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ISL6526CB Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL6526CB Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 15 page 7 FN9055.8 March 20, 2007 ENABLE This pin is the open-collector enable pin. Pulling this pin to a level below 0.8V will disable the controller. Disabling the ISL6526, ISL6526A cause the oscillator to stop, the LGATE and UGATE outputs to be held low, and the soft-start circuitry to re-arm. CT1 and CT2 These pins are the connections for the external charge pump capacitor. A minimum of a 0.1 μF ceramic capacitor is recommended for proper operation of the IC. CPVOUT This pin represents the output of the charge pump. The voltage at this pin is the bias voltage for the IC. Connect a decoupling capacitor from this pin to ground. The value of the decoupling capacitor should be at least 10x the value of the charge pump capacitor. This pin may be tied to the bootstrap circuit as the source for creating the BOOT voltage. CPGND This pin represents the signal and power ground for the charge pump. Tie this pin to the ground island/plane through the lowest impedance connection available. Functional Description Initialization The ISL6526, ISL6526A automatically initialize upon receipt of power. Special sequencing of the input supplies is not necessary. The Power-On Reset (POR) function continually monitors the output voltage of the charge pump. During POR, the charge pump operates on a free running oscillator. Once the POR level is reached, the charge pump oscillator is synched to the PWM oscillator. The POR function also initiates the soft-start operation after the charge pump output voltage exceeds its POR threshold. Soft-Start The POR function initiates the digital soft-start sequence. The PWM error amplifier reference is clamped to a level proportional to the soft-start voltage. As the soft-start voltage slews up, the PWM comparator generates PHASE pulses of increasing width that charge the output capacitor(s). This method provides a rapid and controlled output voltage rise. The soft-start sequence typically takes about 6.5ms. Figure 1 shows the soft-start sequence for a typical application. At t0, the +3.3V VCC voltage starts to ramp. At time t1, the Charge Pump begins operation and the +5V CPVOUT IC bias voltage starts to ramp up. Once the voltage on CPVOUT crosses the POR threshold at time t2, the output begins the soft-start sequence. The triangle waveform from the PWM oscillator is compared to the rising error amplifier output voltage. As the error amplifier voltage increases, the pulse- width on the UGATE pin increases to reach the steady-state duty cycle at time t3. Shoot-Through Protection A shoot-through condition occurs when both the upper MOSFET and lower MOSFET are turned on simultaneously, effectively shorting the input voltage to ground. To protect the regulator from a shoot-through condition, the ISL6526, ISL6526A incorporate specialized circuitry which insures that the complementary MOSFETs are not ON simultaneously. The adaptive shoot-through protection utilized by the ISL6526, ISL6526A look at the lower gate drive pin, LGATE, and the upper gate drive pin, UGATE, to determine whether a MOSFET is ON or OFF. If the voltage from UGATE or from LGATE to GND is less than 0.8V, then the respective MOSFET is defined as being OFF and the complementary MOSFET is turned ON. This method of shoot-through protection allows the regulator to sink or source current. Since the voltage of the lower MOSFET gate and the upper MOSFET gate are being measured to determine the state of the MOSFET, the designer is encouraged to consider the repercussions of introducing external components between the gate drivers and their respective MOSFET gates before actually implementing such measures. Doing so may interfere with the shoot-through protection. Output Voltage Selection The output voltage can be programmed to any level between VIN and the internal reference, 0.8V. An external resistor divider is used to scale the output voltage relative to the reference voltage and feed it back to the inverting input of the error amplifier, see Figure 2. However, since the value of R1 affects the values of the rest of the compensation components, it is advisable to keep its value less than 5k Ω. R4 can be calculated based on the following equation: FIGURE 1. SOFT-START INTERVAL 0V TIME T2 T3 T0 CPVOUT (5V) VCC (3.3V) VOUT (2.50V) (1V/DIV) T1 R4 R1 0.8V × V OUT1 0.8V – -------------------------------------- = (EQ. 2) ISL6526, ISL6526A |
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