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CS52843
CS52843
Current Mode PWM Control Circuit
Description
The CS52843 provides all the necessary features to implement offline fixed frequency current-mode control with a minimum number of external components. The CS52843 incorporates a new precision temperature-controlled oscillator to minimize variations in frequency. An undervoltage lockout ensures that VREF is stabilized before the output stage is enabled. In the CS52843 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 Internally Temperature Compensated Oscillator s VREF Stabilized before Output Stage is Enabled s Very Low Start-up Current 300 A (typ) s Pulse-by-pulse Current Limiting s Improved Undervoltage Lockout s Double Pulse Suppression s 2% 5 Volt 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
VCC VCC Undervoltage Lock-out VCC Pwr
Package Options
8L SO Narrow
COMP VFB
1 2 3 4 8 7 6 5
VREF VCC VOUT Gnd
34V Gnd 8.4V/7.6V VFB Error Amplifier + 2.50V R R
Set/ Reset
5.0 Volt Reference Internal Bias
VREF
Sense OSC
14L SO Narrow
OUTPUT ENABLE
COMP
COMP 1 NC 2
NOR VOUT
14 13 12 11 10 9 8
VREF NC VCC VCC Pwr VOUT Pwr Gnd Gnd
OSC
Oscillator S
VFB 3 NC
4
2R R Sense 1V
Pwr Gnd R PWM Current Sensing Latch Comparator
Sense 5 NC 6 OSC 7
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. 12/23/97
1
A
(R)
Company
CS52843
Electrical Characteristics: -40 TA 85C; VCC = 15V (Note 1); RT = 680; CT = .022F for triangle mode, RT = 10k; CT = 3.3nF 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 Discharge Current Sawtooth Mode, TJ = 25C (Note 1) Triangle Mode, TJ = 25C 12 VCC 25V Sawtooth Mode TMIN TA TMAX Triangle Mode TMIN TA TMAX (Note 1) VOSC (peak to peak) TJ = 25C TMIN TA TMAX 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 Output High Level ISINK = 20mA ISINK = 200mA ISOURCE = 20mA ISOURCE = 200mA 13.0 12.0 0.1 1.5 13.5 13.5 0.4 2.2 V V V V (Notes 2 & 3) VCOMP = 5V (Note 2) 12 VCC 25V (Note 2) VSense=0V TJ=25C (Note 1) 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 2 VOUT 4V (Note 1) 12 VCC 25V 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 2.42 2.50 -0.3 90 1.0 70 6 -0.8 6 0.7 1.1 2.58 -2.0 V A dB MHz dB mA mA V V 7.3 6.8 47 44 52 52 0.2 5 8 1.7 8.3 9.3 9.8 57 60 1.0 kHz kHz % % % V mA mA TJ = 25C, IREF = 1mA 12 VCC 25V 1 IRE F 20mA (Note 1) Line, Load, Temp. (Note 1) 10Hz f 10kHz, TJ = 25C (Note 1) TA = 125C, 1000 Hrs. (Note 1) TA = 25C -30 4.82 50 5 -100 25 -180 4.90 5.00 6 6 0.2 5.10 20 25 0.4 5.18 V mV mV mV/C V V mV mA
2
CS52843
Electrical Characteristics: -40 TA 85C; VCC = 15V (Note 1); RT = 680; CT = .022F for triangle mode, RT = 10k; CT = 3.3nF sawtooth mode unless otherwise stated. PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
Rise Time Fall Time Output Leakage s Total Standby Current Start-Up Current Operating Supply Current VCC Zener Voltage
TJ = 25C, CL = 1nF (Note 1) TJ=25C, CL=1nF (Note 1) UVLO Active VOUT = 0
50 50 -.01
150 150 -10.0
ns ns A
300 VFB=VSense=0V RT=10k, CT=3.3nF ICC=25mA 11 34
500 17
A mA V
s Undervoltage Lockout Section Start Threshold Min. Operating Voltage
Notes:
7.8 After Turn On 7.0
8.4 7.6
9.0 8.2
V V
1.These parameters, although guaranteed, are not 100% tested in production. 2. Parameter measured at trip point of latch with VFB=0. 3. 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 8 9 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
CS52843
Typical Performance Characteristics
100 90
900 800
RT =680
700
80 DUTY CYCLE (%) 70 60 50 40 30
FREQ. (kHz)
600 500
RT =1.5k
400 300 200 100
RT =10k
20 10
.0005
.001
.002
.003
.005
.01
.02
.03 .04 .05
100
200
300 400 500 700
1k
2k
3k 4k 5k
7k
10k
CT (F)
RT ()
Oscillator Frequency vs CT
Oscillator Duty Cycle vs RT
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
VCC
ON/OFF Command to reset of IC
VON =8.4V VOFF = 7.6V
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. 4
ICC
<15mA <500A VON VOFF VCC
Figure 1: Startup voltage for the CS52843.
CS52843
Circuit Description 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 oscillator timing components.
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
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
IO
td = RTCT ln
VO
(
2.3 - 0.0083 RT 4.0 - 0.0083 RT
)
Figure 2: Timing Diagram
For better accuracy RT should be 10k.
Grounding
V REF RT OSC CT Gnd
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.
Vupper
Vlower tc td
Sawtooth Mode
LARGE RT (10k) VOSC
Internal Clock
Triangular Mode
SMALL RT (700k) VREF
Internal Clock
Figure 3: Oscillator Timing Network and Parameters
5
CS52843
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 14L SO Narrow 45 165 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 CS52843ED8 CS52843EDR8 CS52843ED14 CS52843EDR14
Rev. 12/23/97
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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