9

ã 2000 Semtech Corp.

www.semtech.com

SC1531(A)

POWER MANAGEMENT

(Remember that at 125°C, R

value at 25°C.)

Thermal Considerations

When operating from the 5V supply, the power

dissipation in the SC1531(A) is approximately equal to

the product of the output current and the input to

output voltage differential:

()

OUT

OUT

IN

D

I

V

V

P

•

−

≈

The absolute worst-case dissipation is given by:

(

)

)

MAX

)(

AUX

(

Q

)

MAX

(

)

MAX

(

O

)

MIN

(

)

MAX

(

)

MAX

(

D

I

VAUX

I

VO

VIN

P

•

+

•

−

=

Note that the VAUX

VAUX is not available or not connected.

Inserting VIN = 5.5V, VO = 3.234V, I

VAUX = 3.6V and I

W

46

.

0

P

)

MAX

(

D

=

Using this figure, we can calculate the maximum thermal

impedance allowable to maintain T

(

W

/

C

73

R

therefore

,

W

/

C

47

R

W

/

C

120

46

.

0

70

125

P

T

T

R

)

MAX

)(

A

C

(

TH

)

MAX

)(

C

J

(

TH

)

MAX

(

D

)

MAX

(

A

)

MAX

(

J

)

MAX

)(

A

J

(

TH

°

=

°

=

°

=

−

=

−

=

−

−

−

This is readily achievable using pcb copper area to aid in

conducting the heat away from the device (see Figure 1

on page 10). Heatsinking the bypass FET is not

necessary - its power dissipation is given by:

()

)

MAX

)(

ON

(

DS

2

)

MAX

(

O

)

MAX

(

D

R

I

P

•

=

For I

Layout Considerations

While layout for linear devices is generally not as critical

as for a switching application, careful attention to detail

will ensure reliable operation. See Figure 1 on page 10

for a sample layout.

1) Attaching the part to a larger copper footprint will

enable better heat transfer from the device, especially

on PCBs where there are internal ground and power

planes.

2) Place the bulk and decoupling capacitors close to the

device for optimal transient response.

3) If the SENSE lead is being used, route it to the load

using a seperate trace from the main VO path. If it is not

being used, connect to pin 7 as shown.

4) The external bypass FET is shown close to the device

for convenience only. Since it is not being switched, longer

gate drive traces can be used without problem.

Applications Information (Cont.)