This keeps the high frequency content of the input current localized, minimizing EMI and input voltage ripple. The proper placement of the input capacitor (C1) can be seen in the evaluation board layout in Figure 3. A laboratory test set-up typically consists of two long wires running from the bench power supply to the evaluation board input voltage pins. The inductance of these wires, along with the low-ESR ceramic input capacitor, can create a high Q network that may affect converter performance. This problem often becomes apparent in the form of excessive ringing in the output voltage during load transients with errors in loop phase and gain measurements. Since the inductance of a short PCB trace feeding the input voltage is significantly lower than the power leads from the bench power supply, most applications do not exhibit this problem.
CIN =
VO V * 1- O VIN VIN
VPP - ESR * FS IO
VO V 1 * 1- O = VIN VIN 4 VIN = 2 * VO
CIN(MIN) =
1
VPP - ESR * 4 * FS IO
The maximum input capacitor RMS current is:
VO V * 1- O VIN VIN
IRMS = IO *
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
In applications where the input power source lead inductance cannot be reduced to a level that does not affect the converter performance, a high ESR tantalum or aluminum electrolytic capacitor (C3 of Figure 4) should be placed in parallel with the low ESR, ESL bypass ceramic capacitor. This dampens the high Q network and stabilizes the system.
AAT1171
Thermal Calculations
There are three types of losses associated with the AAT1171 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction losses are associated with the RDS(ON) characteristics of the power MOSFET devices. Switching losses are dominated by the gate charge of the power MOSFET devices. The AAT1171 main and synchronous power MOSFETs are sized to have similar RDS(ON) values that track with the input voltage. At full load, assuming continuous conduction mode (CCM), a simplified form of the stepdown converter losses is given by:
DAC Programming Gain
The output voltage is dynamically controlled by the DAC input voltage. The DAC to output gain is fixed at 3. The typical response time for a 0.2V to 1.2V pulsed signal on the DAC input is less than 30s. The DAC gain can be reduced by an external resistive divider at the DAC input, as shown in the evaluation board schematic in Figure 2. For a DAC to output gain of 2 and R2 at 10k, R1 is 4.99k.
PTOTAL = IO2 * RDS(ON) + (tSW * FS * IO + IQ) * VIN
(3- GDAC)R2 (3 - 2)10k R1 = = = 4.99k GDAC 2
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load switching losses, which are dominated by the gate charge losses.
U1 AAT1171
1 2 3 4 5
N/C VOUT VOUT VCC AGND DAC
LX PGND VIN MODE/SYNC BYPASS EN
12 11 10 9 8 7
L1 2.2H VOUT C2 4.7F C1 4.7F GND
GND
6
VIN
DAC
R1
R2
123
321
123
ENABLE Off On
BYPASS SYNC On Off LL PWM
L1 SD3112-2R2 or LPF2010-2R2 C1, COUT 4.7F 10V 0805
Figure 2: AAT1171 Evaluation Board Schematic.
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
For the condition where the buck converter is at 100% duty cycle dropout, the total device dissipation reduces to:
PTOTAL = IO2 * RDS(ON) + IQ * VIN
AAT1171
Layout
The suggested PCB layout for the AAT1171 is shown in Figures 3 and 4. The following guidelines should be used to ensure a proper layout. 1. The input capacitor (C1) should connect as closely as possible to VIN (Pin 10) and PGND (Pin 11). 2. C2 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible. 3. The PCB trace connected to VOUT (Pins 2 and 3) is tied to the bypass path, as well as the feedback path for the control loop. In bypass mode, the full load current is delivered directly from the battery input; therefore, this trace should be sufficient to handle current up to the bypass current limit level. 4. The resistance of the trace from the load return to PGND (Pin 11) should be kept to a minimum. This minimizes any error in DC regulation due to differences in the potential of the internal signal ground and the power ground. 5. For good thermal coupling, PCB vias are required from the pad for the TDFN exposed paddle to the ground plane. The via diameter should be 0.3mm to 0.33mm and positioned on a 1.2mm grid.
In bypass mode, the bypass MOSFET RDS(ON)(bp) is used to determine the losses. The power MOSFET RDS(ON) increases with decreasing input voltage and the associated losses are a maximum at the minimum input voltage (2.7V).
PTOTAL = IO2 * RDS(ON)(bp) + IQ * VIN
Since the RDS(ON), quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input voltage range. After calculating the total losses, the maximum junction temperature can be derived from the JA for the TDFN33-12 package which is typically 50C/W.
TJ(MAX) = PTOTAL * JA + TAMB
Figure 3: AAT1171 Evaluation Board Top Side Layout. 18
Figure 4: AAT1171 Evaluation Board Bottom Side Layout.
