4

Undervoltage Lockout

During Undervoltage Lockout (Figure 1), the output driv-

er is biased to sink minor amounts of current. The output

should be shunted to ground with a resistor to prevent

activating the power switch with extraneous leakage cur-

rents.

PWM Waveform

To generate the PWM waveform, the control voltage from

the error amplifier is compared to a current sense signal

which represents the peak output inductor current (Figure

increase, thus reducing the duty cycle. This is an inherent

feed-forward characteristic of current mode control, since

the control voltage does not have to change during

changes of input supply voltage.

When the power supply sees a sudden large output cur-

rent increase, the control voltage will increase allowing

the duty cycle to momentarily increase. Since the duty

cycle tends to exceed the maximum allowed to prevent

transformer saturation in some power supplies, the inter-

nal oscillator waveform provides the maximum duty cycle

clamp as programmed by the selection of OSC compo-

nents.

Circuit Description

Test Circuit

1k

Ω

1W

0.1

µF

0.1

µF

Gnd

Sense

OSC

COMP

5k

Ω

100k

Ω

4.7k

Ω

1k

Ω

Error Amp

Adjust

4.7k

Ω

Sense

Adjust

2N2222

Gnd

A

CS3845B

8.4V

7.6V

CSX845B

ON/OFF Command

to reset of IC

<15mA

<1mA

Figure 1: Startup voltage for CS3845B.