UC3843T ,HIGH PERFORMANCE CURRENT MODE PWM CONTROLLERBLOCK DIAGRAM (toggle flip flop used only in UC3844T and UC3845T)7ViUVLO34V85VVREF5 S/RREFGROUND 5V ..
UC3843TD ,HIGH PERFORMANCE CURRENT MODE PWM CONTROLLERELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for0 < T ..
UC3843TN ,HIGH PERFORMANCE CURRENT MODE PWM CONTROLLERUC3842T UC3843TUC3844T UC3845T® HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER. TRIMMED OSCILLATOR FO ..
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UN2115 ,Composite DeviceAbsolute Maximum Ratings T = 25°CaParameter Symbol Rating UnitCollector-base voltage (Emitter open ..
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UC3842T-UC3843T-UC3845T
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
UC3 842T UC3843T
UC3 844T UC3845TSeptember 2001
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER 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
DESCRIPTIONThe UC384XT family of control ICs provides the
necessary 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-
out featuring start-up current less than 0.5mA, a pre-
cision reference trimmed for accuracy at the error
amp input, logic to insure latched operation, a PWM
comparator which also provides current limit control,
and a totem pole output stage designed to source
or sink high peak current. The output stage, suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differences between members of this family are the
under-voltage lockout thresholds and maximum duty
cycle ranges. The UC3842T and UC3844T have
UVLO thresholds of 16V (on) and 10V (off), ideally
suited to off-line applications The corresponding
thresholds for the UC3843T and UC3845T are 8.5 V
and 7.9 V. The UC3842T and UC3843T can operate
to duty cycles approaching 100%. A range of zero to
< 50 % is obtained by the UC3844T and UC3845T by
the addition of an internal toggle flip flop which blanks
the output off every other clock cycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844T and UC3845T)1/15
All voltages are with respect to pin 5, all currents are positive into the specified terminal.
PIN CONNECTION (top view)
ORDERING NUMBERS
ABSOLUTE MAXIMUM RATINGS
PIN FUNCTIONS
UC3842T - UC3843T - UC3844T - UC3845T 2/15
ELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for0 < Tamb < 105°C; Vi = 15V (note 5); RT = 10K; CT = 3.3nF)
THERMAL DATA
U3842T - UC3843T - UC3844T - UC3845T3/15
Notes :1. Max package power dissipation limits must be respected; low duty cycle pulse techniques are used during test maintain Tj as
close to Tamb as possible. These parameters, although guaranteed, are not 100% tested in production. Parameter measured at trip point of latch with VPIN2 = 0. Gain defined as :
Δ VPIN1A = ; 0 ≤ VPIN3 ≤ 0.8 VΔ VPIN3 Adjust Vi above the start threshold before setting at 15 V.
ELECTRICAL CHARACTERISTICS (continued)
UC3842T - UC3843T - UC3844T - UC3845T 4/15
Figure 1: Open Loop Test Circuit.High peak currents associated with capacitive loads
necessitate careful grounding techniques. Timing
and bypass capacitors should be connected close
to pin 5 in a single point ground. The transistor and
5 KΩ potentiometer are used to sample the oscillator
waveform and apply an adjustable ramp to pin 3.
10K
D95IN333
(KW)
Figure 2: Timing Resistor vs. Oscillator Fre-
quency 20K 30K
D95IN334
Figure 3: Output Dead-Time vs. Oscillator Fre-quency
U3842T - UC3843T - UC3844T - UC3845T5/15
-55 -25 0 25 50 75 100 TA(˚C)
Idischg
(mA)
D95IN335
Figure 4: Oscillator Discharge Current vs. Tem-perature. 3 5
Dmax
(%)
0.8 RT(KW)
Figure 5: Maximum Output Duty Cycle vs. Tim-ing Resistor. 100 1K 10K 100K 1M f(Hz)
(dB)
D95IN337
Figure 6: Error Amp Open-Loop Gain andPhase vs. Frequency.
0246 VO(V)0.0
Vth
(V)
D95IN338
Figure 7: Current Sense Input Threshold vs. Er-ror Amp Output Voltage. 20406080 100 Iref(mA)
D95IN339
Figure 8: Reference Voltage Change vs.Source Current.
-55 -25 0 25 50 75 100 TA(˚C)
D95IN340
ISC
(mA)
Figure 9: Reference Short Circuit Current vs.Temperature.
UC3842T - UC3843T - UC3844T - UC3845T 6/15
200 400 600 IO(mA)Vsat
(V)
D95IN341
Figure 10: Output Saturation Voltagevs. LoadCurrent.
010 20 30 Vi(V)0
(mA)
D95IN342
Figure 11: Supply Current vs. Supply Voltage.
Figure 12: Output Waveform. Figure 13: Output Cross Conduction
Figure 14: Oscillator and Output Waveforms.50ns/DIV
100ns/DIV
U3842T - UC3843T - UC3844T - UC3845T7/15
Figure 15 : Error Amp Configuration.
Figure 16 : Under Voltage Lockout.
Figure 17 : Current Sense Circuit .Peak current (is) is determined by the formula
1.0 VIS max ≈ RS
A small RC filter may be required to suppress switch transients.
UC3842T - UC3843T - UC3844T - UC3845T 8/15
