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LTM8021 36VIN, 500mA Step-Down DC/DC Module FEATURES
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DESCRIPTION
The LTM(R)8021 is a 36VIN 500mA, step-down DC/DC ModuleTM. Included in the package are the switching controller, power switches, inductor, and all support components. Operating over an input voltage range of 3V to 36V, the LTM8021 supports an output voltage range of 0.8V to 5V, set by a single resistor. Only an output and bulk input capacitor are needed to finish the design. The low profile package (2.82mm) enables utilization of unused space on the bottom of PC boards for high density point of load regulation. A built-in soft-start timer is adjustable with just a resistor and capacitor. The LTM8021 is packaged in a thermally enhanced, compact (11.25mm x 6.25mm) and low profile (2.82mm) overmolded Land Grid Array (LGA) package suitable for automated assembly by standard surface mount equipment. The LTM8021 is RoHS compliant.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Module is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Complete Switch Mode Power Supply Wide Input Voltage Range: 3V to 36V 500mA Output Current 0.8V to 5V Output Voltage Fixed 1.1MHz Switching Frequency Current Mode Control (e4) RoHS Compliant Package with Gold Pad Finish Programmable Soft-Start Tiny, Low Profile (11.25mm x 6.25mm x 2.82mm) Surface Mount LGA Package
APPLICATIONS
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Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation
TYPICAL APPLICATION
7VIN to 36VIN, 5V/500mA Module Regulator
90 VIN* 7V TO 36V LTM8021 IN RUN/SS GND 1F OUT BIAS ADJ 19.1k
8021 TA01a
Efficiency and Power Loss
450 400 EFFICIENCY EFFICIENCY (%) 70 60 50 40 30 1.00 POWER LOSS 350 POWER LOSS (mW) 300 250 200 150 100 50
VOUT 5V AT 500mA
80
2.2F
*RUNNING VOLTAGE RANGE. PLEASE REFER TO THE APPLICATIONS INFORMATION SECTION FOR START-UP DETAILS.
10.00 100.00 LOAD CURRENT (mA)
0 1000.00
8021 TA01b
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LTM8021 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW 5 4 3 ADJ 2 RUN/SS 1 A B C D E F G H GND BANK 3 VIN BANK 1
VIN, RUN/SS Voltage .................................................40V RUN/SS Above VIN ......................................................3V ADJ Voltage ................................................................5V BIAS Voltage ...............................................................7V VOUT Voltage .............................................................10V Internal Operating Temperature Range (Note 2) ..................................-40C to 125C Maximum Solder Temperature .............................. 260C Storage Temperature Range...................-55C to 125C
VOUT BANK 2
BIAS
LGA PACKAGE 35-LEAD (11.25mm 6.25mm
2.82mm)
TJMAX = 125C, JA = 24.9C/W, WEIGHT = 0.49g JA DERIVED FROM 6.35cm x 6.35cm; 4-LAYER PCB
ORDER INFORMATION
LEAD FREE FINISH LTM8021EV#PBF LTM8021IV#PBF PART MARKING* LTM8021V LTM8021V PACKAGE DESCRIPTION 35-Lead (11.25mm x 6.25mm x 2.82mm) 35-Lead (11.25mm x 6.25mm x 2.82mm) TEMPERATURE RANGE (NOTE 2) -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
ELECTRICAL CHARACTERISTICS
SYMBOL VIN VOUT RADJ(MIN) ILK IOUT IQVIN IQBIAS VOUT/VOUT VOUT/VOUT PARAMETER Input DC Voltage Output DC Voltage Minimum Allowable RADJ Leakage from IN to OUT Continuous Output DC Current Quiescent Current into VIN Quiescent Current into BIAS Line Regulation Load Regulation
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C, VIN = 10V, VRUN/SS = 10V, VBIAS = 3V, RADJ = 31.6k.
