RT9618GS datasheet(9/12 Pages) RICHTEK

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RT9618/A

DS9618/A-04 March 2007

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

12V

(3)

(4)

gd2

12V

(A)

0.88

2200

(A)

1.428

1660

-12

gs2

gs1

(5)

(6)

(7)

(8)

(9)

(10)

gs1

gs2

gs1

dVg2

dVg1

(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

12)

(12

500

(A)

0.326

380

-12

gd2

gd1

(A)

1.28

0.4)

(0.88

(A)

1.754

0.326)

(1.428

gd2

gs2

gd1

gs1

BOOT

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 1

μF 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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