sc1402 Semtech Corporation, sc1402 Datasheet - Page 13

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

I
I
I
If your ESR value varies significantly from the calcu-
lated value and you don’t want to add more capaci-
tance or add a series resistor in the capacitor path as
described above. We recommend that you bench test
the supply over temperature to verify transient re-
sponse and operation of the SMPS.
Input Capacitor Selection
Input capacitor is selected based upon the input ripple
current demand of the converter. First determine the
input ripple current expected and then choose a capac-
itor to meet that demand.
The input RMS ripple current can be calculated as fol-
lows:
The worse case input RMS ripple current occurs at
50% duty cycle (D = 0.5 or Vin = 2 Vout) and therefore
under this condition the I
proximated by:
Therefore, for a maximum load current of 3.0A , the
input capacitors should be able to safely handle 1.5A
of ripple current. For the EVAL board there are two
such regulators that operate simultaneously. Each ca-
pable of 1.5A of ripple current, although it is impossible
for both regulators to be at 50% duty cycle at the same
time since they have different output voltages. For the
EVAL board, we chose four 10uF, 30V OS-CON ca-
pacitors, two for each supply. Each capacitor has a rip-
ple current capability of 1.38A at 100KHz, 45°C. Fol-
lowing the capacitor-derating chart for temperature and
frequency operation at 300KHz, two of these capaci-
tors in parallel will suffice, as calculated below:
The RMS ripple current is under a worst-case condition
at full load, 3A each when both SMPSs are on.
When the 5V output is at maximum ripple of 1.5A (D =
50%), the 3.3V output adds 1.41A of ripple current.
The maximum ripple current is then calculated by:
RMS(MAX)
© 2000 SEMTECH CORP.
RMS
RMS
December 15, 2000
I
LOAD
2
V
OUT
1.5
2
(V
IN
1.41
V
2
OUT
2.06A
)
RMS
I
V
OUT
ripple current can be ap-
IN
Multi-Output, Low-Noise Power Supply
Controller for Notebook Computers
652 MITCHELL ROAD
I
P
P
P
P
P
Conversely:
When the 3V output is at maximum ripple 1.5A (D =
50%), the 5V output adds 1.29A of ripple current.
The worse case ripple current is then calculated by:
Clearly, the combined input capacitor bank must be
chosen to handle 2A of ripple current under worst-
case conditions.
MOSFET Switches
After selecting the voltage and current requirements
of each MOSFET device for the upper and lower
switches, the next step is to determine their power
handling capability. For the EVAL board the IRF7413
met the voltage and current requirements. These are
30V, 9A FET’s. Based on 85
150
their power handling is calculated as follows:
Power Limit for Upper & Lower FET:
T
Each FET must not exceed 1.3W of power dissipa-
tion. The conduction losses for the upper & lower
FET can be determined. For the calculations below,
a nominal input voltage of 12V, for Vout = 3.3V, Iout
= 3A and f = 300KHz. The Rdson value for the upper
& lower FET is 11m . We will calculate the conduc-
tion losses and switching losses for each FET. From
the calculations below we are well within the 1.3W
dissipation limit as calculated above.
Conduction Losses Upper FET:
Conduction Losses Lower FET:
RMS(MAX)
CL
T
CU
CL
CU
J
= 150
0
C junction temperature and thermal resistance,
T
R
0.011
J
0.011
R

DS
JA
DS
0
C; T
T
A
(1
1.5
D
æ
ç
è
150
A
D)
2
3.3
1
12
= 85
I
2
50
1.29
3.3
I
12
2
0
85
3
C;
2
ö
2
3
NEWBURY PARK CA 91320
1.3W
0.027W
ja
2
1.98A
= 50°C/W
0
C ambient temperature,
0.072W
SC1402
13