sc1402 Semtech Corporation, sc1402 Datasheet - Page 14

• Manufacturer Part Number: sc1402
• Description: Multi-output, Low-noise Power Supply Controller For Notebook Computers
• Manufacturer: Semtech Corporation

P
© 2000 SEMTECH CORP.
December 15, 2000
SU
Switching Losses Upper FET:
Note: switching losses exist on the upper FET only
because the clamp diode across the lower FET will
turn on prior to the lower FET turning on.
Where Ig is the gate driver current. This is equal to
1A for the SC1402.
Crss is the reverse transfer capacitance of the FET;
in this case it equals 240pF for the IRF7413.
So the total FET losses equate to:
P
Note that as Vin increases the power dissipation from
switching losses will also increase. This is especially
important if the input to the supply is from an AC
adapter. Therefore, it is necessary to check the cal-
culations with your maximum input voltage specifica-
tion. In addition, the distribution of power in the upper
and lower FET will change as input voltage in-
creases.
Other losses to consider are gate charge losses, in-
ductor switching and copper losses, and losses in the
input and output capacitors. All these items will de-
crease efficiency and need to be carefully analyzed to
obtain the highest efficiency possible, especially if
running off battery power.
Basic Application Circuit
The basic dual-output 3.3V / 5V synchronous buck
converter is shown in Figure 2. This circuit shows the
minimum requirements for successful operation. For
varying current levels, Table 3 provides a useful se-
lection of components for varying supply require-
ments and is based on the calculations described
previously. Input voltage ranges for all designs in the
table are 6.5V to 28V. Frequency used in calculations
is 300KHz.
FETS
C
RSS
= 0.027+0.072+0.031 = 0.130W
V
IN
I
G
2
F
I
OUT
240
10
12
12
1
2
300000
Multi-Output, Low-Noise Power Supply
Controller for Notebook Computers
3
0.031W
652 MITCHELL ROAD
High & Low
Side FET’s
Input
Capacitor
Output
Capacitor
Resistor
Inductor
*10V for 5V SMPS, 6.3V for 3V SMPS
IR IRF7901D1
Siliconix Si4412
10uF, 30V Sanyo
OS-CON
330uF, 6.3V/10V*
AVX TPS
0.033
WSL2010-R033-F
15uF, Coilcraft
DO-3316P-153
2A
Dale
IR IRF7413
Siliconix Si4412
2 X 10uF, 30V Sanyo
OS-CON
2X 330uF, 6.3V/10V*
AVX TPS
0.02 , Dale
WSL2010-R020-F
10uF, Coilcraft
DO3316P-103
Table 3
NEWBURY PARK CA 91320
3A
IR IRF7805
Siliconix Si4410
3 X 10uF, 30V Sanyo
OS-CON
4X 330uF, 6.3V/10V*
AVX TPS
0.012 , Dale
WSL2512-R012-F
4.7uF, Coilcraft
DO3316P-472
SC1402
4A
14