PWM CONTROL

1. Oscillator

Frequency =

2. Ramp Generator

Develops a linear ramp with a slope defined externally by

dv

dt

=

sense voltage

3. Error Amplifier

Conventional operational amplifier for closed-loop gain and phase compensation.

Low output impedance; unity-gain stable.

The output is held low by the slow start voltage at turn on in order to minimize overshoot.

4. Reference Generator

Precision 5.0V for internal and external usage to 50mA.

Tracking 3.0V reference for internal usage only with nominal accuracy of

40V clamp zener for chip OV protection, 100mA maximum current.

5. PWM Comparator

Generates output pulse which starts at termination of clock pulse and ends when the ramp

input crosses the lowest of two positive inputs.

6. PWM Latch

Terminates the PWM output pulse when set by inputs from either the PWM comparator, the

pulse-by-pulse current limit comparator, or the error latch. Resets with each internal clock

pulse.

7. PWM Output Switch

Transistor capable of sinking current to ground which is off during the PWM on-time and turns

on to terminate the power pulse. Current capacity is 400mA saturated with peak

capacitance discharge in excess of one amp.

SEQUENCING FUNCTIONS

1. Start/UV Sense

With an increasing voltage, it generates a turn-on signal and releases the slow-start clamp at

a start threshold.

With a decreasing voltage, it generates a turn-off command at a lower level separated by a

200

2. Drive Switch

Disables most of the chip to hold internal current consumption low, and Driver Bias OFF, until

input voltage reaches start threshold.

3. Driver Bias

Supplies drive current to external power switch to provide turn-on bias.

4. Slow Start

increase of output pulse width.

Can also be used as an alternate maximum duty cycle clamp with an external voltage divider.

PROTECTION FUNCTIONS

1. Error Latch

When set by momentary input, this latch insures immediate PWM shutdown and hold off until

reset. Inputs to Error Latch are:

a. OV > 3.2V (typically 3V)

b. Stop > 2.4V (typically 1.6V)

c. Current Sense 400mV over threshold (typical).

Error Latch resets when slow start voltage falls to 0.4V if Reset Pin 5 < 2.8V. With Pin 5 >

3.2V, Error Latch will remain set.

2. Current Limiting

Differential input comparator terminates individual output pulses each time sense voltage

rises above threshold.

When sense voltage rises to 400mV (typical) above threshold, a shutdown signal is sent to

Error Latch.

3. External Stop

A voltage over 1.2V will set the Error Latch and hold the output off.

A voltage less than 0.8V will defeat the error latch and prevent shutdown.

A capacitor here will slow the action of the error latch for transient protection by providing a

typical delay of 13ms/

UC1841

UC2841

UC3841

FUNCTIONAL DESCRIPTION

4