ꢀ
ꢀ
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢂ
ꢆ
ꢇ
ꢃ
ꢃ
ꢃ
ꢄ
ꢄ
ꢄ
ꢅ
ꢅ
ꢅ
SLUS157L − DECEMBER 1999 − REVISED APRIL 2008
DETAILED PIN DESCRIPTION (continued)
Soft-Start/Disable (SS/DISB)
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
RT
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
RT
the part switches into low I
current mode, though, is when the part is in undervoltage lockout.
DD
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
RT
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.
Oscillator Synchronization (SYNC)
This pin is bidirectional (refer to Figure 1). When used as an output, SYNC can be used as a clock, which is the
same as the device’s internal clock. When used as an input, SYNC overrides the chip’s internal oscillator and
act as it’s 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.
T
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.
Chip Supply (VDD)
This is the input pin to the chip. VDD must be bypassed with a minimum of 1.0 µF low ESR, low ESL capacitor
to ground. The addition of a 10−µF low ESR, low ESL between VDD and PGND is recommended.
11
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