SC1404(2003) Просмотр технического описания (PDF) - Semtech Corporation
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SC1404 Datasheet PDF : 27 Pages
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SC1404
POWER MANAGEMENT
Functional Information
SHDN# ON3 ON5
The REF output is accurate to +/- 2% over temperature. It is ca-
pable of delivering 5mA max and should be bypassed with 1uF
Low
X
X
minimum. Loading the REF pin will reduce the REF voltage slightly.
Loading
511
Resistance (Ω)
2.67K 49.9K 255K 1Meg
Deviation from 8.3mV 3.1mV 0.5mV 0.3mV 0mV
Vref = 2.4920V
High Low Low
High High High
PRELIMINARY
MODE
DESCRIPTION
Shut-down
Standby
Run
Mode
Minimum bias
current
VREF and VL
regulator
enable
Both SMPS
Running
Current Sense (CSH, CSL)
The output current of the power supply is sensed as the voltage
drop across an external resistor between the CSH and CSL pins.
Overcurrent is detected when the current sense voltage exceeds
+/- 50mV. A positive overcurrent will turn off the high-side driver, a
negative overcurrent will turn off the low-side driver; each on a
cycle-by-cycle basis.
Oscillator
Power up Controls and Soft Start
The user has control of the SC1404 RESET# by setting the SEQ,
ON3 and ON5 pins as described in the following table.
At startup, the RESET# pin is held low for 32K switching cycles.
Another timer is used to enable the undervoltage protection. The
undervoltage protection circuitry is enabled after 6144 switching
cycles, at which time the SMPS should be in regulation.
When the SYNC pin is set high the oscillator runs at 300KHz;
when SYNC is set low the frequency is 200KHz. The oscillator can
also be synchronized to the falling edge of a clock on the SYNC pin
with a frequency between 240KHz and 350KHz. In general,
200KHz operation is used for highest efficiency while 300KHz
leads to less output ripple and/or smaller filter components.
Fault Protection
In addition to cycle-by-cycle current limit, the SC1404 monitors
over-temperature, and output overvoltage and undervoltage
conditions. The overtemperature detection will shut the part down
if the die temperature exceeds 150°C with 10°C of hysteresis.
When SEQ is set to REF, the RESET# only monitors the 3.3V SMPS
and the 5V SMPS is ignored.
Each SMPS contains its own counter and DAC to gradually increase
the current limit at startup to prevent surge currents. The current
limit is increased from 0, 20%, 40%, 60%, 80%, to 100% linearly
over the course of 512 switching cycles.
12OUT Supply
The 12OUT linear regulator is capable of supplying 200mA. The
input voltage to the 12OUT regulator is generated by a secondary
winding on the 5V SMPS inductor.
If either SMPS output is more than 10% above its nominal value,
both SMPS are latched off and the low-side MOSFETS are latched
on. To prevent the output from ringing too far below ground in a
fault condition, a 1A Schottky diode should be placed across each
output. Two different levels of undervoltage are detected. If the
output falls 10% below its nominal output, the RESET# output is
pulled low.If the output falls 25% below its nominal output following
a start-up delay, both SMPS are latched off. Both of the latched
fault modes persist until SHDN# or ON3 is toggled, or the V+ input
is brought below 1V.
Shutdown and Operating Modes
A heavy load on the 12OUT regulator when the 5V SMPS is in
PSAVE will cause the VDD input to drop, browning out the regulator.
If the output drops 0.8% from its nominal value, the 5V SMPS is
forced out of PSAVE mode and into continuous conduction mode
for several cycles. This recharges the bulk input capacitor on the
VDD. The 12OUT linear regulator also has a current limit to prevent
damage under short circuit conditions.
Over-voltage protection is provided on the VDD input. If the VDD
input is above 19V, an over-voltage is detected and a 10mA current
shunt load is applied to VDD. The over-voltage threshold has a
0.5V hysteresis.
Holding the SHDN# pin low disables the SC1404, reducing the V+
input current to less than 10uA. When SHDN goes high, the part
enters a standby mode where the VL regulator and VREF are
enabled. Turning on either SMPS will put the SC1404 in run mode.
2003 Semtech Corp.
10
www.semtech.com
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