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CS5422 Datasheet(PDF) 9 Page - ON Semiconductor

Part No. CS5422
Description  Dual Out−of−Phase Synchronous Buck Controller with Current Limit
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS5422 Datasheet(HTML) 9 Page - ON Semiconductor

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9
offset threshold and the artificial ramp, the PWM
comparator terminates the initial pulse.
Figure 4. Idealized Waveforms
8.6 V
0.45 V
VIN
VCOMP
VFB
GATE(H)1
GATE(H)2
UVLO
STARTUP
NORMAL OPERATION
tS
Normal Operation
During normal operation, the duty cycle of the gate drivers
remains approximately constant as the V2 control loop
maintains the regulated output voltage under steady state
conditions. Variations in supply line or output load
conditions will result in changes in duty cycle to maintain
regulation.
Gate Charge Effect on Switching Times
When using the onboard gate drivers, the gate charge has
an important effect on the switching times of the FETs. A
finite amount of time is required to charge the effective
capacitor seen at the gate of the FET. Therefore, the rise and
fall times rise linearly with increased capacitive loading,
according to the following graphs.
Figure 5. Average Rise and Fall Times
90
80
70
60
50
40
30
20
10
0
0
1
23
4
5
67
8
Load (nF)
Average Fall Time
Average Rise Time
Transient Response
The 150 ns reaction time of the control loop provides fast
transient response to any variations in input voltage and
output current. Pulse−by−pulse adjustment of duty cycle is
provided to quickly ramp the inductor current to the required
level. Since the inductor current cannot be changed
instantaneously, regulation is maintained by the output
capacitors during the time required to slew the inductor
current. For better transient response, several high
frequency and bulk output capacitors are usually used.
Out−of−Phase Synchronization
In out−of−phase synchronization, the turn−on of the
second channel is delayed by half the switching cycle. This
delay is supervised by the oscillator, which supplies a clock
signal to the second channel which is 180° out of phase with
the clock signal of the first channel.
The advantages of out−of−phase synchronization are
many. Since the input current pulses are interleaved with one
another, the overlap time is reduced. The effect of this
overlap reduction is to reduce the input filter requirement,
allowing the use of smaller components. In addition, since
peak current occurs during a shorter time period, emitted
EMI is also reduced, thereby reducing shielding
requirements.
Overvoltage Protection
Overvoltage Protection (OVP) is provided as a result of
the normal operation of the V2 control method and requires
no additional external components. The control loop
responds to an overvoltage condition within 150 ns, turning
off the upper MOSFET and disconnecting the regulator
from its input voltage. This results in a crowbar action to
clamp the output voltage preventing damage to the load. The
regulator remains in this state until the overvoltage
condition ceases.
Hiccup Overcurrent Protection
A lossless hiccup mode short circuit protection feature is
provided on the chip. The only external component required
is the COMP1 capacitor. Any overcurrent condition results
in the immediate shutdown of both output phases.
A comparator between the IS+ and IS− on each output
phase detects a short circuit when the voltage difference
between the two pins exceeds 70 mV and sets the fault latch.
The fault latch immediately turns off the error amplifier and
discharges both COMP capacitors. The capacitor connected
to COMP1 is discharged through a 5.0 μA current sink in
order to provide timing for the reset cycle. When COMP1
has fallen below 0.25 V, a comparator resets the fault latch
and error amplifier 1 begins to charge COMP1 with a 30 μA
source current. When COMP1 exceeds the feedback voltage
plus the PWM Comparator offset voltage, the normal
switching cycle will resume.
If the short circuit condition persists through the restart
cycle, the overcurrent reset cycle will repeat itself until the
short circuit is removed, resulting in small hiccup output
pulses while the COMP capacitor charges, as shown in
Figure 6.


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