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RT9618A Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT9618A Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 12 page  RT9618/A 9 DS9618/A-06 April 2011 www.richtek.com the total current required from the gate driving source is 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 RT9618/A. The VCB(the voltage difference between BOOT and PHASE on RT9618/A) 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 CB has to be selected properly. It is determined by following constraints. Before driving the gate of the high side MOSFET up to 12V (or 5V), the low side MOSFET has to be off; and the high side MOSFET is turned off before the low side is turned on. From Figure 1, the body diode "D2" had been turned on before high side MOSFETs turned 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 It is helpful to calculate these currents in a typical case. Assume a synchronous rectified buck converter, input voltage VIN = 12V, Vg1 = Vg2 = 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) gd1 gd1 gd1 r1 dV 12V IC C dt t == (3) (4) r2 gd2 gd2 gd2 t 12V Vi C dt dV C I + = = (A) 0.88 10 30 12 10 2200 I (A) 1.428 10 14 12 10 1660 I 9 - 12 - 9 - -12 gs2 gs1 = × × × = = × × × = (5) (6) (7) (8) (9) (10) r2 gs1 gs1 gs2 r1 gs1 gs1 gs1 t 12 C dt dVg2 C I t 12 C dt dVg1 C I × = = × = = (1) (2) the low side power MOSFETs, respectively and referred to the data sheets as "Crss" 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 showed below : (A) 0.4 10 30 12) (12 10 500 I (A) 0.326 10 14 12 10 380 I 9 - 12 - 9 - -12 gd2 gd1 = × + × × = = × × × = (A) 1.28 0.4) (0.88 I I I (A) 1.754 0.326) (1.428 I I I gd2 gs2 g2 gd1 gs1 g1 = + = + = = + = + = V IN C B V CB + - BOOT V CC V CC UGATE PHASE LGATE PGND 1N4148 Figure 2. Part of Bootstrap Circuit of RT9618/A In practice, a low value capacitor CB will lead the over- charging that could damage the IC. Therefore to minimize the risk of overcharging and reducing the ripple on VCB, the bootstrap capacitor should not be smaller than 0.1 μF, and the larger the better. In general design, using 1uF can provide better performance. At least one low-ESR capacitor should be used to provide good local de-coupling. Here, to adopt either a ceramic or tantalum capacitor is suitable. Power Dissipation For not exceeding the maximum allowable power dissipation to drive the IC beyond the maximum recommended operating junction temperature of 125 °C, it is necessary to calculate power dissipation appro-priately. |
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