SC1154EVB データシートの表示（PDF） - Semtech Corporation

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SC1154EVB

Programmable Synchronous DC/DC Hysteretic Controller for Advanced Processors

Semtech Corporation 

SC1154

POWER MANAGEMENT

Applications Information - .unctional Description (Cont.)

PRELIMINARY

current transient. The overshoot during a high to low

load current transient is reduced by subtracting the volt-

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

will be independent of VRE.. Thus, CSS can remain the

same value for all VID settings. The soft start charging

current is determined by the following equation:

ISS = IRE.B/5. Where IRE.B is the current flowing out of the

VRE.B pin. It is recommended that no additional loads

be connected to VRE.B, other than the resistor divider

for setting the hysteresis voltage. Thus these resistor

values will determine the soft start charging current. The

maximum current that can be sourced by VRE.B is

500µA.

The inhibit pin is a TTL compatible digital pin that is used

to enable the controller. When INHIBIT 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 IQ state. When INHIBIT 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 INHIBIT, it controls power sequencing by locking out

controller operation until the system logic supply exceeds

the input threshold voltage of the INHIBIT 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

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 IQ 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

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 15% above VRE.,

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 2µs deglitch timer is in-

cluded for noise immunity.

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 IOUT

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 2µs 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).

The soft start circuit controls the rate at which VOUT pow-

ers up. A capacitor is connected between SO.TST and

AGND and is charged by an internal current source. The

value of the current source is proportional to the refer-

ence voltage so the charging rate of CSS is also propor-

tional to the reference voltage. By making the charging

current proportional to VRE., the power-up time for VOUT

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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