SC4607
POWER MANAGEMENT
Application Information (Cont.)
SC4607 application:
Where:
fs = the switching frequency and
Dmax = maximum duty ratio, 0.97 for the SC4607.
Input Capacitor Selection:
The required minimum capacitance for boost capacitor
will be:
The input capacitor selection is based on its ripple cur-
rent level, required capacitance and voltage rating. This
capacitor must be able to provide the ripple current by
the switching actions. For the continuous conduction
mode, the RMS value of the input capacitor can be cal-
culated from:
IB
VD
Cboost
=
TW
Where:
IB = the boost current and
VD= discharge ripple voltage.
Vout (V − Vout
)
in
ICIN
= IOMAX
(RMS)
2
V
in
With fs = 300kH, VD=0.3V and IB=50mA, the required
capacitance for the boost capacitor is:
This current gives the capacitor’s power loss as follows:
IB
VD fs
1
0.05
0.3 300k
1
PCIN = I2
RCIN(ESR)
Cboost
=
Dmax
=
0.97 = 540nF
CIN(RMS)
This capacitor’s RMS loss can be a significant part of the
total loss in the converter and reduce the overall con-
verter efficiency. The input ripple voltage mainly depends
on the input capacitor’s ESR and its capacitance for a
given load, input voltage and output voltage. Assuming
that the input current of the converter is constant, the
required input capacitance for a given voltage ripple can
be calculated by:
Power MOSFET Selection:
The SC4607 can drive an N-MOSFET at the high side
and an N-MOSFET synchronous rectifier at the low side.
The use of the high side N-MOSFET will significantly re-
duce its conduction loss for high current. For the top
MOSFET, its total power loss includes its conduction loss,
switching loss, gate charge loss, output capacitance loss
and the loss related to the reverse recovery of the bot-
tom diode, shown as follows:
D (1−D)
fs (∆V −IOMAX RCIN
CIN = IOMAX
)
I
(ESR)
Where:
D = Vout/Vin , duty ratio and
∆VI = the given input voltage ripple.
ITOP _PEAK
V
fs
in
PTOP _TOTAL = I2
RTOP _ON
+
TOP _RMS
V
GATE RG
Because the input capacitor is exposed to the large surge
current, attention is needed for the input capacitor. If
tantalum capacitors are used at the input side of the
converter, one needs to ensure that the RMS and surge
ratings are not exceeded. For generic tantalum capaci-
tors, it is wise to derate their voltage ratings at a ratio of
2 to protect these input capacitors.
(QGD + QGS2 ) + QGT VGATE fs + (QOSS + Qrr ) V fs
in
Where:
RG = gate drive resistor,
QGD = the gate to drain charge of the top MOSFET,
QGS2 = the gate to source charge of the top MOSFET,
QGT = the total gate charge of the top MOSFET,
QOSS = the output charge of the top MOSFET and
Qrr = the reverse recovery charge of the bottom diode.
Boost Capacitor Selection:
The boost capacitor selection is based on its discharge
ripple voltage, worst case conduction time and boost
current. The worst case conduction time Tw can be esti-
mated as follows:
For the top MOSFET, it experiences high current and high
voltage overlap during each on/off transition. But for the
1
fs
Tw =
Dmax
2005 Semtech Corp.
11
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