12

ã 2001 Semtech Corp.

www.semtech.com

SC1548

POWER MANAGEMENT

It is possible to adjust the output voltage of the fixed

voltage options, by applying an external resistor divider

to the sense pin (please refer to Figure 1 below). Since

the sense pin sinks a nominal 100µA, the resistor

values should be selected to allow 10mA to flow through

the divider. This will ensure that variations in this current

do not adversely affect output voltage regulation. Thus a

target value for R2 (maximum) can be calculated:

Ω

≤

mA

10

V

2

R

)

FIXED

(

OUT

The output voltage can only be adjusted upwards from the fixed

output voltage, and can be calculated using the

following equation:

Volts

A

100

1

R

2

R

1

R

1

V

V

)

FIXED

(

OUT

)

ADJUSTED

(

OUT

µ

•

+











 +

•

=

Adjustable Output Voltage Option

The adjustable output voltage option does not have an

internal resistor divider. The adjust pin connects directly

to the inverting input of the error amplifier, and the

output voltage is set using external resistors (please

refer to Figure 2 above). In this case, the adjust pin

sources a nominal 0.5µA, so the resistor values should

be selected to allow 50µA to flow through the divider.

Again, a target value for R2 (maximum) can be

calculated:

Ω

µ

≤

A

50

V

263

.

1

2

R

The output voltage can be calculated as follows:

1

R

A

5

.

0

2

R

1

R

1

263

.

1

V

OUT

•

µ

−











 +

•

=

Applications Infomation (Cont.)

VOUT

VPWR

12V IN

ENABLE

R1

R2

Q1

C4

0.1uF

+

C3

22uF

+

C2

100uF

+

C1

100uF

U1

SC1548CSK-X.X

1

2

3

4

5

SNS

GND

DRV

IN

EN

Figure 1: Adjusting The Output Voltage of Fixed Output Voltage Options

VOUT

VPWR

12V IN

ENABLE

Q1

C4

0.1uF

+

C1

100uF

+

C2

100uF

+

C3

22uF

R1

R2

U1

SC1548CSK

1

2

3

4

5

ADJ

GND

DRV

IN

EN

Figure 2: Setting The Output Voltage of the Adjustable Output Voltage Option