Typical Performance Characteristics:
Oscillator Frequency vs CT
Oscillator Duty Cycle vs RT
100
90
900
800
700
600
500
400
300
200
100
80
RT =680Ω
70
60
50
40
RT =1.5kΩ
30
20
10
RT =10kΩ
100
200 300 400 500 700 1k
RT (Ω)
2k
3k 4k 5k 7k 10k
.0005
.001
.002 .003
.005
.01
.02
.03 .04 .05
CT (µF)
Test Circuit
VREF
RT
2N2222
VCC
A
100kΩ
COMP
VREF
4.7kΩ
1kΩ
ERROR AMP
ADJUST
0.1µF
CS-3842B
CS-3843B
VFB
VCC
1kΩ
1W
0.1µF
5kΩ
Sense
4.7kΩ
VOUT
VOUT
Sense
ADJUST
OSC
Gnd
Gnd
CT
Circuit Description
Undervoltage Lockout
VCC
ON/OFF Command
to reset of IC
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.
CS3842B CS3843B
VON
VOFF
16V
10V
8.4V
7.6V
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.
ICC
<15mA
<0.5mA
VCC
VON VOFF
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
Figure 1: Typical Undervoltage Characteristics
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
