sc1154 Semtech Corporation, sc1154 Datasheet - Page 12

• Manufacturer Part Number: sc1154
• Description: Programmable Synchronous Dc/dc Hysteretic Controller For Advanced Processors
• Manufacturer: Semtech Corporation

PRELIMINARY - March 1, 2000
FUNCTIONAL DESCRIPTION (cont.)
resistor divider, and connected to the DROOP pin.
Thus, under loaded conditions, VOUT is regulated to
Vout = Vref • (1+R7/R8) - IOUT • R2/(R1+R2).
Inhibit
The inhibit pin is a TTL compatible digital pin that is
used to enable the controller. When INH is low, the
output drivers are low, the soft start capacitor is dis-
charged, the soft start current source is disabled, and
the controller is in a low I
the short across the soft start capacitor is removed, the
soft start current source is enabled, and normal con-
verter operation begins. When the system logic supply
is connected to INH, it controls power sequencing by
locking out controller operation until the system logic
supply exceeds the input threshold voltage of the INH
circuit; thus the +12V supply and the system logic sup-
ply (either +5V or 3.3V) must be above UVLO thresh-
olds before the controller is allowed to start up.
VIN
The VIN undervoltage lockout circuit disables the con-
troller while the +12V supply is below the 10V start
threshold during power-up. While the controller is dis-
abled, the output drivers will be low, the soft start ca-
pacitor will be shorted and the soft start current is dis-
abled and the controller will be in a low I
VIN exceeds the start threshold, the short across the
soft start capacitor is removed, the soft start current
source is enabled and normal converter operation be-
gins. There is a 2V hysteresis in the undervoltage
lockout circuit for noise immunity.
Soft Start
The soft start circuit controls the rate at which VOUT
powers up. A capacitor is connected between SS and
AGND and is charged by an internal current source.
The value of the current source is proportional to the
reference voltage so the charging rate of C
proportional to the reference voltage. By making the
charging current proportional to VREF, the power-up
time for VOUT will be independent of VREF. Thus, C
can remain the same value for all VID settings. The
soft start charging current is determined by the follow-
ing equation: I
flowing out of the REFB pin. It is recommended that
no additional loads be connected to REFB, other than
the resistor divider for setting the hysteresis voltage.
© 2000 SEMTECH CORP.
SS
= I
REFB
/5. Where I
Q
state. When INH goes high,
REFB
is the current
Q
state. When
SS
PROGRAMMABLE SYNCHRONOUS
DC/DC HYSTERETIC CONTROLLER
FOR ADVANCED PROCESSORS
is also
SS
Thus these resistor values will determine the soft start
charging current. The maximum current that can be
sourced by REFB is 500µA.
Power Good
The power good circuit monitors for an undervoltage
condition on VOUT. If VSENSE is 7% (nominal) below
VREF, then the power good pin is pulled low. The
PWRGD pin is an open drain output.
Overvoltage Protection
The overvoltage protection circuit monitors VOUT for
an overvoltage condition. If VSENSE is 15% above
VREF, than a fault latch is set and both output drivers
are turned off. The latch will remain set until VIN goes
below the undervoltage lockout value. A 1ms deglitch
timer is included for noise immunity.
Overcurrent Protection
The overcurrent protection circuit monitors the current
through the high side FET. The overcurrent threshold
is adjustable with an external resistor divider between
IO and AGND, with the divider voltage connected to
the OCP pin. If the voltage on the OCP pin exceeds
100mV, then a fault latch is set and the output drivers
are turned off. The latch will remain set until VIN goes
below the undervoltage lockout value. A 1ms deglitch
timer is included for noise immunity. The OCP circuit
is also designed to protect the high side FET against a
short-to-ground fault on the terminal common to both
power FETs (VPHASE).
Drive Regulator
The drive regulator provides drive voltage to the low
side driver, and to the high side driver when the high
side driver is configured as a floating driver. The mini-
mum drive voltage is 7V. The minimum short circuit
current is 100mA.
652 MITCHELL ROAD NEWBURY PARK CA 91320
SC1154
12