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SC4605IMSTRT Datasheet(PDF) 9 Page - Semtech Corporation

Part # SC4605IMSTRT
Description  Low Input, High Efficiency Synchronous, Step Down Controller
PDF  19 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC4605IMSTRT Datasheet(HTML) 9 Page - Semtech Corporation

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9
 2004 Semtech Corp.
www.semtech.com
SC4605
POWER MANAGEMENT
mode, the RMS value of the input capacitor can be cal-
culated from:
I
2
O
I
O
OMAX
)
RMS
(
CIN
V
)
V
V
(
V
I
I
−
⋅
⋅
=
This current gives the capacitor’s power loss as follows:
)
ESR
(
CIN
)
RMS
(
CIN
2
CIN
R
I
P
⋅
=
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:
)
R
I
V
(
fs
)
D
1
(
D
I
C
)
ESR
(
CIN
OMAX
I
OMAX
IN
⋅
−
∆
⋅
−
⋅
⋅
=
Where:
D = VO/VI , duty ratio and
DVI = the given input voltage ripple.
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.
Power Mosfet Selection
The SC4605 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:
Applications Information (Cont.)
s
I
rr
OSS
s
GATE
GT
2
GS
GD
G
GATE
s
I
PEAK
_
TOP
ON
_
TOP
RMS
_
TOP
2
TOTAL
_
TOP
f
V
)
Q
Q
(
f
V
Q
)
Q
Q
(
R
V
f
V
I
R
I
P
⋅
⋅
+
+
⋅
⋅
+
+
⋅
⋅
⋅
+
⋅
=
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.
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:
F
AVG
D
s
GATE
GB
ON
_
BOT
RMS
_
BOT
2
TOTAL
_
BOT
V
_
I
f
V
Q
R
I
P
⋅
+
⋅
⋅
+
⋅
=
Where:
QGB = the total gate charge of the bottom MOSFET and
VF = the forward voltage drop of the bottom diode.
For a low voltage and high output current application such
as the 3.3V/1.5V@12A case, the conduction loss is of-
ten dominant and selecting low RDS(ON) MOSFETs will
noticeably improve the efficiency of the converter even
though they give higher switching losses.
The gate charge loss portion of the top/bottom MOSFET’s
total power loss is derived from the SC4605. This gate
charge loss is based on certain operating conditions (fs,
VGATE, and I
O).
The thermal estimations have to be done for both
MOSFETs to make sure that their junction temperatures
do not exceed their thermal ratings according to their
total power losses PTOTAL, ambient temperature Ta and
their thermal resistance R
θja as follows:
ja
TOTAL
a
(max)
j
R
P
T
T
θ
+
<



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