RT9625B datasheet(13/14 Pages) RICHTEK

13 / 14 page

RT9625B

DS9625B-02

September 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Layout Consideration

Figure 6 shows the schematic circuit of a synchronous

buck converter to implement the RT9625B. The converter

operates from 5V to 12V of input Voltage.

For the PCB layout, 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 location

of QUGx, QLGx, Lx should be very close.

Next, the trace from UGATEx, and LGATEx should also

be short to decrease the noise of the driver output signals.

PHASEx 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 CVCC

should be connected to GND directly. Furthermore, the

bootstrap capacitors (CBOOTx) should always be placed as

close to the pins of the IC as possible.

Figure 6. Synchronous Buck Converter Circuit

Figure 5. Derating Curve of Maximum Power Dissipation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

100

125

Ambient Temperature (°C)

Four-Layer PCB

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

For recommended operating condition specifications of

the RT9625B, the maximum junction temperature is 125

°C.

The junction to ambient thermal resistance,

θJA, is layout

dependent. For WQFN-16L 4x4 packages, the thermal

resistance,

θJA, is 54°C/W on a standard JEDEC 51-7

four-layer thermal test board. The maximum power

dissipation at TA= 25

°C can be calculated by the following

formula :

PD(MAX) = (125

°C - 25°C) / (54°C/W) = 1.852W for

WQFN-16L 4x4 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance,

θJA. The derating curve in Figure 5 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

BOOTx

UGATEx

PHASEx

LGATEx

VCC

RT9625B

GND

CBOOTx

12V

LIN

COUT

VIN

VOUT

CIN2

PHB83N03LT

PHB95N03LT

QLGx

QUGx

CIN

12V

PWMx

CVCC

RVCC

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