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

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:

PSU = 0.065W

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.

TJ = 150oC

TA = 85oC

!JA = 50oC/W

PT

=

TJ − TA

θ JA

=

150 −

50

85

= 1.3W

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:

PCU = R DS • D • I2

PCU

= 0.011

•

2

12

•

72

So the total FET losses equate to:

PFETS = 0.09 + 0.225 + 0.065 = 0.38W.

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.

PCU = 0.090W

Conduction Losses Lower FET:

PCU = RDS • (1− D) • I2

( ) PCL = 0.0055

•

1−

2

12

• 72

PCL = 0.225W

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.

PSU = CRSS

• VIN 2

IG

• F • IO

=

215

• 10 −12

• 12 2

1

• 300000

•7

© 2000 SEMTECH CORP.

16

652 MITCHELL ROAD NEWBURY PARK CA 91320

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