UC3845B STMicroelectronics, UC3845B Datasheet

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UC3845B

Manufacturer Part Number
UC3845B
Description
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
Manufacturer
STMicroelectronics
Datasheet

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TheUC384xB family ofcontrolICs providesthe nec-
essary features to implement off-line or DC to DC
fixed frequency current mode control schemes with
a minimal external parts count. Internally imple-
mented circuits include a trimmed oscillator for pre-
cise DUTY CYCLE CONTROL under voltage lock-
outfeaturingstart-up current less than0.5mA,a pre-
cision reference trimmed for accuracy at the error
amp input, logicto insure latched operation,a PWM
BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B)
March 1999
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
DESCRIPTION
TRIMMED OSCILLATOR FOR PRECISE FRE-
QUENCY CONTROL
OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz
CURRENT MODE OPERATION TO 500kHz
AUTOMATIC FEED FORWARD COMPENSA-
TION
LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGE LOCKOUT
HIGH CURRENT TOTEM POLE OUTPUT
UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
LOW START-UP AND OPERATING CURRENT
CURRENT
GROUND
SENSE
RT/CT
COMP
VFB
Vi
7
5
4
2
1
3
34V
+
-
ERROR AMP.
OSC
2.50V
UVLO
2R
R
S/R
REF
5V
1V
VREF GOOD
LOGIC
comparatorwhich alsoprovidescurrent limit control,
and a totem pole output stage designed to source
or sink high peakcurrent. The outputstage, suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differences between members of this family are the
under-voltagelockout thresholds and maximum duty
cycle ranges. The UC3842B and UC3844B have
UVLO thresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh-
oldsforthe UC3843BandUC3845Bare8.5 V and7.9
V. The UC3842B and UC3843B can operate to duty
cycles approaching 100%. A range of the zero to <
50 % is obtained by the UC3844B and UC3845B by
the addition of an internal toggle flip flop which blanks
the output off every otherclock cycle.
COMPARATOR
CURRENT
SENSE
UC2842B/3B/4B/5B
UC3842B/3B/4B/5B
S
R
INTERNAL
T
BIAS
Minidip
LATCH
PWM
UC3842B
D95IN331
8
6
5V 50mA
OUTPUT
SO8
VREF
1/15

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UC3845B Summary of contents

Page 1

... DUTY CYCLE CONTROL under voltage lock- outfeaturingstart-up current less than0.5mA,a pre- cision reference trimmed for accuracy at the error amp input, logicto insure latched operation,a PWM BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B UVLO 34V ...

Page 2

... UC2844BD1; UC3844BD1 UC2845BD1; UC3845BD1 2/15 Self Limiting – 0 (SO8) – 150 – 150 Minidip/SO8 REF OUTPUT 4 5 GROUND D95IN332 Description to ground. Operation to 500kHz is possible. T Minidip UC2842BN; UC3842BN UC2843BN; UC3843BN UC2844BN; UC3844BN UC2845BN; UC3845BN Valu 1.25 W 800 300 C through resistor ...

Page 3

THERMAL DATA Symbo l Description R Thermal Resistance Junction-ambient. th j-amb ELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for -25 < T < for UC284XB; 0 < T amb amb Symbo l Parameter REFERENCE ...

Page 4

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B ELECTRICAL CHARACTERISTICS (continued) Symbo l Parameter OUTPUT SECTION V Output Low Level I OL SINK I SINK V Output High Level I OH SOURCE I SOURCE V UVLO Saturation VCC = 6V; I OLS t Rise Time ...

Page 5

Figure 1: Open Loop Test Circuit. 4.7K R 2N2222 100K COMP ERROR AMP. ADJUST SENSE 4.7K SENSE ADJUST D95IN343 High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors ...

Page 6

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B Figure 4: Oscillator Discharge Current vs. Tem- perature. I dischg D95IN335 (mA) V =15V i V =2V OSC 8.5 8.0 7.5 7.0 -55 - 100 T Figure 6: Error Amp Open-Loop Gain and ...

Page 7

Figure 10: Output Saturation Voltagevs. Load Current. V sat D95IN341 (V) Source Saturation V i (Load to Ground = =- =15V Pulsed Load 120Hz Rate 3 T =-40 ...

Page 8

... GND D95IN347 Peak current ( determined by the formula s 1 max small RC filter may be required to suppress switch transients. 8/15 2.5V 1mA + COMP 1 D95IN345 I CC <17mA UC3843B UC3845B 8.4V <0.5mA V V OFF 7.6V During UVLO, the Output is low D95IN346 ERROR AMPL CURRENT SENSE COMPARATOR ...

Page 9

Figure 18 : Slope Compensation Techniques. V REG UC3842B R C SLOPE SENSE Figure 19 : Isolated MOSFET Drive and Current Transformer ...

Page 10

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B Figure 20 : Latched Shutdown. 2N 3905 SCR must be selected for a holding current of less than 0.5mA at T The simple two transistor circuit can be used in place of the SCR as shown. All ...

Page 11

Figure 22: External Clock Synchronization EXTERNAL SYNC INPUT 0. The diode clamp is required if the Sync amplitude is large enough to cause the bottom side more than 300mV below ground T ...

Page 12

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B Figure 24: Soft-Start Circuit Figure 25: Soft-Start and Error Amplifier Output Duty Cycle Clamp 1mA + ...

Page 13

DIM. MIN. TYP. MAX. MIN. TYP. A 1.75 a1 0.1 0.25 0.004 a2 1.65 a3 0.65 0.85 0.026 b 0.35 0.48 0.014 b1 0.19 0.25 0.007 C 0.25 0.5 0.010 c1 45 (typ.) D (1) 4.8 5.0 0.189 ...

Page 14

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B mm inch DIM. MIN. TYP. MAX. MIN. TYP. A 3.32 0.131 a1 0.51 0.020 B 1.15 1.65 0.045 b 0.356 0.55 0.014 b1 0.204 0.304 0.008 D 10.92 E 7.95 9.75 0.313 e 2.54 0.100 e3 7.62 ...

Page 15

... No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMi- croelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics ...

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