CS3842B datasheet(4/6 Pages) CHERRY

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Undervoltage Lockout

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

er is biased to a high impedance state. VOUT should be

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

(Figure 2). An increase in VCC causes the 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

Test Circuit

VREF

VCC

VOUT

1kΩ

0.1µF

VREF

VCC

VOUT

Gnd

VFB

Sense

OSC

COMP

5kΩ

100kΩ

4.7kΩ

1kΩ

ERROR AMP

ADJUST

4.7kΩ

Sense

ADJUST

2N2222

Gnd

CS-3842B

CS-3843B

Circuit Description

Typical Performance Characteristics:

Oscillator Duty Cycle vs RT

Oscillator Frequency vs CT

VCC

ON/OFF Command

to reset of IC

VON

16V

8.4V

VOFF

10V

7.6V

CS3842B CS3843B

<0.5mA

<15mA

VON VOFF

ICC

VCC

Figure 1: Typical Undervoltage Characteristics

.0005

.001

.002

.003

.005

.01

.02

.03

.04

800

900

CT (µF)

700

600

500

400

300

200

100

.05

RT =1.5kΩ

RT =680Ω

RT =10kΩ

100

200

700

10k

RT (Ω)

500

400

300

100

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