CONDITIONS VRUN/SS = 5V, RADJ = Open 0 < IOUT < 500mA; RADJ Open 0 < IOUT < 500mA; RADJ = 19.1k, 0.1% Note 3 RUN/SS = VBIAS = 0V, RADJ Open 5V VIN 36V, VBIAS = VOUT RUN/SS = 0.2V, VBIAS , RADJ Open Not Switching Not Switching 5V VIN 36V, IOUT = 500mA RADJ = Open VIN = 24V, 0 IOUT 500mA, VBIAS = VOUT 0 0.1 1.5 0.15 0.5 0.35 18 2.7 6 500 1 2.5 MIN 3 0.8 5 TYP MAX 36 UNITS V V V k A mA A mA A % %
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LTM8021 ELECTRICAL CHARACTERISTICS
SYMBOL VOUT(DC) VOUT(AC_RMS) fSW IOSC IISC ADJ VBIAS(MIN) IADJ IRUN/SS VIH(RUN/SS) VIL(RUN/SS) RFB PARAMETER DC Output Voltage Output Voltage Ripple (RMS) Switching Frequency Short-Circuit Output Current Short-Circuit Input Current Voltage at ADJ Pin Minimum BIAS Voltage for Proper Operation Current Out of ADJ Pin RUN/SS Pin Current RUN/SS Input High Voltage RUN/SS Input Low Voltage Internal Feedback Resistor
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C, VIN = 10V, VRUN/SS = 10V, VBIAS = 3V, RADJ = 31.6k.
CONDITIONS VIN = 24V, 0 IOUT 500mA RADJ = 31.6k, 0.1% VIN = 24V, IOUT = 250mA COUT = 2.2F VBIAS = VOUT , IOUT = 500mA VIN = 36V, VBIAS = VOUT = 0V VIN = 36V, VBIAS = VOUT = 0V RADJ Open IOUT = 500mA VOUT = 5V, VADJ = 0V, RUN/SS = 0V VRUN/SS = 2.5V, RADJ Open RADJ Open, IOUT = 500mA RADJ Open, IOUT = 500mA RUN/SS = VBIAS = VADJ = 0V 100 1.6 0.5
l
MIN
TYP 3.3 1
MAX
UNITS V mV
0.9
1.1 900 25
1.3
MHz mA mA
0.79
0.80 2.2 50 23
0.83 3
V V A A V V k
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTM8021E is guaranteed to meet performance specifications from 0C to 125C internal. Specifications over the full -40C to 125C internal operating temperature range are assured by design, characteriza-
tion and correlation with statistical process controls. The LTM8021I is guaranteed to meet specifications over the full -40C to 125C internal operating temperature range. Note that the maximum internal temperature is determined by specific operating conditions in conjunction with board layout, the rated package thermal resistance and other environmental factors. Note 3: Guaranteed by design.
