RT9603CS datasheet(7/10 Pages) RICHTEK

7 / 10 page

RT9603

Preliminary

DS9603-00

November 2003

www.richtek.com

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 "D

" 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

gd 2

gd2

V i

12V

(4)

d tt

g d1

gd1

d V

1 2V

(3)

d t

(5)

(A)

1.428

1660

gs1

−

(6)

(A)

0.88

2200

gs2

−

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

Figure 2. Two-Phase Synch. Buck Converter Circuit

from equation. (3) and (4)

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.

(7)

(A)

0.326

380

gs1

−

(9)

1.745(A)

0.326)

(1.428

gd1

gs1

(10)

1.28(A)

0.4)

(0.88

gd2

gs2

Layout Consideration

Figure 2 shows the schematic circuit of a two-phase

synchronous buck converter to implement the RT9603.

The converter operates from 5V to 12V of VIN.

When layout the PC board, it should be very careful. The

power-circuit section is the most critical one. If not

configured properly, it will generate a large amount of EMI.

The junction of Q1, Q2, L2 should be very close.

Next, the trace from DRVH, and DRVL should also be

short to decrease the noise of the driver output signals.

SW signals from the junction of the power MOSFET,

carrying the large gate drive current pulses, should be

as heavy as the gate drive trace. The bypass capacitor

C4 should be connected to PGND directly. Furthermore,

the bootstrap capacitors (CB) should always be placed

as close to the pins of the IC as possible.

Select the Bootstrap Capacitor

Figure 3 shows part of the bootstrap circuit of RT9603.

The VCB (the voltage difference between BST and SW on

RT9603) 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.

(8)

0.4(A)

12)

(12

500

-12

gd2

−

BST

DRVH

DRVL

VCC

PGND

1uF

12V

1uF

PHB83N03LT

PHB95N03LT

2uH

1uF

1000uF

1.2uH

12V

1500uF

PWM

VIN

VCORE

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