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UCC3895PWG4 Datasheet(PDF) 11 Page - Texas Instruments |
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UCC3895PWG4 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 25 page UCC1895 UCC2895 UCC3895 SLUS157L − DECEMBER 1999 − REVISED APRIL 2008 11 www.ti.com 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 × IRT). 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 IRT. The only time the part switches into low IDD 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 IRT. 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. 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 CT 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 k Ω 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. |
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