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LTM8021 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
90 85 80 EFFICIENCY (%) 75 70 65 60 55 50 45 40 0 50 100 150 200 250 300 350 400 450 500 LOAD CURRENT (mA)
8021 G01
TA = 25C, unless otherwise noted
Efficiency vs Load Current
90 VOUT = 2.5V 90 VIN = 5V VIN = 12V 80 VIN = 24V VIN = 36V EFFICIENCY (%) 75 70 65 60 55 0 50 100 150 200 250 300 350 400 450 500 LOAD CURRENT (mA)
8021 G02
Efficiency vs Load Current
VOUT = 3.3V VIN = 5V VIN = 12V VIN = 24V VIN = 36V 85
VOUT = 1.8V VIN = 5V VIN = 12V EFFICIENCY (%) VIN = 24V VIN = 36V
85 80 75 70 65 60 55 50
0
50 100 150 200 250 300 350 400 450 500 LOAD CURRENT (mA)
8021 G03
Efficiency vs Load Current
90 85 EFFICIENCY (%) 80 75 70 65 60 VOUT = 5V 6 VIN = 12V VIN = 24V VIN = 36V BIAS CURRENT (mA) 5 4
IBIAS vs Load Current
VOUT = 0.8V VIN = 3.4V BIAS CURRENT (mA) 9 8 7 6 5 4 3 2 1 VIN = 24V 0 1 0 0 100 300 400 200 LOAD CURRENT (mA) 500 600 VIN = 5V 3 2 VIN = 12V
IBIAS vs Load Current
VOUT = 1.8V VIN = 3.4V
VIN = 5V
VIN = 12V
VIN = 24V 0 100 300 400 200 LOAD CURRENT (mA) 500 600
0
50 100 150 200 250 300 350 400 450 500 LOAD CURRENT (mA)
8021 G04
8021 G05
8021 G06
IBIAS vs Load Current
8 7 BIAS CURRENT (mA) 6 5 4 VIN = 12V 3 2 1 0 0 100 300 400 200 LOAD CURRENT (mA) 500 600 VIN = 24V VOUT = 2.5V VIN = 5V BIAS CURRENT (mA) 10 9 8 7 6 5 4 3 2 1 0
IBIAS vs Load Current
VOUT = 3.3V VIN = 5V
VIN = 12V VIN = 24V
0
100
300 400 200 LOAD CURRENT (mA)
500
600
8021 G07
8021 G08
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LTM8021 TYPICAL PERFORMANCE CHARACTERISTICS
Input Current vs Output Current
400 350 INPUT CURRENT (mA) 300 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 450 500 OUTPUT CURRENT (mA)
8021 G09
TA = 25C, unless otherwise noted
Input Current vs Output Current
300 VIN = 12V VOUT = 5V INPUT CURRENT (mA) 100 80 VOUT = 3.3V VOUT = 2.5V 140 120 250 INPUT CURRENT (mA) 200 150 100 50 0 0 50 100 150 200 250 300 350 400 450 500 OUTPUT CURRENT (mA)
8021 G10
Input Current vs Output Current
VIN = 24V VOUT = 5V VOUT = 3.3V VOUT = 2.5V 60 40 20 0
VIN = 5V
VOUT = 3.3V
VOUT = 2.5V VOUT = 1.8V
VOUT = 1.8V
VOUT = 1.8V
0
50 100 150 200 250 300 350 400 450 500 OUTPUT CURRENT (mA)
8021 G11
Input Quiescent Current vs Input Voltage
3000 INPUT QUIESCENT CURRENT (A) 2500 2000 1500 1000 500 0 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 INPUT VOLTAGE (V) VO = 3.3V 7 6 5 4 3 2 1 0
Minimum Input Running Voltage vs Output Voltage
IOUT = 500mA 900 880 OUTPUT CURRENT (mA) 860 840 820 800 780 760 740 720 0 1 3 4 2 OUTPUT VOLTAGE (V) 5
8021 G13
Output Short-Circuit Current vs Input Voltage
VOUT = 3.3V
6
4
8
12
16
20
24
28
32
36
INPUT VOLTAGE (V)
8021 G14
8021 G12
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LTM8021 PIN FUNCTIONS
VIN (Bank 1): The VIN pin supplies current to the LTM8021's internal regulator and to the internal power switch. This pin must be locally bypassed with an external, low ESR capacitor of at least 1F. VOUT (Bank 2): Power Output Pins. An external capacitor is connected from VOUT to GND in most applications. Apply output load between these pins and GND pins. BIAS (Pin H3): The BIAS pin connects to the internal boost Schottky diode and to the internal regulator. Tie to VOUT when VOUT > 3V or to another DC voltage greater than 3V otherwise. When BIAS > 3V the internal circuitry will be powered from this pin to improve efficiency. Main regulator power will still come from VIN. RUN/SS (Pin A1): Tie RUN/SS pin to ground to shut down the LTM8021. Tie to 1.6V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. The RUN/SS also provides soft-start and frequency foldback. To use the soft-start function, connect a resistor and capacitor to this pin. Do not allow the RUN/SS pin to rise above VIN. See the Applications Information section. GND (Bank 3): The GND connections serve as the main signal return and the primary heatsink for the LTM8021. Tie the GND pins to a local ground plane below the LTM8021 and the circuit components. Return the feedback divider to this signal. ADJ (Pin A2): The LTM8021 regulates its ADJ pin to 0.8V. Connect the adjust resistor from this pin to ground. The value of RADJ is given by the equation, RADJ = 80/(VOUT - 0.8), where RADJ is in k.
