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CS3845GDR14 Datasheet(PDF) 6 Page - ON Semiconductor

No. de Pieza. CS3845GDR14
Descripción  Current Mode PWM Control Circuit
Descarga  12 Pages
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Fabricante  ONSEMI [ON Semiconductor]
Página de inicio  http://www.onsemi.com
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CS3845GDR14 Datasheet(HTML) 6 Page - ON Semiconductor

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CS2844, CS3844, CS2845, CS3845
http://onsemi.com
6
CIRCUIT DESCRIPTION
CSX845
8.4 V
7.6 V
CSX844
16 V
10 V
VON
VOFF
ON/OFF Command
to reset of IC
VCC
< 1.0 mA
< 15 mA
VON VOFF
VCC
ICC
Figure 3. Startup Voltage for CSX844 and CSX845
Undervoltage Lockout
During Undervoltage Lockout (Figure 3), 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
4). 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.
Figure 4. Timing Diagram
VOSC
OSC
RESET
Toggle
F/F Output
EA Output
Switch
Current
VCC
IO
VO
Figure 5. Duty Cycle Parameters
Vupper
Vlower
ton
tc
toff
td
ton = tc
toff = tc + 2td
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 OSC components.
Setting the Oscillator
The times tc and td can be determined as follows:
tc + RTCT ln
VREF * Vlower
VREF * Vupper
td + RTCT ln
VREF * IdRT * Vlower
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
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 k
Ω potentiometer are used to
sample the oscillator waveform and apply an adjustable
ramp to Sense.


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