SC1401ISSTR View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC1401ISSTR

HIGH PERFORMANCE SYNCHRONOUS BUCK CONTROLLER WITH LDO FOR PORTABLE POWER

Semtech Corporation 

HIGH PERFORMANCE SYNCHRONOUS BUCK

CONTROLLER WITH LDO FOR PORTABLE

SC1401

POWER

November 21, 2000

any resistive drops due to winding resistance and

MOSFET on resistance.

Where:

D=

VO ,

VIN

t=

1f ,

and

f = 300kHz

L1

=

(28

−

2.0)

•

2.0

28

•

1.5

3.33

•

10 −6

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:

RSENSE

=

100mV

IPEAK

Where IPEAK equals:

IPEAK

= IO

+

∆IO

2

IPEAK = 7 + 0.75

IPEAK = 7.75 Amps

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:

IPEAK

=

0.14

0.0135

=

10 .4 Amps

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.

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:

IRMS

=

IO

VIN

VO • (VIN − VO )

IRMS

=

7

6

•

2 • (6 - 2)

IRMS = 3.3 Amps

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:

IRMS

=

IO

2

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.

For the EVAL board, we chose four 22uF, 35V

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

© 2000 SEMTECH CORP.

15

652 MITCHELL ROAD NEWBURY PARK CA 91320

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