SC1403ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC1403ITSTRT' PDF : 31 Pages View PDF
SC1403
POWER MANAGEMENT
Applications Information (Cont.)
Input ripple current calculations: The following equations provide
quick approximations for input ripple current:
D3 = 3.3V / VIN = 3V duty cycle
D5 = 5V / VIN = 5V duty cycle
I3 = 3V DC load current
I5 = 5V DC load current
DOVL = overlapping duty cycle of the 3V and 5V pulses
(varies according to input voltage)
DOVL = 0 for 9.6V ≤ VIN
DOVL = (D5 - 0.41) for 6.7V ≤ VIN < 9.6V
DOVL = (D5 - 0.36) for VIN < 6.7
IIN = Average DC input current
IIN = I3⋅D3 + I5⋅D5
and the Schottky diode. The current rating of the Schottky diode
can be determined by the following equation:
IF
_
AVG
=
ILOAD
⋅
100n
TS
=
0.2A
where 100nsec is the estimated time between the mosfet turn-
ing off and the Schottky diode taking over and Ts = 3.33uS. There-
fore a Schottky diode with a forward current of 0.5A is sufficient
for this design.
External Feedback Design
In order to optimize the ripple voltage during Power Save mode, it
is strongly recommended to use external voltage dividers (R10
and R9 for 5V power train; R8 and R11 for 3.3V power train) to
achieve the required output voltages. In addition a 56pF (C22 for
5V and C21 for 3.3V) cap is recommended connecting from the
output to both feedback pins (pin # 3 and #12). The signal-to-
noise ratio is therefore increased due to the added zeros.
ISW _ RMS = RMS current drawn from VIN
ISW _ RMS2 = D3⋅I32 + D5⋅I52 + 2 ⋅DOVL ⋅I3⋅I5
+ IRMS_CAP = ISW_RMS 2 IIN_AVE 2
The worst-case ripple current varies by application. For the case
of I3 = I5 = 6A, the worst-case ripple occurs at Vin = 7.5V, at which
point the rms capacitor ripple current is 4.2A. To handle this the
reference design uses 4 paralleled ceramic capacitors, (Murata
GRM32NF51E106Z, 10 uF 25V, size 1210). Each capacitor is
rated at 2.2A.
Choosing Synchronous mosfet and Schottky Diode
Since this is a buck topology, the voltage and current ratings of the
synchronous mosfet are the same as the main switching mosfet.
It makes sense cost- and volume-wise to use the same mosfet for
the main switch as for the synchronous mosfet. Therefore, IRF7413
is used again in the design for synchronous mosfet.
To improve overall efficiency, an external Schottky diode is used in
parallel to the synchronous mosfet. The freewheeling current is
going into the Schottky diode instead of the body diode of the
synchronous mosfet, which usually has very high forward drop and
slow transient behavior. It is really important when laying out the
board to place both the synchronous mosfet and Schottky diode
close to each other to reduce the current ramp-up and ramp-down
time due to parasitic inductance between the channel of the mosfet
2004 Semtech Corp.
21
United States Patent No. 6,377,032
www.semtech.com
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