SC1401 Semtech Corporation, SC1401 Datasheet - Page 15

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

November 21, 2000
© 2000 SEMTECH CORP.
R
L1
I
I
any resistive drops due to winding resistance and
MOSFET on resistance.
Where:
L1 = 4µH
From this calculation we choose L1 to be 3.9uH.
For a lower operating frequency, f = 200kHz, L1
calculates to be 6uH.
Choosing Inductor & FET Current Rating
The current carrying capability of the inductor and
MOSFETs should be sized to handle the maximum
current of the supply set by the current sense
resistor. For the SC1401 EVAL board Rsense equals
the parallel combination of R7A & R7B. Here we use
the minimum current threshold value:
Where I
I
I
Here we chose a sense resistor of 13.5 milliohms.
Now we can determine FET and inductor current
carrying capability by using the maximum current
limit threshold:
From this value choose an inductor with Isat > 10.4
Amps, and for the FET choose a continuous
conduction current rating Ic > 10.4 Amps.
PEAK
PEAK
PEAK
PEAK
SENSE
(28
= 7 + 0.75
= 7.75 Amps
I
0.0135
PEAK
O
100mV
0.14
D
I
PEAK
2.0)
equals:
∆I
2
O
V
V
IN
O
t ,
2.0
10
28
1.5
4 .
1
f
Amps
,
3.33
and
10
f
6
300kHz
HIGH PERFORMANCE SYNCHRONOUS BUCK
CONTROLLER WITH LDO FOR PORTABLE
POWER
I
I
I
Input Capacitor Selection
Input capacitor is selected based upon the input
ripple current demands of the converter. First
determine the input ripple current expected and then
choose a capacitor to meet that demand.
The input RMS ripple current can be calculated as
follows:
I
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 IRMS ripple current
can be approximated by:
However, this condition will not occur because the
duty cycle is set by Vo/Vin, 2/6 = 0.33. If the output
voltage is changed where D approaches 50% the
input capacitor ripple current may need to be
recalculated and capacitors added.
Therefore, for a maximum load current of 7 amps,
the input capacitors should be able to safely handle
3.3 amps of ripple current.
Tantantalum capacitors. Each capacitor has a ripple
current capability of 0.908 amps at 100kHz, 25°C.
Four of these capacitors in parallel will meet the
requirements.
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
IRF7805 meet the voltage and current requirements.
However, the lower FET is a combination of two
IRF7805 devices in parallel for improved efficiency
RMS
RMS
652 MITCHELL ROAD
RMS
RMS
For the EVAL board, we chose four 22uF, 35V
= 3.3 Amps
V
I
I
7
6
2
O
O
IN
V
2
O
(6
(V
IN
-
2)
V
O
NEWBURY PARK CA 91320
)
SC1401
15