SC1401 Semtech Corporation, SC1401 Datasheet - Page 16

• Manufacturer Part Number: SC1401
• Description: HIGH PERFORMANCE SYNCHRONOUS BUCK CONTROLLER WITH LDO FOR PORTABLE POWER
• Manufacturer: Semtech Corporation

November 21, 2000
© 2000 SEMTECH CORP.
P
P
P
P
P
P
and to handle any possible short circuit condition.
These are 30V, 9A FET’s. Based on 85°C ambient
temperature, 150°C junction temperature and
thermal resistance, their power handling is calculated
as follows:
Power Limit for Upper & Lower FET:
T
Each FET must not exceed 1.3W of power
dissipation.
The conduction losses for the upper & lower FET can
be determined. For the calculations below, a
nominal input voltage of 12V, for Vout = 2.0V, Iout =
7A and f = 300kHz. The Rdson value for the upper &
lower FET is 11milliohms. We will calculate the
conduction losses and switching losses for each
FET. From the calculations below we are well within
the 1.3 watt dissipation limit as calculated above.
Conduction Losses Upper FET:
P
Conduction Losses Lower FET:
P
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.
CU
CL
SU
T
CU
CU
J
CU
CL
= 150
= 0.225W
= 0.090W
T
C
R
0.0055
R
0.011
J
RSS
θ
DS
o
DS
C
JA
T
A
1 (
D
V
I
IN
G
T
D
2
150
12
(
I
A
2
)
2
1
= 85
F
50
I
2
12
7
2
I
O
o
2
85
C
)
215
7
2
1.3W
!
10
JA
= 50
12
HIGH PERFORMANCE SYNCHRONOUS BUCK
CONTROLLER WITH LDO FOR PORTABLE
POWER
o
1
12
C/W
2
300000
7
P
Crss is the reverse transfer capacitance of the FET;
in this case it equals 215pF for the IRF7805.
Where Ig is the gate driver current. This is equal to
2A for the SC1401.
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 calculations with your maximum input voltage
specification. In addition, the distribution of power in
the upper and lower FET will change as input voltage
increases.
Other losses to consider are gate charge losses,
inductor switching and copper losses, and losses in
the input and output capacitors. All these items will
decrease efficiency and need to be carefully
analyzed to obtain the highest efficiency possible,
especially if running off battery power.
652 MITCHELL ROAD
SU
FETS
= 0.065W
= 0.09 + 0.225 + 0.065 = 0.38W.
NEWBURY PARK CA 91320
SC1401
16