sc4609mltrt Semtech Corporation, sc4609mltrt Datasheet - Page 10

• Manufacturer Part Number: sc4609mltrt
• Description: Sc4609 Low-input, Mhz Operation, High-efficiency Synchronous Buck
• Manufacturer: Semtech Corporation

This current gives the capacitor’s power loss as follows:
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:
Where:
D = V
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.
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 T
mated as follows:
Where:
f
Dmax = maximum duty ratio.
The required minimum capacitance for boost capacitor
will be:
POWER MANAGEMENT
Application Information (Cont.)
s
V
2006 Semtech Corp.
= the switching frequency and
I
= the given input voltage ripple.
O
/V
I
, duty ratio and
C
I
CIN
IN
(
RMS
I
OMAX
P
)
CIN
I
OMAX
C
Tw
fs
boost
I
2
CIN
(
(
RMS
V
f
1
V
s
I
V
I
OUT
D
B
)
D
D
I
OMAX
R
max
1 (
T
CIN
(
W
V
V
IN
(
D
2
ESR
IN
R
)
CIN
)
V
OUT
(
ESR
w
)
)
)
can be esti-
10
Where:
I
V
With f
capacitance for the boost capacitor is:
Power MOSFET Selection
The SC4609 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:
Where:
R
Q
Q
Q
Q
Q
For the top MOSFET, it experiences high current and high
voltage overlap during each on/off transition. But for the
bottom MOSFET, its switching voltage is the bottom
diode’s forward drop during its on/off transition. So the
switching loss for the bottom MOSFET is negligible. Its
total power loss can be determined by:
Where:
Q
V
C
P
B
D
F
G
GD
GS2
GT
OSS
rr
BOT
GB
boost
= the boost current and
= discharge ripple voltage.
= the forward voltage drop of the bottom diode.
= gate drive resistor,
= the reverse recovery charge of the bottom diode.
= the total gate charge of the top MOSFET,
= the total gate charge of the bottom MOSFET and
= the gate to drain charge of the top MOSFET,
_
= the gate to source charge of the top MOSFET,
= the output charge of the top MOSFET and
P
(
TOTAL
Q
s
TOP
GD
= 300kH, V
V
I
B
D
_
TOTAL
f
Q
I
1
s
2
BOT
GS
D
2
max
_
)
I
RMS
2
TOP
Q
D
=0.3V and I
GT
_
R
. 0
0
RMS
BOT
05
3 .
V
GATE
_
R
ON
300
TOP
1
k
_
f
Q
s
ON
GB
. 0
B
(
=50mA, the required
95
Q
I
V
TOP
OSS
GATE
V
_
528
PEAK
GATE
SC4609
www.semtech.com
Q
f
s
nF
rr
R
)
V
I
G
D
I
V
_
I
f
s
AVG
f
s
V
F