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CS52845
CS52845
Current Mode PWM Control Circuit with 50% Max Duty Cycle
Description
The CS52845 provides all the necessary features to implement offline fixed frequency current-mode control with a minimum number of external components. The CS52845 incorporates a new precision temperature-controlled oscillator to minimize variations in frequency. An internal toggle flipflop, which blanks the output every other clock cycle, limits the duty-cycle range to less than 50%. An undervoltage lockout ensures that VREF is stabilized before the output stage is enabled. In the CS52845 turn on is at 8.4V and turn off at 7.6V. Other features include low start-up current, pulse-by-pulse current limiting, and a high-current totem pole output for driving capacitive loads, such as gate of a power MOSFET. The output is low in the off state, consistent with N-channel devices.
Features
s Optimized for Off-line Control s Temperature Compensated Oscillator s 50% Maximum Duty-cycle Clamp s VREF Stabilized before Output Stage is Enabled s Low Start-up Current s Pulse-by-pulse Current Limiting s Improved Undervoltage Lockout s Double Pulse Suppression s 1% Trimmed Bandgap Reference s High Current Totem Pole Output
Absolute Maximum Ratings Supply Voltage (ICC<30mA) ..........................................................Self Limiting Supply Voltage (Low Impedance Source)...................................................30V Output Current ...............................................................................................1A Output Energy (Capacitive Load) .................................................................5J Analog Inputs (VFB, VSENSE)...........................................................-0.3V to 5.5V Error Amp Output Sink Current...............................................................10mA Lead Temperature Soldering Reflow (SMD styles only) ...........60 sec. max above 183C, 230C peak Block Diagram
Package Options
8L SO Narrow
COMP
1 2 3 4 8 7 6 5
VREF VCC VOUT Gnd
VCC
VCC Undervoltage Lock-out
VCC Pwr
VFB Sense
34V Gnd 8.4V/7.6V VFB Error Amplifier + 2.50V R R
Set/ Reset
5.0 Volt Reference Internal Bias
VREF
OSC
14L SO Narrow
COMP 1 NC 2 VFB 3 NC 4 Sense 5
Pwr Gnd
14 13 12 11 10 9 8
COMP
VREF Undervoltage Lockout
VREF NC VCC VCC Pwr VOUT Gnd Pwr Gnd
OSC
Oscillator
Toggle Flip-Flop S R
NOR
VOUT
2R R Sense 1V
NC 6 OSC
7
PWM Current Sensing Latch Comparator
Cherry Semiconductor Corporation 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885-3600 Fax: (401)885-5786 Email: info@cherry-semi.com Web Site: www.cherry-semi.com
Rev. 3/4/99
1
A
(R)
Company
CS52845
Electrical Characteristics: -40 TA 85C; VCC = 15V (Note 1); RT = 10k; CT = 3.3nF for sawtooth mode.unless otherwise stated. PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
s Reference Section Output Voltage Line Regulation Load Regulation Temperature Stability Total Output Variation Output Noise Voltage Long Term Stability Output Short Circuit s Oscillator Section Initial Accuracy Voltage Stability Temperature Stability Amplitude s Error Amp Section Input Voltage Input Bias Current AVOL Unity Gain Bandwidth PSRR Output Sink Current Output Source Current VOUT HIGH VOUT LOW s Current Sense Section Gain Maximum Input Signal PSRR Input Bias Current Delay to Output s Output Section Output Low Level ISINK=200mA Output High Level Rise Time Fall Time ISINK=20mA ISOURCE=20mA ISOURCE=200mA TJ=25C, CL=1nF (Note 2) TJ=25C, CL=1nF (Note 2) 13.0 12.0 0.1 1.5 13.5 13.5 50 50 150 150 0.4 2.2 V V V V ns ns (Notes 3 & 4) VCOMP=5V (Note 3) 12VCC25V (Note 3) VSense=0V TJ=25C (Note 2) 2.85 0.9 3.00 1.0 70 -2 150 -10 300 3.15 1.1 V/V V dB A ns VCOMP=2.5V VFB=0V 2VOUT4V (Note 2) 12VCC25V VFB=2.7V, VCOMP=1.1V VFB=2.3V, VCOMP=5V VFB=2.3V, RL=15k to Gnd VFB=2.7V, RL=15k to VREF 65 0.7 60 2 -0.5 5 2.45 2.50 -0.3 90 1.0 70 6 -0.8 6 0.7 1.1 2.55 -1.0 V A dB MHz dB mA mA V V Sawtooth Mode, TJ=25C 12VCC25V Sawtooth Mode TMINTATMAX (Note 2) VOSC (peak to peak) 47 52 0.2 5 1.7 57 1.0 kHz % % V TJ=25C, IREF=1mA 12VCC25V 1IREF20mA (Note 2) Line, Load, Temp. (Note 2) 10Hzf10kHz, TJ=25C (Note 2) TA=125C, 1000 Hrs. (Note 2) TA=25C -30 4.90 50 5 -100 25 -180 4.95 5.00 6 6 0.2 5.05 20 25 0.4 5.10 V mV mV mV/C V V mV mA
2
Electrical Characteristics: Unless otherwise stated, specifications apply for -40 TA 85C; VCC = 15V (Note 1); RT = 10k; CT = 3.3nF for sawtooth mode. PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
s Total Standby Current Start-Up Current Operating Supply Current VCC Zener Voltage s PWM Section Maximum Duty Cycle Minimum Duty Cycle s Undervoltage Lockout Section Start Threshold Min. Operating Voltage
Notes:
0.5 VFB=VSense=0V RT=10k, CT=3.3nF ICC=25mA 11 34
1.0 17
mA mA V
46 0
48
50
% %
7.8 After Turn On 7.0
8.4 7.6
9.0 8.2
V V
1. Adjust VCC above the start threshold before setting at 15V. 2.These parameters, although guaranteed, are not 100% tested in production.
