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ISL6207HBZ Datasheet(PDF) 6 Page - Renesas Technology Corp |
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ISL6207HBZ Datasheet(HTML) 6 Page - Renesas Technology Corp |
6 / 10 page ISL6207 FN9075 Rev 8.00 Page 6 of 10 December 2, 2005 A falling transition on PWM indicates the turn-off of the upper MOSFET and the turn-on of the lower MOSFET. A short propagation delay [tPDLUGATE] is encountered before the upper gate begins to fall [tFUGATE]. Again, the adaptive shoot- through circuitry determines the lower gate delay time tPDHLGATE. The upper MOSFET gate-to-source voltage is monitored, and the lower gate is allowed to rise, after the upper MOSFET gate-to-source voltage drops below 1V. The lower gate then rises [tRLGATE], turning on the lower MOSFET. This driver is optimized for converters with large step down ratio, such as those used in a mobile-computer core voltage regulator. The lower MOSFET is usually sized much larger. This driver is optimized for converters with large step down compared to the upper MOSFET because the lower MOSFET conducts for a much longer time in a switching period. The lower gate driver is therefore sized much larger to meet this application requirement. The 0.4 on-resistance and 4A sink current capability enable the lower gate driver to absorb the current injected to the lower gate through the drain-to-gate capacitor of the lower MOSFET and prevent a shoot through caused by the high dv/dt of the phase node. Three-State PWM Input A unique feature of the ISL6207 and other Intersil drivers is the addition of a shutdown window to the PWM input. If the PWM signal enters and remains within the shutdown window for a set holdoff time, the output drivers are disabled and both MOSFET gates are pulled and held low. The shutdown state is removed when the PWM signal moves outside the shutdown window. Otherwise, the PWM rising and falling thresholds outlined in the ELECTRICAL SPECIFICATIONS determine when the lower and upper gates are enabled. During start-up, PWM should be in the three-state position (1/2 VCC). However, with rising VCC, the active tracking elements for PWM are not active until VCC > 1.2V, which leaves PWM in a high impedance (undetermined) state; therefore, a 500k resistor must be place from the PWM pin to GND. Adaptive Shoot-Through Protection Both drivers incorporate adaptive shoot-through protection to prevent upper and lower MOSFETs from conducting simultaneously and shorting the input supply. This is accomplished by ensuring the falling gate has turned off one MOSFET before the other is allowed to turn on. During turn-off of the lower MOSFET, the LGATE voltage is monitored until it reaches a 1V threshold, at which time the UGATE is released to rise. Adaptive shoot-through circuitry monitors the upper MOSFET gate-to-source voltage during UGATE turn-off. Once the upper MOSFET gate-to-source voltage has dropped below a threshold of 1V, the LGATE is allowed to rise. Internal Bootstrap Diode This driver features an internal bootstrap Schottky diode. Simply adding an external capacitor across the BOOT and PHASE pins completes the bootstrap circuit. The bootstrap capacitor must have a maximum voltage rating above the maximum battery voltage plus 5V. The bootstrap capacitor can be chosen from the following equation: where QGATE is the amount of gate charge required to fully charge the gate of the upper MOSFET. The VBOOT term is defined as the allowable droop in the rail of the upper drive. As an example, suppose an upper MOSFET has a gate charge, QGATE, of 25nC at 5V and also assume the droop in the drive voltage over a PWM cycle is 200mV. One will find that a bootstrap capacitance of at least 0.125 F is required. PWM UGATE LGATE tPDLLGATE tFLGATE tPDHUGATE tRUGATE tPDLUGATE tFUGATE tPDHLGATE tRLGATE 1V 1V FIGURE 1. TIMING DIAGRAM CBOOT QGATE V BOOT ------------------------ |
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