BLOCK DIAGRAM
VIN 0.1F 10H 15pF 100k 1% VOUT
10F
BIAS
RUN/SS
CURRENT MODE CONTROLLER
GND
ADJ
8021 BD
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LTM8021 OPERATION
The LTM8021 is a stand alone non-isolated step down switching DC/DC power supply. It can deliver up to 500mA of DC output current with only bulk external input and output capacitors. This module provides a precisely regulated output voltage programmable via one external resistor from 0.8VDC to 5VDC . The input voltage range is 3V to 36V. Given that the LTM8021 is a step down converter, make sure that the input voltage is high enough to support the desired output voltage and load current. Please refer to the simplified Block Diagram. The LTM8021 contains a current mode controller, power switching element, power inductor, power Schottky diode and a modest amount of input and output capacitance. With its high performance current mode controller and internal feedback loop compensation, the LTM8021 module has sufficient stability margin and good transient performance under a wide range of operating conditions with a wide range of output capacitors, even all ceramic ones (X5R or X7R). Current mode control provides cycle-by-cycle fast current limit, and automatic current limiting protects the module in the event of a short-circuit or overload fault. The LTM8021 is based upon a 1.1MHz fixed frequency PWM current mode controller, equipped with cycle skip capability for low voltage outputs or light loads. A frequency foldback scheme helps to protect internal components from overstress under heavy and short-circuit output loads. The drive circuit for the internal power switching element is powered through the BIAS pin. Power this pin with at least 3V.
APPLICATIONS INFORMATION
For most applications, the design process is straight forward, summarized as follows: 1. Refer to Table 1 for the row that has the desired input range and output voltage. 2. Apply the recommended CIN, COUT, and RADJ values. 3. Connect BIAS as indicated. While these component combinations have been tested for proper operation, it is incumbent upon the user to verify proper operation over the intended system's line, load and environmental conditions. If the desired output voltage is not listed in Table 1, set the output by applying an RADJ resistor whose value is given by the equation, RADJ = 80/(VOUT - 0.80), where RADJ is in k and VOUT is in volts. Verify the LTM8021's operation over the system's intended line, load and environmental conditions. Minimum Duty Cycle The LTM8021 has a fixed 1.1MHz switching frequency. For any given output voltage, the duty cycle falls as the input voltage rises. At very large VIN to VOUT ratios, the duty cycle can be very small. Because the LTM8021's internal controller IC has a minimum on-time, the regulator will skip cycles in order to maintain output voltage regulation. This will result in a larger output voltage ripple and possible disturbances during recovery from a transient load step. The component values provided in Table 1 allow for skip cycle operation, but hold the resultant output ripple to around 50mV, or less. If even less ripple is desired, then more output capacitance may be necessary. Adding a feedforward capacitor has been empirically shown to modestly extend the input voltage range to where the LTM8021 does not skip cycles. Apply the feedforward capacitor between the VOUT pins and ADJ. This injects perturbations into the control loop, therefore, values larger than 50pF are not recommended. A good value to start with is 12pF.
