Datasheet

Since Pin 10 is a direct input to the PWM comparator, this
point can also serve as a current sense port for current mode
control. In this application, current sensing is ground refer-
enced through R
CS. Resistor R1 sets a 400mV offset across
R2 (assuming R2 > R
CS) so that both the Error Amplifier and
Fault Shutdown can force the current completely to zero. R2 is
also used along with C
F as a small filter to attenuate leading-
edge spikes on the load current waveform. In this mode,
current limiting can be accomplished by divider R3/R4 which
forms a clamp overriding the output of the Error Amplifier.
In this circuit, R1 is used in conjunction with C
R (not shown) to
establish a minimum ramp charging current such that the ramp
voltage reaches 4.2V at the required maximum output pulse
width.
The purpose of Q1 is to provide an increasing ramp current
above a threshold established by R2 and R3 such that the duty
cycle is further reduced with increasing VIN.
The minimum ramp current is:
l
R(MIN) =
V
REF − VIN SENSE
R1
≈
4 V
R1
The threshold where VIN begins to add extra ramp current is:
V
IN ≈ 5.6V



R2 + R3
R3



Above the threshold, the ramp current will be:
l
R (VARIA B ) ≈
4
R1
+
VIN − 5.6
R2
−
5.6
R3
UC1841
UC2841
UC3841
ERROR LATCH INTERNAL CIRCUITRY
PROGRAMMABLE SOFT START AND
RESTART DELAY CIRCUIT
The Error Latch consists of Q5 and Q6 which, when both on,
turns off the PWM Output and pulls the Slow-Start pin low. This
latch is set by either the Over-Voltage or Current Shutdown
comparators, or by a high signal on Pin 4. Reset is accom-
plished by either the Reset comparator or a low signal on Pin
4. An activation time delay can be provided with an external
capacitor on Pin 4 in conjunction with the ≈ 100µA collector
current from Q4.
UNITRODE INTEGRATED CIRCUITS
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. [phone] • FAX [phone]
VOLTAGE FEED-FORWARD COMBINED WITH
MAXIMUM DUTY-CYCLE CLAMP
CURRENT MODE CONTROL
8