Typical Performance Characteristics:
Oscillator Duty Cycle vs RT
Oscillator Frequency vs CT
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
2k
3k 4k 5k 7k 10k
.0005
.001
.002 .003
.005
.01
.02
.03 .04 .05
RT (Ω)
CT (µF)
Test Circuit
VREF
RT
2N2222
VCC
A
100kΩ
COMP
VFB
VREF
4.7kΩ
1kΩ
0.1µF
0.1µF
VCC
Error Amp
Adjust
1kΩ
1W
5kΩ
Sense
Adjust
4.7kΩ
VO
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
VON = 8.0V
V
OFF= 7.4V
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.
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-
ICC
<15mA
<1mA
VCC
7.4V 8.0V
Figure 1: Typical Undervoltage Characteristics
4