9

 2005 Semtech Corp.

www.semtech.com

SC1456

POWER MANAGEMENT

Thermal Considerations

The worst-case power dissipation for this part is given

by:

()

()

)

MAX

(

2

Q

)

MAX

(

2

IN

)

MAX

(

2

OUT

)

MIN

(

2

OUT

)

MAX

(

2

IN

)

MAX

(

1

Q

)

MAX

(

1

IN

)

MAX

(

1

OUT

)

MIN

(

1

OUT

)

MAX

(

1

IN

)

MAX

(

D

I

V

I

V

V

I

V

I

V

V

P

•

+

•

−

+

•

+

•

−

=

For all practical purposes, it can be reduced to:

()

()

)

MAX

(

2

OUT

)

MIN

(

2

OUT

)

MAX

(

2

IN

)

MAX

(

1

OUT

)

MIN

(

1

OUT

)

MAX

(

1

IN

)

MAX

(

D

I

V

V

I

V

V

P

•

−

+

•

−

=

Looking at a typical application:

V

V

I

T

This gives us:

()

mW

300

100

.

0

702

.

2

2

.

4

2

P

)

MAX

(

D

=

•

−

•

=

Using this figure, we can calculate the maximum thermal

impedance allowable to maintain T

W

/

C

133

300

.

0

85

125

P

T

T

)

MAX

(

D

)

MAX

(

A

)

MAX

(

J

)

MAX

)(

A

J

(

°

=

−

=

−

=

θ

−

Applications Information (Cont.)

With the standard MSOP-10 Land Pattern shown at the

end of this datasheet, and minimum trace widths, the

thermal impedance junction to ambient for SC1456 is

113°C/W. Thus with no additional heatsinking,

T

The junction temperature can be reduced further by the

use of larger trace widths, and connecting pcb copper

area to the GND pins (pins 2 and 7), which connect

directly to the device substrate. Lower junction

temperatures improve overall output voltage accuracy.

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.

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 input and output capacitors close to the

device for optimal transient response and device

behavior.

3) Connect all ground connections directly to the ground

plane. If there is no ground plane, connect to a common

local ground point before connecting to board ground.

VOUT1 (SEE EQUATION)

VOUT2 = PRESET

VIN1

VIN2

U1

SC1456

1

2

3

4

5

6

7

8

9

10

EN1

GND

SET1

OUT2

IN2

EN2

GND

SET2

OUT1

IN1

C1

1uF

C3

2.2uF

C4

1uF

C2

1uF

R1

R2

(OR VIN1)

Figure 1: Output 1 Set Externally and Output 2 Set Internally