CS3842B Datasheet, PDF (4/6 Pages) Cherry Semiconductor Corporation – Off-Line Current Mode PWM Control Circuit Off-Line Current Mode PWM Control Circuit

Typical Performance Characteristics:
Oscillator Frequency vs C_T
Oscillator Duty Cycle vs R_T
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
R_T
2N2222
100kΩ
COMP
V_REF
1kΩ
ERROR AMP
ADJUST
4.7kΩ
5kΩ
Sense
ADJUST
CS-3842B
V_{FB CS-3843B} V_CC
Sense
V _{O UT}
OSC
Gnd
A
0.1µF
0.1µF
1kΩ
1W
V_REF
V_CC
V _{O UT}
Gnd
C_T
Circuit Description
V_CC
ON/OFF Command
to reset of IC
V_ON
V_OFF
CS3842B CS3843B
16V
10V
8.4V
7.6V
I_CC
<15mA
<0.5mA
V_ON V_OFF
Figure 1: Typical Undervoltage Characteristics
V_CC
Undervoltage Lockout
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
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