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LM27222MX Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM27222MX Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 11 page  Application Information GENERAL The LM27222 is designed for high speed and high operating reliability. The driver can handle very narrow, down to zero, PWM pulses in a guaranteed, deterministic way. Therefore, the HG and LG outputs are always in predictable states. No latches are used in the HG and LG control logic so the drivers cannot get "stuck" in the wrong state. The driver design allows for powering up with a pre-biasing voltage being present at the regulator output. To reduce conduction losses in DC-DC converters with low duty factors the LM27222 driver can be powered from a 6.5V ±5% power rail. It is recommended to use the same power rail for both the controller and driver. If two different power rails are used, never allow the PWM pulse magnitude at the IN input or the control voltage at the LEN input to be above the driver V CC voltage or unpredictable HG and LG outputs pulse widths may result. MINIMUM PULSE WIDTH As the input pulse width to the IN pin is decreased, the pulse width of the high-side gate drive (HG-SW) also decreases. However, for input pulse widths 60ns and smaller, the HG-SW remains constant at 30ns. Thus the minimum pulse width of the driver output is 30ns. Figure 5 shows an input pulse at the IN pin 20ns wide, and the output of the driver, as measured between the nodes HG and SW is a 30ns wide pulse. Figure 6 shows the variation of the SW node pulse width vs IN pulse width. At the IN pin, if a falling edge is followed by a rising edge within 5ns, the HG may ignore the rising edge and remain low until the IN pin toggles again. If a rising edge is followed by a falling edge within 5ns, the pulse may be completely ignored. ADAPTIVE SHOOT-THROUGH PROTECTION The LM27222 prevents shoot-through power loss by ensur- ing that both the high- and low-side MOSFETs are not con- ducting at the same time. When the IN signal rises, LG is first pulled down. The adaptive shoot-through protection circuit waits for LG to reach 0.9V before turning on HG. Similarly, when IN goes low, HG is pulled down first, and the circuit turns LG on only after the voltage difference between the high-side gate and the switch node, i.e. HG-SW, has fallen to 0.9V. It is possible in some applications that at power-up the driver’s SW pin is above 3V in either buck or boost com- verter applications. For instance, in a buck configuration a pre-biasing voltage can be either a voltage from anothert power rail connected to the load, or a leakage voltage through the load, or it can be an output capacitor pre- charged above 3V while no significant load is present. In a boost application it can be an input voltage rail above 3V. In the case of insufficient initial CB-SW voltage (less than 2V) such as when the output rail is pre-biased, the shoot- through protection circuit holds LG low for about 170ns, beginning from the instant when IN goes high. After the 170ns delay, the status of LG is dictated by LEN and IN. Once LG goes high and SW goes low, the bootstrap capaci- tor will be charged up (assuming SW is grounded for long enough time). As a result, CB-SW will be close to 5V and the LM27222 will now fully support synchronous operation. The dead-time between the high- and low-side pulses is kept as small as possible to minimize conduction through the body diode of the low-side MOSFET(s). 20117905 FIGURE 5. Min On Time 20117906 FIGURE 6. www.national.com 7 |
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