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LTM8021 APPLICATIONS INFORMATION
Table 1. Recommended Component Values and Configuration
VIN RANGE 3.4V to 36V 3.4V to 36V 3.4V to 36V 3.4V to 36V 3.5V to 36V 4V to 36V 4V to 36V 5V to 36V 7V to 36V 3.5V to 32V 3.75V to 31V 3.4V to 15V 3.4V to 15V 3.4V to 15V 3.4V to 15V 3.5V to 15V 4V to 15V 4V to 15V 5V to 15V 7V to 15V 9V to 24V 9V to 24V 9V to 24V 9V to 24V 9V to 24V 9V to 24V 9V to 24V 9V to 24V 9V to 24V 18V to 36V 18V to 36V 18V to 36V 18V to 36V 18V to 36V 18V to 36V 18V to 36V 18V to 36V 18V to 36V VOUT 0.8V 1.2V 1.5V 1.8V 2V 2.2V 2.5V 3.3V 5V -3.3V -5V 0.8V 1.2V 1.5V 1.8V 2V 2.2V 2.5V 3.3V 5V 0.8V 1.2V 1.5V 1.8V 2V 2.2V 2.5V 3.3V 5V 0.8V 1.2V 1.5V 1.8V 2V 2.2V 2.5V 3.3V 5V CIN 4.7F 4.7F 4.7F 2.2F 2.2F 1F 1F 1F 1F 1F 1F 4.7F 4.7F 4.7F 2.2F 2.2F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1uF 1uF 1uF 1uF 1uF 1uF 1uF 1uF 1uF COUT 100F 1210 100F 1210 100F 1210 100F 1210 100F 1210 22F 1206 10F 0805 4.7F 0805 2.2F 0805 4.7F 0805 4.7F 0805 100F 1210 100F 1210 47F 1206 47F 1206 22F 1206 22F 1206 10F 0805 2.2F 0805 1F 0805 100F 1210 100F 1210 47F 1206 47F 1206 22F 1206 22F 1206 10F 0805 2.2F 0805 1F 0805 100F 1210 100F 1210 100F 1210 100F 1210 100F 1210 22F 1206 10F 0805 4.7F 0805 2.2F 0805 RADJ 8.2M 200k 115k 78.7k 66.5k 57.6k 47.5k 32.4k 19.1k 32.4k 19.1k 8.2M 200k 115k 78.7k 66.5k 57.6k 47.5k 32.4k 19.1k Open 200k 115k 78.7k 66.5k 57.6k 47.5k 32.4k 19.1k Open 200k 115k 78.7k 66.5k 57.6k 47.5k 32.4k 19.1k BIAS 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V VOUT VOUT GND GND 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V VOUT VOUT 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V VOUT VOUT 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V 3V to 7V VOUT VOUT
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LTM8021 APPLICATIONS INFORMATION
Capacitor Selection Considerations The CIN and COUT capacitor values in Table 1 are the minimum recommended values for the associated operating conditions. Applying capacitor values below those indicated in Table 1 is not recommended, and may result in undesirable operation. Using larger values is generally acceptable, and can yield improved dynamic response or fault recovery, if it is necessary. Again, it is incumbent upon the user to verify proper operation over the intended system's line, load and environmental conditions. Ceramic capacitors are small, robust and have very low ESR. However, not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and applied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application circuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. Ceramic capacitors are also piezoelectric. At light loads, the LTM8021 skips switching cycles in order to maintain regulation. The resulting bursts of current can excite a ceramic capacitor at audio frequencies, generating audible noise.
8 VOUT = 5V 7 INPUT VOLTAGE (V) 6 5 4 3 2 0.001 RUN/SS ENABLED TO RUN TO START 5.0 INPUT VOLTAGE (V) 4.5 4.0 3.5 3.0 2.5 0.01 0.1 LOAD CURRENT (A) 1 2.0 0.001 0.01 0.1 LOAD CURRENT (A)
8021 F01
If this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. This output capacitor can be a parallel combination of a 1F ceramic capacitor and a low cost electrolytic capacitor. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM8021. A ceramic input capacitor combined with trace or cable inductance forms a high Q (under damped) tank circuit. If the LTM8021 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the device's rating. This situation is easily avoided; see the Hot-Plugging Safely section. Minimum Input Voltage The LTM8021 is a step-down converter, so a minimum amount of headroom is required to keep the output in regulation. For most applications at full load, the input must be about 1.5V above the desired output. In addition, it takes more input voltage to turn on than is required for continuous operation. This is shown in Figure 1.
