CS2842A Datasheet, PDF (4/6 Pages) Cherry Semiconductor Corporation – Off-Line Current Mode PWM Control Circuit with Undervoltage Lockout

Typical Performance Characteristics
Oscillator Frequency vs C_T
Oscillator Duty Cycle vs R_T
900
800
R_T =680Ω
700
600
500
R_T =1.5kΩ
400
300
200
R_T =10kΩ
100
.0005
.001
.002 .003 .005
.01
C_T (µF)
.02 .03 .04 .05
100
90
80
70
60
50
40
30
20
10
100
Test Circuit
4.7kΩ
R_T
2N2222
100kΩ
COMP
1kΩ
ERROR AMP
ADJUST
4.7kΩ
5kΩ
Sense
ADJUST
V_FB
Sense
OSC
V_REF
V_CC
V _{O UT}
Gnd
200 300 400 500 700 1k
2k
R_T (Ω)
3k 4k 5k 7k 10k
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
CSX842A CSX843A
16V
10V
8.4V
7.6V
I_CC
<15mA
<1mA
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. The output should
be shunted to ground with a resistor to prevent output
leakage 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 control, 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 cycle
tends to exceed the maximum allowed to prevent trans-
4