SC1548
POWER MANAGEMENT
Typical Characteristics
(Cont.)
(1)
Output Short Circuit Glitch Immunity
Note:
(1) See Applications Circuit on page 1.
Trace 1: V
DRV
, 5V/div.
Trace 2: V
OUT
, 1V/div.
Timebase: 1ms/div
SC1548 enabled, then shorted, therefore
V
OUT
< V
TH(OC)
immediately the short is applied. This
device shuts down after 5ms.
Applications Infomation
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.
ã
2001 Semtech Corp.
11
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:
Output Voltage
1.515V
1.818V
R 1 (k
W)
2.52
5.55
R 1 (k
W)
12.63
12.63
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