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RT9624C Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT9624C Datasheet(HTML) 10 Page - Richtek Technology Corporation |
10 / 13 page RT9624C 10 DS9624C-00 September 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Figure 1. Equivalent Circuit and Waveforms (VCC = 12V) In Figure 1, the current Ig1 and Ig2 are required to move the gate up to 12V. The operation consists of charging Cgd1, Cgd2 , Cgs1 and Cgs2. Cgs1 and Cgs2 are the capacitors from gate to source of the high side and the low side power MOSFETs, respectively. In general data sheets, the Cgs1 and Cgs2 are referred as “ Ciss” which are the input capacitors. Cgd1 and Cgd2 are the capacitors from gate to drain of the high side and the low side power MOSFETs, respectively and referred to the data sheets as “ C rss ” the reverse transfer capacitance. For example, tr1 and tr2 are the rising time of the high side and the low side power MOSFETs respectively, the required current Igs1 and Igs2, are shown as below : Before driving the gate of the high side MOSFET up to 12V, the low side MOSFET has to be off; and the high side MOSFET will be turned off before the low side is turned on. From Figure 1, the body diode “ D2” will be turned on before high side MOSFETs turn on. Before the low side MOSFET is turned on, the Cgd2 have been charged to VIN. Thus, as Cgd2 reverses its polarity and g2 is charged up to 12V, the required current is gd1 gd1 gd1 r1 dV 12 I = C = C (3) dt t It is helpful to calculate these currents in a typical case. Assume a synchronous rectified Buck converter, input voltage VIN = 12V, Vgs1 = 12V, Vgs2 = 12V. The high side MOSFET is PHB83N03LT whose Ciss = 1660pF, Crss = 380pF, and tr = 14ns. The low side MOSFET is PHB95N03LT whose Ciss = 2200pF, Crss = 500pF and tr = 30ns, from the equation (1) and (2) we can obtain from equation. (3) and (4) IN gd2 gd2 gd2 r2 V12 dV I C C (4) dt t + == the total current required from the gate driving source can be calculated as the following equations. ( ) () =+ = + = =+ = + = g1 gs1 gd1 g2 gs2 gd2 I I I 1.428 0.326 1.754 (A) (9) I I I 0.88 0.4 1.28 (A) (10) By a similar calculation, we can also get the sink current required from the turned off MOSFET. Select the Bootstrap Capacitor Figure 2 shows part of the bootstrap circuit of the RT9624C. The VCB (the voltage difference between BOOT and PHASE on RT9624C) provides a voltage to the gate of the high side power MOSFET. This supply needs to be ensured that the MOSFET can be driven. For this, the capacitance CBOOT has to be selected properly. It is determined by the following constraints. g1 gs1 gs1 gs1 r1 g2 gs1 gs2 gs1 r2 dV C x 12 IC dt t dV C x 12 IC dt t == == (1) (2) -12 gs1 -9 -12 gs2 -9 1660 x 10 x 12 I 1.428 (A) 14 x 10 2200 x 10 x 12 I 0.88 (A) 30 x 10 == == (5) (6) () -12 gd1 -9 -12 gd2 -9 380 x 10 x 12 I 0.326 (A) 14 x 10 500 x 10 x 12+12 I 0.4 (A) 30 x 10 == == (7) (8) 12V t t Vg2 Vg1 VPHASE +12V L d2 s2 Cgs2 g2 Ig2 Igd2 Igs2 Cgd2 Cgs1 Cgd1 Igd1 Igs1 Ig1 D2 VOUT s1 VIN d1 GND g1 VPHASE |
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