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ISL9305IRTWBNLZ-T Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL9305IRTWBNLZ-T Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 17 page ISL9305 7 FN7605.1 August 15, 2011 Theory of Operation DCD1 and DCD2 Introduction Both the DCD1 and DCD2 converters on ISL9305 use the peak-current-mode pulse-width modulation (PWM) control scheme for fast transient response and pulse-by-pulse current limiting. Both converters are able to supply up to 800mA load current. The default output voltage ranges from 0.8V to 3.6V depending on the factory pre-set configuration and can be programmed via the I2C interface in the range of 0.825V to 3.6V at 25mV/step with a programmable slew rate. An open-drain DCDPG (DCD Power-Good) signal is also provided to monitor the DCD1 and DCD2 output voltages. Optionally, both DCD1 and DCD2 can be programmed to be actively discharged via an on-chip bleeding resistor (typical 115Ω) when the converter is disabled. Skip Mode (PFM Mode) for DCD1/DCD2 Under light load condition, the DCD1 and DCD2 can be programmed to automatically enter a pulse-skipping mode to minimize the switching loss by reducing the switching frequency. Figure 2 illustrates the skip mode operation. A zero-cross sensing circuit monitors the current flowing through the SW node for zero crossing. When it is detected to cross zero for 16 consecutive cycles, the regulator enters the skip mode. During the 16 consecutive cycles, the inductor current could be negative. The counter is reset to zero when the sensed current flowing through the SW node does not cross zero during any cycle within the 16 consecutive cycles. Once the converter enters the skip mode, the pulse modulation is controlled by an internal comparator while each pulse cycle remains synchronized to the PWM clock. The P-Channel MOSFET is turned on at the rising edge of the clock and turned off when its current reaches ~20% of the peak current limit. As the average inductor current in each cycle is higher than the average current of the load, the output voltage rises cycle-over-cycle. When the output voltage is sensed to reach 1.5% above its nominal voltage, the P-Channel MOSFET is turned off immediately and the inductor current is fully discharged to zero and stays at zero. The output voltage reduces gradually due to the load current discharging the output capacitor. When the output voltage drops to the nominal voltage, the P-Channel MOSFET will be turned on again, repeating the previous operations. The regulator resumes normal PWM mode operation when the output voltage is sensed to drop below 1.5% of its nominal voltage value as shown in Figure 3. 16 CYCLES CLOCK IL VOUT 0 VOUT_NOMINAL 20% PEAK CURRENT LIMIT 1.015*VOUT_NOMINAL FIGURE 2. SKIP MODE OPERATION WAVEFORMS vEAMP d iL vOUT vCSA FIGURE 3. PWM OPERATION WAVEFORMS |
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