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L6398DTR Fiches technique(PDF) 11 Page - STMicroelectronics |
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L6398DTR Fiches technique(HTML) 11 Page - STMicroelectronics |
11 / 16 page L6398 Bootstrap driver Doc ID 18199 Rev 3 11/16 8 Bootstrap driver A bootstrap circuitry is needed to supply the high voltage section. This function is normally accomplished by a high voltage fast recovery diode (Figure 6). In the L6398 a patented integrated structure replaces the external diode. It is realized by a high voltage DMOS, driven synchronously with the low side driver (LVG), with diode in series, as shown in Figure 7. An internal charge pump (Figure 7) provides the DMOS driving voltage. 8.1 CBOOT selection and charging To choose the proper CBOOT value the external MOS can be seen as an equivalent capacitor. This capacitor CEXT is related to the MOS total gate charge: Equation 1 The ratio between the capacitors CEXT and CBOOT is proportional to the cyclical voltage loss. It has to be: Equation 2 CBOOT >>> CEXT e.g.: if Qgate is 30 nC and Vgate is 10 V, CEXT is 3 nF. With CBOOT = 100 nF the drop would be 300 mV. If HVG has to be supplied for a long time, the CBOOT selection has to take into account also the leakage and quiescent losses. e.g.: HVG steady state consumption is lower than 190 μA, so if HVG T ON is 5 ms, CBOOT has to supply 1 μC to C EXT. This charge on a 1 μF capacitor means a voltage drop of 1V. The internal bootstrap driver gives a great advantage: the external fast recovery diode can be avoided (it usually has great leakage current). This structure can work only if VOUT is close to GND (or lower) and in the meanwhile the LVG is on. The charging time (Tcharge) of the CBOOT is the time in which both conditions are fulfilled and it has to be long enough to charge the capacitor. The bootstrap driver introduces a voltage drop due to the DMOS RDSon (typical value: 120 Ω). At low frequency this drop can be neglected. Anyway increasing the frequency it must be taken in to account. The following equation is useful to compute the drop on the bootstrap DMOS: Equation 3 C EXT Q gate V gate -------------- = V drop I ch e arg Rdson V drop → Q gate T ch e arg ------------------R dson == |
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