6.0 5.5
VOUT = 3.3V TO START
RUN/SS ENABLED TO RUN
1
Figure 1. The LTM8021 Requires More Voltage to Start Than to Run
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LTM8021 APPLICATIONS INFORMATION
Soft-Start The RUN/SS pin can be used to soft-start the LTM8021, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC filter to create a voltage ramp at this pin. Figure 2 shows the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 80A when the RUN/SS pin reaches 2V.
RUN 15k RUN/SS 0.22F GND
Shorted Input Protection Care needs to be taken in systems where the output will be held high when the input to the LTM8021 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LTM8021's output. If the VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to VIN), then the LTM8021's internal circuitry will pull its quiescent current through its internal power switch. This is fine if your system can tolerate a few milliamps in this state. If the RUN/SS pin is grounded, the internal power switch current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LTM8021 can pull large currents from the output through the internal power switch and the VIN pin. Figure 3 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
LTM8021 VIN 4V TO 36V VIN VOUT VOUT
8021 F02
Figure 2. To Soft-Start the LTM8021, Add a Resisitor and Capacitor to the RUN/SS Pin
RUN/SS GND CIN
BIAS RT RADJ COUT
8021 F03
Figure 3. The Input Diode Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LTM8021 Runs Only When the Input is Present
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LTM8021 APPLICATIONS INFORMATION
PCB Layout Most of the problems associated with the PCB layout have been alleviated or eliminated by the high level of integration of the LTM8021. The LTM8021 is nevertheless a switching power supply, and care must be taken to minimize EMI and ensure proper operation. Even with the high level of integration, one may fail to achieve a specified operation with a haphazard or poor layout. See Figure 4 for a suggested layout. Ensure that the grounding and heatsinking are acceptable. A few rules to keep in mind are: 1. Place the CIN capacitor as close as possible to the VIN and GND connection of the LTM8021. 2. Place the COUT capacitor as close as possible to the VOUT and GND connection of the LTM8021. 3. Place the CIN and COUT capacitors such that their ground currents flow directly adjacent to, or underneath, the LTM8021. 4. Connect all of the GND connections to as large a copper pour or plane area as possible on the top layer. Avoid breaking the ground connection between the external components and the LTM8021. Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LTM8021. However, these capacitors can cause problems if the LTM8021 is plugged into a live supply (see the Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an under damped tank circuit, and the voltage at the VIN pin of the LTM8021 can ring to twice the nominal input voltage, possibly exceeding the LTM8021's rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LTM8021 into an energized supply, the input network should be designed to prevent this overshoot. Figure 5 shows the waveforms that result when an LTM8021 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2F ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 5b an aluminum electrolytic capacitor has been added. This capacitor's high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 5c. A 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1F capacitor improves high frequency filtering. This solution is smaller and less expensive than the electrolytic capacitor. For high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5V output at full load operating from 24V.
VIN CIN
GND
PLANE VOUT COUT
RADJ RUN/SS
FB
BIAS 8021 F04
Figure 4. Layout Showing Suggested External Components, GND Plane and Thermal Vias
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LTM8021 APPLICATIONS INFORMATION
CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LTM8021 DANGER VIN 20V/DIV RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING
+
4.7F
LOW IMPEDANCE ENERGIZED 24V SUPPLY
STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR
IIN 10A/DIV 20s/DIV
(5a)
VIN 20V/DIV
LTM8021
+
+
22F AI.EI.
