RT9629B datasheet(11/14 Pages) RICHTEK

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RT9629B

DS9629B-06

July 2015

www.richtek.com

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

= C

(3)

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)

the total current required from the gate driving source can

be calculated as following equations.







gs1

gd1

gs2

gd2

1.428

0.326

1.754 (A)

(9)

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.



gs1

x 12

(1)

(2)

-12

gs1

-9

-12

gs2

-9

1660 x 10

x 12

1.428

(A)

14 x 10

2200 x 10

x 12

0.88

(A)

30 x 10



(5)

(6)



-12

gd1

-9

-12

gd2

-9

380 x 10

x 12

0.326 (A)

14 x 10

500 x 10

x 12+12

0.4 (A)

30 x 10



(7)

(8)



gs1

gs2

gs1

x 12

12V

Vg2

Vg1

VPHASEx +12V

Cgs2

Ig2 Igd2

Igs2

Cgd2

Cgs1

Cgd1

Igd1

Igs1

Ig1

VOUT

VIN

GND

VPHASEx

gd2

V12

(4)





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