3. Parameter measured at trip point of latch with VFB=0. 4. Gain defined as: A = VCOMP VSense ; 0 VSense 0.8V.
Package Pin Description
PACKAGE PIN # PIN SYMBOL FUNCTION
8L SO Narrow 1 2 3 4 5 5 6 7 7 8
14L SO Narrow 1 3 5 7 9 8 10 11 12 14 2,4,6,13 COMP VFB Sense OSC Gnd Pwr Gnd VOUT VCCPwr VCC VREF NC Error amp output, used to compensate error amplifier. Error amp inverting input. Noninverting input to Current Sense Comparator. Oscillator timing network with Capacitor to Ground, resistor to VREF. Ground. Output driver Ground. Output drive pin. Output driver positive supply. Positive power supply. Output of 5V internal reference. No Connection.
3
Test Circuit Open Loop Laboratory Test Fixture
V REF RT
2N2222 100k 4.7k 1k Error Amp Adjust 4.7k 5k
A COMP V REF
0.1F
V CC
V FB
V CC
0.1F 1k 1W
Sense Adjust
Sense
V OUT
V OUT
OSC
Gnd
Gnd CT
Circuit Description Undervoltage Lockout During Undervoltage Lockout (Figure 1), the output driver is biased to sink minor amounts of current. The output should be shunted to ground with a resistor to prevent activating the power switch with extraneous leakage currents. PWM Waveform To generate the PWM waveform, the control voltage from the error amplifier is compared to a current sense signal which represents the peak output inductor current (Figure 2). An increase in VCC causes the inductor current slope to increase, thus reducing the duty cycle. This is an inherent feed-forward characteristic of current mode control, since the control voltage does not have to change during changes of input supply voltage. When the power supply sees a sudden large output current increase, the control voltage will increase allowing the duty cycle to momentarily increase. Since the duty cycle tends to exceed the maximum allowed to prevent transformer saturation in some power supplies, the internal oscillator waveform provides the maximum duty cycle clamp as programmed by the selection of OSC components.
VCC
ON/OFF Command to reset of IC
VON = 8.4V VOFF = 7.6V
ICC
<15mA <1mA VON VOFF VCC
Figure 1: Startup voltage for the CS52845.
4
CS52845
Circuit Description:: continued
VOSC OSC RESET Toggle F/F Output EA Output Switch Current VCC
Setting the Oscillator The times Tc and Td can be determined as follows: t c = RTCT ln
t d = RTCT ln
( (
VREF - Vlower VREF - Vupper
) )
VREF - IdRT - Vlower VREF - IdRT - Vupper
IO
Substituting in typical values for the parameters in the above formulas: VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA, then tc 0.5534RTCT td = RTCT ln
VO
Figure 2: Timing Diagram
(
2.3 - 0.0083 RT 4.0 - 0.0083 RT
)
Vupper
For better accuracy RT should be 10k.
Vlower ton toff
Grounding
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected close to Gnd in a single point ground. The transistor and 5k potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Sense.
tc ton=tc toff=tc+2td
Figure 3: Duty Cycle parameters.
td
5
CS52845
Package Specification
PACKAGE DIMENSIONS IN mm (INCHES) PACKAGE THERMAL DATA
D Lead Count 8L SO Narrow 14L SO Narrow Metric Max Min 5.00 4.80 8.75 8.55 English Max Min .197 .189 .344 .337
Thermal Data RJC RJA typ typ
8L SO Narrow 45 165
14L SO Narrow 30 125
C/W C/W
Surface Mount Narrow Body (D); 150 mil wide
4.00 (.157) 3.80 (.150)
6.20 (.244) 5.80 (.228)
0.51 (.020) 0.33 (.013)
1.27 (.050) BSC
1.75 (.069) MAX 1.57 (.062) 1.37 (.054) 1.27 (.050) 0.40 (.016) 0.25 (.010) 0.19 (.008) D REF: JEDEC MS-012
0.25 (0.10) 0.10 (.004)
Ordering Information
Part Number CS52845ED8 CS52845EDR8 CS52845ED14 CS52845EDR14
Rev. 3/4/99
Description 8L SO Narrow 8L SO Narrow (tape & reel) 14L SO Narrow 14L SO Narrow (tape & reel) 6
Cherry Semiconductor Corporation reserves the right to make changes to the specifications without notice. Please contact Cherry Semiconductor Corporation for the latest available information.
(c) 1999 Cherry Semiconductor Corporation

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