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L6387DTR Arkusz danych(PDF) 4 Page - STMicroelectronics |
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4 / 9 page Obsolete Product(s) - Obsolete Product(s) Obsolete Product(s) - Obsolete Product(s) For both high and low side buffers @25˚C Tamb 0 1 2 3 4 5 C (nF) 0 50 100 150 200 250 time (nsec) Tr D99IN1054 Tf Figure 1. Typical Rise and Fall Times vs. Load Capacitance 0 2468 10 12 14 16 VS(V) 10 102 103 104 Iq ( µA) D99IN1055 Figure 2. Quiescent Current vs. Supply Voltage Input Logic L6387 Input Logic is VCC (17V) compatible. An in- terlocking features is offered (see truth table be- low) to avoid undesired simultaneous turn ON of both Power Switches driven. Table 1. Input HIN 00 11 LIN 0 1 0 1 Output HVG 0 0 1 0 LVG 0 1 0 0 BOOTSTRAP DRIVER A bootstrap circuitry is needed to supply the high voltage section. This function is normally accom- plished by a high voltage fast recovery diode (fig. 3a). In the L6387 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 in series a diode, as shown in fig. 3b An internal charge pump (fig. 3b) provides the DMOS driving voltage . The diode connected in series to the DMOS has been added to avoid undesirable turn on of it. 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 : CEXT = Qgate Vgate The ratio between the capacitors CEXT and CBOOT is proportional to the cyclical voltage loss . It has to be: CBOOT>>>CEXT e.g.: if Qgate is 30nC and Vgate is 10V, CEXT is 3nF. With CBOOT = 100nF the drop would be 300mV. If HVG has to be supplied for a long time, the CBOOT selection has to take into account also the leakage losses. e.g.: HVG steady state consumption is lower than 200 µA, so if HVG TON is 5ms, CBOOT has to supply 1 µC to CEXT. This charge on a 1µF ca- pacitor means a voltage drop of 1V. The internal bootstrap driver gives great advan- tages: 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: 125 Ohm). At low frequency this drop can be ne- glected. Anyway increasing the frequency it must be taken in to account. The following equation is useful to compute the L6387 4/9 |
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