Undervoltage Lockout

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

shunted to ground with a resistor to prevent output leak-

age current from activating the power switch.

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

slope to increase, thus reducing the duty cycle. This is an

inherent feed-forward characteristic of current mode con-

trol, 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

4

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

CS-3842B

CS-3843B

Circuit Description

Typical Performance Characteristics:

ON/OFF Command

to reset of IC

16V

8.4V

10V

7.6V

CS3842B CS3843B

<0.5mA

<15mA

Figure 1: Typical Undervoltage Characteristics

.0005

.001

.002

.003

.005

.01

.02

.03

.04

800

900

700

600

500

400

300

200

100

.05

100

200

700

1k

2k

5k

7k

10k

80

90

70

60

50

40

30

20

10

4k

3k

500

400

300

100