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-

Fault Shutdown can force the current completely to zero. R2 is

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.

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

The minimum ramp current is:

≈

4

V

≈ 5.6V







R2

+ R3

R3







Above the threshold, the ramp current will be:

(VARIAB)

≈

4

R1

+

− 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. (603) 424-2410

• FAX (603) 424-3460

VOLTAGE FEED-FORWARD COMBINED WITH

MAXIMUM DUTY-CYCLE CLAMP

CURRENT MODE CONTROL

8