Datasheet

UCC1895, UCC2895, UCC3895

SLUS157P –DECEMBER 1999–REVISED JUNE 2013

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SS/DISB (Soft-Start/Disable)

This pin combines two independent functions.

Disable Mode: A rapid shutdown of the chip is accomplished by externally forcing SS/DISB below 0.5 V,

externally forcing REF below 4 V, or if VDD drops below the undervoltage lockout threshold. In the case of REF

being pulled below 4 V or an undervoltage condition, SS/DISB is actively pulled to ground via an internal

MOSFET switch.

If an overcurrent fault is sensed (CS = 2.5 V), a soft-stop is initiated. In this mode, SS/DISB sinks a constant

current of (10 × I

). The soft-stop continues until SS/DISB falls below 0.5 V. When any of these faults are

detected, all outputs are forced to ground immediately.

NOTE

If SS/DISB is forced below 0.5 V, the pin starts to source current equal to I

. The only

time the part switches into low I

current mode, though, is when the part is in

undervoltage lockout.

Soft-start Mode: After a fault or disable condition has passed, VDD is above the start threshold, and, or,

SS/DISB falls below 0.5 V during a soft-stop, SS/DISB switches to a soft-start mode. The pin then sources

current, equal to I

. A user-selected resistor/capacitor combination on SS/DISB determines the soft start time

constant.

NOTE

SS/DISB actively clamps the EAOUT pin voltage to approximately the SS/DISB pin

voltage during both soft-start, soft-stop, and disable conditions.

SYNC (Oscillator Synchronization)

This pin is bidirectional (refer to Figure 3). When used as an output, SYNC is used as a clock, which is the same

as the internal clock of the device. When used as an input, SYNC overrides the internal oscillator of the chip and

acts as the clock signal. This bidirectional feature allows synchronization of multiple power supplies. Also, the

SYNC signal internally discharge the C

capacitor and any filter capacitors that are present on the RAMP pin.

The internal SYNC circuitry is level sensitive, with an input-low threshold of 1.9 V, and an input-high threshold of

2.1 V. A resistor as small as 3.9 kΩ may be tied between SYNC and GND to reduce the sync pulse width.

VDD (Chip Supply)

This is the input pin to the chip. VDD must be bypassed with a minimum of 1-μF low ESR, low ESL capacitor to

ground. The addition of a 10-μF low ESR, low ESL between VDD and PGND is recommended.

Product Folder Links: UCC1895 UCC2895 UCC3895