1171.2006.06.1.0
600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch PA Step-Down Converter Design Example
Specifications
VO(BUCK) 0.6V to 3.4V with RL =10 VIN FS TAMB 2.7V to 4.2V (3.6V nominal) 2.0MHz 85C
AAT1171
Output Inductor
L1 = 2.2H For Copper Electronics SD3112, 2.2H, DCR = 140m.
IL1(MAX) =
VO V 2.1V 2.1V 1- O = 1= 239mA L FS VIN 2.2H 2.0MHz 4.2V
The maximum inductor ripple current occurs at 50% duty cycle at the maximum input voltage.
IPKL1 = IO +
IL1(MAX) = 0.6A + 0.118A = 0.718A 2
PL1 = IO2 DCR = 0.6A2 140m = 50mW
Output Capacitor
Specify that VDROOP = 0.2V for a 600mA load pulse.
3 * ILOAD 3 * 0.6A = = 4.5F 0.2V * 2.0MHz VDROOP * FS 1 2* 3 * (VO) * (VIN(MAX) - VO) 1 3.4V * (4.2V - 3.4V) * = 69mArms = L1 * FS * VIN(MAX) 2 * 3 4.7H * 2.0MHz * 4.2V
COUT = IRMS =
PESR = ESR * IRMS2 = 5m * (69mA)2 = 24W
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
Input Capacitor
Specify a maximum input voltage ripple of VPP = 25mV.
AAT1171
CIN(MIN) =
1
VPP - ESR * 4 * FS IO
=
1 = 3.4F 25mV - 5m * 4 * 2.0MHz 0.6A
IRMS =
IO = 0.3Arms 2
P = ESR * IRMS2 = 5m * (0.3A)2 = 0.45mW
AAT1171 Losses
PTOTAL = IO2 * RDS(ON) + (tsw * FS * IO + IQ) * VIN = 0.62 * 0.29 + (5ns * 2.0MHz * 0.6A + 60A) * 4.2V = 104mW
TJ(MAX) = PTOTAL * JA + TAMB = 104mW * 50C/W = 5.2C + 70C = 75.2C
AAT1171 Dropout Losses
PTOTAL = IO2 * RDS(ON)(HS) + IQ * VIN = 0.62 * 310m + 100A * 3.5V = 112mW
TJ(MAX) = PTOTAL * JA + TAMB = 112mW * 50C/W = 5.6C + 70C = 75.6C
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch
Manufacturer
Murata www.murata.com TDK www.tdk.com Taiyo Yuden www.t-yuden.com
AAT1171
Value
4.7F 4.7F 4.7F
Device
Output or Input Capacitor Input Capacitor Output or Input Capacitor Input Capacitor Output or Input Capacitor Input Capacitor
Voltage
10V 6.3V 10V 6.3V 10V 6.3V
Case Size
0805 0603 0805 0603 0805 0603
Part Number
GRM21BR61A475KA73L GRM188R60J475KE19D C2012X5R1A475K C1608X5ROJ475K LMK212BJ475MG JMK107BJ475MA
Manufacturer
Cooper Electronics www.cooperet.com Sumida www.sumida.com ABCO Electronics www.abco.co.kr
Value
2.2H 2.2H 2.2H 2.2H
Part Number
SD3118-2R2 CDRH2D11/HP LPF2010-2R2M LPF2010-2R2M
ISAT
1.12A 1.1A
IRMS
0.91A 1.3A 0.52A 0.55A
DCR
140m 96m 200m 110m
Case Size (mm)
3.1x3.1x1.2 3.2x3.2x1.2 2.0x2.0x1.0 2.0x2.0x1.4
Table 1: Suggested Component Selection.
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600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch Ordering Information
Package
TDFN33-12
AAT1171
Marking1
RXXYY
Part Number (Tape and Reel)2
AAT1171IWP-1-T1
All AnalogicTech products are offered in Pb-free packaging. The term "Pb-free" means semiconductor products that are in compliance with current RoHS standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more information, please visit our website at http://www.analogictech.com/pbfree.
Package Information
Index Area (D/2 x E/2)
TDFN33-12
Detail "B"
3.00 0.05
2.40 0.05
0.3 0.10 0.16 0.375 0.125 0.075 0.075 0.1 REF
Top View
Bottom View
Pin 1 Indicator (optional)
7.5 7.5
+ 0.05 0.8 -0.20
0.229 0.051
0.05 0.05
Option A: C0.30 (4x) max Chamfered corner
Option B: R0.30 (4x) max Round corner
Detail "B"
Side View
All dimensions in millimeters. 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD.
(c) Advanced Analogic Technologies, Inc.
Detail "A"
AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech's standard warranty. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 22
1171.2006.06.1.0
0.23 0.05
0.45 0.05
Detail "A"
3.00 0.05
1.70 0.05

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