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ISL6535IBZ Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL6535IBZ Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 14 page 6 FN9255.0 January 17, 2006 ISL6535 OCSET (Pin 2/15) The current limit is programmed by connecting this pin with a resistor and capacitor to the drain of the high side MOSEFT. A 200µA current source develops a voltage across the resis- tor which is then compared with the voltage developed across the high side MOSFET. A blanking period of 120ns is provided for noise immunity. SS (Pin 3/1) Connect a capacitor from this pin to ground. This capacitor, along with an internal 30 µA current source, sets the soft-start interval of the converter. COMP (Pin 4/2) and FB (Pin 5/3) COMP and FB are the available external pins of the error amplifier. The FB pin is the inverting input of the error ampli- fier and the COMP pin is the error amplifier output. These pins are used to compensate the voltage-control feedback loop of the converter. EN (Pin 6/4) This pin is a TTL compatible input. Pull this pin below 0.8V to disable the converter. In shutdown the soft-start pin is dis- charged and the UGATE and LGATE pins are held low. GND (Pin 7/6) Signal ground for the IC. All voltage levels are measured with respect to this pin. PHASE (Pin 8/7) This pin connects to the source of the high side MOSFET and the drain of the low side MOSFET. This pin represents the return path for the high side gate driver. During normal switching, this pin is used for high side current sensing. UGATE (Pin 9/8) Connect UGATE to the upper MOSFET gate. This pin pro- vides the gate drive for the upper MOSFET. BOOT (Pin 10/9) This pin provides bias to the upper MOSFET driver. A boot- strap circuit may be used to create a BOOT voltage suitable to drive a standard N-Channel MOSFET. PGND (Pin 11/10) This is the power ground connection. Tie the lower MOSFET source and board ground to this pin. LGATE (Pin 12/11) Connect LGATE to the lower MOSFET gate. This pin pro- vides the gate drive for the lower MOSFET. PVCC (Pin 13/12) Provide a 12V ±10% bias supply for the lower gate drive to this pin. This pin should be bypassed with a capacitor to PGND. VCC (Pin 14/13) Provide a 12V bias supply for the chip to this pin. The pin should be bypassed with a capacitor to GND. REFIN (QFN ONLY Pin 5) Upon enable if REFIN is less than 2.2V, the external refer- ence pin is used as the control reference instead of the inter- nal 0.597V reference. An internal 6µA pull up to 5V is provided for disabling this functionality. SSDONE (QFN ONLY Pin 16) Provides an open drain signal at the end of soft-start. Functional Description Initialization The ISL6535 automatically initializes upon receipt of power. Special sequencing of the input supplies is not necessary. The Power-On Reset (POR) function continually monitors the bias voltage at the VCC pin and the driver input on the PVCC pin. When the voltages at VCC and PVCC exceed their rising POR thresholds, a 30 µA current source driving the SS pin is enabled. Upon the SS pin exceeding 1V, the ISL6535 begins ramping the non-inverting input of the error amplifier from GND to the System Reference. During initialization the MOSFET drivers pull UGATE to PHASE and LGATE to PGND. Soft-start During soft-start, an internal 30 µA current source charges the external capacitor (CSS) on the SS pin up to ~4V. If the ISL6535 is utilizing the internal reference, then as the SS pin’s voltage ramps from 1V to 3V, the soft-start function scales the reference input (positive terminal of error amp) from GND to VREF (0.597V nominal). If the ISL6535 is utilizing an externally supplied reference, when the voltage on the SS pin reaches 1V, the internal reference input (into of the error amp) ramps from GND to the externally supplied reference at the same rate as the voltage on the SS pin. Figure 3 shows a FIGURE 3. TYPICAL SOFT-START INTERVAL VEN VOUT VSS tSS |
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