4.7F
IIN 10A/DIV
(5b)
20s/DIV
0.7 LTM8021
VIN 20V/DIV
+
0.1F 4.7F IIN 10A/DIV
(5c)
20s/DIV
8021 F05
Figure 5. Ensures Reliable Operation When the LTM8021 is Connected to a Live Supply
High Temperature Considerations The die temperature of the LTM8021 must be lower than the maximum rating of 125C, so care should be taken in the layout of the circuit to ensure good heat sinking of the LTM8021. To estimate the junction temperature, approximate the power dissipation within the LTM8021 by applying the typical efficiency stated in this datasheet to the desired output power, or, if one has an actual module, by taking a power measurement. Then, calculate the temperature rise of the LTM8021 junction above the surface
of the printed circuit board by multiplying the module's power dissipation by the thermal resistance. The actual thermal resistance of the LTM8021 to the printed circuit board depends on the layout of the circuit board, but the thermal resistance given on page 2, which is based upon a 40.3cm2 4-layer FR4 PC board, can be used a guide. Finally, be aware that at high ambient temperatures the internal Schottky diode will have significant leakage current (see the Typical Performance Characteristics) increasing the quiescent current of the LTM8021.
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LTM8021 TYPICAL APPLICATIONS
0.8V Step-Down Converter
LTM8021 VIN 5V BIAS RUN/SS 1F GND ADJ 1F
8021 TA02
1.8V Step-Down Converter
LTM8021 VIN 5V 100F BIAS RUN/SS GND ADJ 78.7k 100F VOUT
VIN* 3.4V TO 36V
VOUT
VOUT 0.8V AT 500mA
VIN* 3.4V TO 36V
VOUT 1.8V AT 500mA
*RUNNING VOLTAGE RANGE. PLEASE REFER TO THE APPLICATIONS INFORMATION SECTION FOR START-UP DETAILS.
8021 TA03
*RUNNING VOLTAGE RANGE. PLEASE REFER TO THE APPLICATIONS INFORMATION SECTION FOR START-UP DETAILS.
5V Step-Down Converter
LTM8021 VIN VOUT
VIN* 7V TO 36V
VOUT 5V AT 500mA
RUN/SS
BIAS
GND 1F
ADJ 19.1k 2.2F
8021 TA04
*RUNNING VOLTAGE RANGE. PLEASE REFER TO THE APPLICATIONS INFORMATION SECTION FOR START-UP DETAILS.
-5V Positive-to-Negative Converter
600 VIN* 3.75V TO 31V LTM8021 VIN VOUT 500 400 4.7F GND 1F ADJ 19.1k -5V
8021 TA05
Load Current vs Input Voltage
RUN/SS
BIAS
OPTIONAL SCHOTTKY CLAMP ILOAD (mA)
300 200 100 0 0 5 10 15 VIN (V)
8021 TA05b
*RUNNING VOLTAGE RANGE. PLEASE REFER TO THE APPLICATIONS INFORMATION SECTION FOR START-UP DETAILS.
