11

ã 2001 Semtech Corp.

www.semtech.com

SC1548

POWER MANAGEMENT

Theory Of Operation

The SC1548 linear FET controller provides a simple way

to drive an N-channel MOSFET to produce a tightly

regulated output voltage from an available, higher,

supply voltage. It takes its power from a 12V supply,

drawing typically 2mA while operating.

It contains an internal bandgap reference which is

compared to the output voltage via a resistor divider.

This resistor divider is internal on the fixed output

voltage options, and user selectable on the adjustable

option. Since the drive pin can pull up to a 9V

guaranteed minimum, the device can be used to

regulate a large range of output voltages by careful

selection of the external MOSFET (see component

selection, below).

The SC1548 includes an active high enable control with

an internal pullup resistor. If this pin is pulled low, the

drive pin is pulled low, turning off the N-channel MOSFET.

If the pin is left open or pulled up to 2.5V, 3.3V or 5V,

then the drive pin will be enabled.

Also included is an overcurrent protection circuit that

monitors the output voltage. If the output voltage drops

below 50% of nominal, as would occur during an

overcurrent or short condition, the device will pull the

drive pin low and latch off.

Fixed Output Voltage Options

Please refer to the Application Circuit on Page 1. The

fixed output voltage parts have an internal resistor

divider that draws a nominal 100µA from the output.

The voltage at the common node of the resistor divider

is then compared to the bandgap reference voltage of

1.263V. The drive pin voltage is then adjusted to

maintain the output voltage set by the resistor divider.

Referring to the block diagram on page 5, the nominal

resistor values are:

e

g

a

tl

o

V

t

u

p

t

u

Ok

(

1

R

W)k

(

1

R

W)

V

5

1

5

.

12

5

.

23

6

.

2

1

V

8

1

8

.

15

5

.

53

6

.

2

1

Output Short Circuit Glitch Immunity

Trace 1: V

Trace 2: V

Timebase: 1ms/div

SC1548 enabled, then shorted, therefore

V

device shuts down after 5ms.

Note:

(1) See Applications Circuit on page 1.

Applications Infomation