CS3842B/3843B
Typical Performance Characteristics:
Oscillator Frequency vs C
T
100
900
800
90
Oscillator Duty Cycle vs R
T
R
T
=680Ω
DUTY CYCLE (%)
700
FREQ. (kHz)
600
500
80
70
60
50
40
30
20
R
T
=1.5kΩ
400
300
200
100
R
T
=10kΩ
10
100
.0005
.001
.002
.003
.005
.01
.02
.03 .04 .05
200
300 400 500 700
1k
2k
3k 4k 5k
7k
10k
C
T
(µF)
R
T
(Ω)
Test Circuit
V
REF
R
T
2N2222
100kΩ
4.7kΩ
1kΩ
ERROR AMP
ADJUST
4.7kΩ
5kΩ
A
COMP
CS-3842B
V
FB
CS-3843B
V
CC
0.1µF
1kΩ
1W
V
CC
V
REF
0.1µF
Sense
ADJUST
Sense
V
OUT
V
OUT
OSC
Gnd
Gnd
C
T
Circuit Description
V
CC
Undervoltage Lockout
ON/OFF Command
to reset of IC
CS3842B CS3843B
V
ON
V
OFF
16V
10V
8.4V
7.6V
During Undervoltage Lockout (Figure 1), the output driv-
er is biased to a high impedance state. V
OUT
should be
shunted to ground with a resistor to prevent output leak-
age current from activating the power switch.
PWM Waveform
I
CC
<15mA
<0.5mA
V
ON
V
OFF
V
CC
Figure 1: Typical Undervoltage Characteristics
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 V
CC
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
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