20
25
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13
LTM8021 PACKAGE DESCRIPTION
LGA Package 35-Lead (11.25mm x 6.25mm x 2.82mm)
(Reference LTC DWG # 05-08-1805 Rev A)
aaa Z
11.250 BSC
X
2.72 - 2.92
Y
6.250 BSC 2.40 - 2.60
bbb Z
MOLD CAP
SUBSTRATE
0.27 - 0.37
Z
PAD 1 CORNER
4
DETAIL A
aaa Z
PACKAGE TOP VIEW
DETAIL A PACKAGE SIDE VIEW 0.0000
4.445
3.175
1.905
0.635
0.635
1.905
3.175
4.445
PADS SEE NOTES 3
8.890 BSC 0.605 - 0.665 5
2.540
1.270
0.605 - 0.665 5.080 BSC
4
0.0000 0.9525 1.270 1.5875 2.540
3
2 1.270 BSC 1 H G F E D C B A PAD 1 C (0.30)
0.9525 0.635 0.3175
PACKAGE BOTTOM VIEW
SUGGESTED PCB LAYOUT TOP VIEW
NOTES: 1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994 2. ALL DIMENSIONS ARE IN MILLIMETERS 3 4 LAND DESIGNATION PER JESD MO-222, SPP-010 AND SPP-020 DETAILS OF PAD #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE PAD #1 IDENTIFIER MAY BE EITHER A MOLD OR A MARKED FEATURE
COMPONENT PIN "A1"
5. PRIMARY DATUM -Z- IS SEATING PLANE 6. THE TOTAL NUMBER OF PADS: 35
LTMXXXXXX Module
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14
LTM8021 PACKAGE DESCRIPTION
LTM8021 Pinout (Sorted by Pin Number)
PIN A1 A2 A4 A5 B1 B2 B4 B5 C1 C2 D1 D2 D3 D4 D5 E1 E2 E3 E4 E5 F1 F2 F3 F4 F5 G1 G2 G3 G4 G5 H1 H2 H3 H4 H5 SIGNAL DESCRIPTION RUN/SS ADJ VIN VIN GND GND VIN VIN GND GND GND GND GND GND GND GND GND GND GND GND GND GND VOUT VOUT VOUT GND GND VOUT VOUT VOUT GND GND BIAS VOUT VOUT
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTM8021 TYPICAL APPLICATION
3.3V Step-Down Converter
VIN* 5.5V TO 36V
LTM8021 VIN VOUT
VOUT 3.3V AT 500mA
RUN/SS
BIAS
GND 1F
ADJ 32.4k 4.7F
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*RUNNING VOLTAGE RANGE. PLEASE REFER TO THE APPLICATIONS INFORMATION SECTION FOR START-UP DETAILS.
RELATED PARTS
PART NUMBER LTM4600 LTM4600HVMPV LTM4601/ LTM4601A LTM4602 LTM4603 LTM4604 LTM4605 LTM4607 LTM4608 LTM8020 LTM8022 LTM8023 DESCRIPTION 10A DC/DC Module Military Plastic 10A DC/DC Module 12A DC/DC Module with PLL, Output Tracking/Margining and Remote Sensing 6A DC/DC Module 6A DC/DC Module with PLL and Output Tracking/ Margining and Remote Sensing 4A Low VIN DC/DC Module 5A to 12A Buck-Boost Module 5A to 12A Buck-Boost Module 8A Low VIN DC/DC Module 36V, 200mA DC/DC Module 1A, 36V DC/DC Module 2A, 36V DC/DC Module COMMENTS Basic 10A DC/DC Module, 15mm x 15mm x 2.8mm LGA -55C to 125C Operation, 15mm x 15mm x 2.8mm LGA Synchronizable, PolyPhase(R) Operation, LTM4601-1 Version Has No Remote Sensing Pin-Compatible with the LTM4600 Synchronizable, PolyPhase Operation, LTM4603-1 Version Has No Remote Sensing, Pin-Compatible with the LTM4601 2.375V VIN 5V, 0.8V VOUT 5V, 9mm x 15mm x 2.3mm LGA High Efficiency, Adjustable Frequency, 4.5V VIN 20V, 0.8V VOUT 16V, 15mm x 15mm x 2.8mm High Efficiency, Adjustable Frequency, 4.5V VIN 36V, 0.8V VOUT 25V, 15mm x 15mm x 2.8mm 2.375V VIN 5V, 0.8V VOUT 5V, 9mm x 15mm x 2.8mm LGA 4V VIN 36V, 1.25V VOUT 5V, 6.25mm x 6.25mm x 2.3mm LGA Adjustable Frequency, 0.8V VOUT 5V, 11.25mm x 9mm x 2.82mm, Pin-Compatible to the LTM8023 Adjustable Frequency, 0.8V VOUT 5V, 11.25mm x 9mm x 2.82mm, Pin-Compatible to the LTM8022
PolyPhase is a trademark of Linear Technology Corporation
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16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1208 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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