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SD46520 Datasheet, PDF (6/8 Pages) Silan Microelectronics Joint-stock – 400KHZ 2A STEP-DOWN DC-DC CONVERTER
◁
SD46520
Where, ΔVO is output voltage ripple, ΔIL is inductor current ripple, ESR is the equivalent serial resistor of
output capacitor.
The freewheeling diode selection
Select the schottky diode as freewheeling diode to reduce the loss caused by forward voltage drop of the diode.
At the same time, the maximum current of schottky should be higher than 2.4A.
Compensation network
The resistor-capacitor compensation network connected to
COMP pin make sure the stability of the control loop circuit.
As the right figure:
The DC gain of the control loop is:
A VDC
=
VFB
VO
⋅ A VEA
⋅ GCS
⋅RL
Where, VFB is feedback voltage, 1.22V; VO is setting output
voltage; AVEA is the voltage gain of error amplifier, 400V/V; GCS is the trans-conductance of current sense(about
the value of the output current divided by COMP pin voltage), 1.95A/V; RL is load resistor.
The control loop has two poles of inportance.
The first
pole P1 is decided by the following formula:
fP1
=
1
2π ⋅ROEA
⋅ CC1
,
where,
ROEA
=
A VEA
GEA
, GEA is the trans-conductance of error amplifier, 770uA/V.
The second pole
P2
is
decided
by:
fP2
=
1
2π⋅RL
⋅ CO
。
The
control
loop
has
a
main
zero
Z1,
which
is
decided
by:
fZ1
=
1
2π⋅RC ⋅ CC1
。
When the output capacitor ESR is large, another zero Z2 is introduced which is decided by:
fZ2
=
1
2π⋅ESR ⋅CO
.
At this time, it needs to add another compensation capacitor Cc2 to introduce an additional pole P3 compensate
this
zero ,
and this pole is decided by:
fP3
=
1
2π⋅RC ⋅ CC2
The system crossover frequency fC is very important. If it’s too high , it will cause the system unstable. If it’s too
low, it will slower the system transient response. In general, fC is about 1/8~1/10 of the switch frequency.
Use the following procedure to choose the value of the compensation components.
First, according to the selected crossover crossover fC, choose the value of compensation resistor Rc:
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0 2009.04.07
Page 6 of 8
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