Typical Performance Characteristics
Oscillator Duty Cycle vs RT
Oscillator Frequency vs CT
100
90
900
800
700
600
500
400
300
200
100
R
T =680Ω
80
70
60
50
40
RT =1.5kΩ
30
20
10
RT =10kΩ
.0005
.001
.002 .003
.005
.01
.02
.03 .04 .05
100
200 300 400 500 700 1k
2k
3k 4k 5k 7k 10k
CT (µF)
RT (Ω)
Test Circuit
VREF
RT
2N2222
VCC
A
100kΩ
COMP
VREF
4.7kΩ
1kΩ
0.1µF
0.1µF
VFB
VCC
ERROR AMP
ADJUST
1kΩ
1W
5kΩ
Sense
ADJUST
4.7kΩ
VOUT
VOUT
Sense
OSC
Gnd
Gnd
CT
Circuit Description
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.
VCC
ON/OFF Command
to reset of IC
CSX842A CSX843A
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 control, since
the control voltage does not have to change during
changes of input supply voltage.
ICC
<15mA
<1mA
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-
VCC
VON VOFF
Figure 1: Typical Undervoltage Characteristics
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
