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| CS2841B Datasheet, PDF (4/6 Pages) Cherry Semiconductor Corporation – Automotive Current Mode PWM Control Circuit | |||
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Typical Performance Characteristics:
Oscillator Frequency vs CT
Oscillator Duty Cycle vs RT
900
800
RT =680â¦
700
600
500
RT =1.5kâ¦
400
300
200
RT =10kâ¦
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 (â¦)
Test Circuit
4.7kâ¦
1kâ¦
Error Amp
Adjust
4.7kâ¦
RT
2N2222
100kâ¦
COMP
5kâ¦
Sense
Adjust
VFB
Sense
OSC
VREF
VCC
V O UT
Gnd
A
0.1µF
1kâ¦
0.1µF
1W
VREF
VCC
VO
Gnd
CT
Circuit Description
VCC
ON/OFF Command
to reset of IC
VON = 8.0V
VOFF= 7.4V
ICC
<15mA
<1mA
7.4V 8.0V
Figure 1: Typical Undervoltage Characteristics
VCC
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 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-
4
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