SC1155_01 Datasheet, PDF (13/20 Pages) Semtech Corporation – Programmable Synchronous DC/DC Hysteretic Controller with VRM 9.0VID Range

SC1155
POWER MANAGEMENT
Applications Information - .unctional Description (Cont.)
PRELIMINARY
age that is on the DROOP pin from VRE.. The voltage on
the IO pin is divided down with an external resistor di-
vider, and connected to the DROOP pin. Thus, under
loaded conditions, VOUT is regulated to:
VOUT = VRE. • (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 discharged,
the soft start current source is disabled, and the control-
ler is in a low I_Q state. When INH goes high, the short
across the soft start capacitor is removed, the soft start
current source is enabled, and normal converter opera-
tion begins. When the system logic supply is connected
to INH, it controls power sequencing by locking out con-
troller operation until the system logic supply exceeds
the input threshold voltage of the INH circuit; thus the
+12V supply and the system logic supply (either +5V or
3.3V) must be above UVLO thresholds before the con-
troller is allowed to start up.
VIN
value for all VID settings. The soft start charging current
is determined by the following equation: I_SS = I_RE.B/5.
Where I_RE.B is the current flowing out of the RE.B pin. It
is recommended that no additional loads be connected
to RE.B, other than the resistor divider for setting the
hysteresis voltage. Thus these resistor values will deter-
mine the soft start charging current. The maximum cur-
rent that can be sourced by RE.B is 500µA.
Power Good
The power good circuit monitors for an undervoltage con-
dition on VOUT. If VSENSE is 7% (nominal) below VRE.,
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 greater then 2.15V
(nominal), 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.
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_Q state. When
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 lock-
out circuit for noise immunity.
Soft Start
The soft start circuit controls the rate at which VOUT pow-
ers 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_SS is also proportional to
the reference voltage. By making the charging current
proportional to VRE., the power-up time for VOUT will be
independent of VRE.. Thus, C_SS can remain the same
Overcurrent Protection
The overcurrent protection circuit monitors the current
through the high side .ET. 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 in-
cluded for noise immunity. The OCP circuit is also de-
signed to protect the high side .ET against a short-to-
ground fault on the terminal common to both power .ETs
(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 minimum
drive voltage is 7V. The minimum short circuit current is
100mA.
 2001 Semtech Corp.
13
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