CS52843 データシートの表示（PDF） - ON Semiconductor
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CS52843 Datasheet PDF : 9 Pages
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CS52843
VCC
ON/OFF Command to reset of IC
VON = 8.4 V
VOFF = 7.6 V
ICC
< 15 mA
< 500 μA
VON VOFF
VCC
Figure 5. Startup Voltage for the CS52843
Undervoltage Lockout
During Undervoltage Lockout (Figure 5), the output
driver is biased to sink minor amounts of current. The output
should be shunted to ground with a resistor to prevent
activating the power switch with extraneous leakage
currents.
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
6). 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 current
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 transformer
saturation in some power supplies, the internal oscillator
waveform provides the maximum duty cycle clamp as
programmed by the selection of oscillator timing
components.
VOSC
OSC
RESET
Toggle
F/F Output
EA Output
Switch
Current
VCC
IO
VO
Figure 6. Timing Diagram
Setting the Oscillator
The times tc and td can be determined as follows:
ǒ Ǔ tc + RTCT ln
VREF * VLOWER
VREF * VUPPER
ǒ Ǔ td + RTCT ln
VREF * IdRTVLOWER
VREF * IdRT * VUPPER
Substituting in typical values for the parameters in the
above formulas:
VREF + 5.0 V, VUPPER + 2.7 V,
VLOWER + 1.0 V, Id + 8.3 mA
then
tc [ 0.5534RTCT
ǒ Ǔ td + RTCT ln
2.3 * 0.0083RT
4.0 * 0.0083RT
For better accuracy RT should be ≥ 10 kΩ.
Grounding
High peak currents associated with capacitive loads
necessitate careful grounding techniques. Timing and
bypass capacitors should be connected close to GND in a
single point ground.
The transistor and 5.0 kΩ potentiometer are used to
sample the oscillator waveform and apply an adjustable
ramp to Sense.
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