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CS3843BGD14

Off-Line Current Mode PWM Control Circuit with Very Low Start Up Current

Cherry semiconductor 

Typical Performance Characteristics:

Oscillator Frequency vs CT

Oscillator Duty Cycle vs RT

900

800

RT =680Ω

700

600

500

RT =1.5kΩ

400

300

200

RT =10kΩ

100

.0005

.001

.002 .003 .005

.01

CT (µF)

.02 .03 .04 .05

100

90

80

70

60

50

40

30

20

10

100

200 300 400 500 700 1k

2k 3k 4k 5k 7k 10k

RT (Ω)

Test Circuit

4.7kΩ

RT

2N2222

100kΩ

COMP

VREF

1kΩ

ERROR AMP

ADJUST

4.7kΩ

5kΩ

Sense

ADJUST

CS-3842B

VFB CS-3843B VCC

Sense

V O UT

OSC

Gnd

A

0.1µF

0.1µF

1kΩ

1W

VREF

VCC

V O UT

Gnd

CT

Circuit Description

VCC

ON/OFF Command

to reset of IC

VON

VOFF

CS3842B CS3843B

16V

10V

8.4V

7.6V

ICC

<15mA

<0.5mA

VON VOFF

Figure 1: Typical Undervoltage Characteristics

VCC

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

4

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