RT9618GS datasheet(10/12 Pages) RICHTEK

10 / 12 page

RT9618/A

DS9618/A-04 March 2007

www.richtek.com

The operating junction temperature can be calculated from

the power dissipation curves (Figure 4). Assume

VCC =12V, operating frequency is 200kHz and the

CU=CL=1nF which emulate the input capacitances of the

high side and low side power MOSFETs. From Figure 4,

the power dissipation is 100mW. For RT9618/A, the

package thermal resistance

θJA is 160°C/W, the operating

junction temperature is calculated as :

100

200

300

400

500

600

700

800

900

1000

200

400

600

800

1000

Frequency (kHz)

CU=CL=3nF

CU=CL=1nF

CU=CL=2nF

Power Dissipation vs. Frequency

VCC

VIN

PGND

BOOT

UGATE

PHASE

LGATE

RT9618/A

1uF

3nF

2N7002

+12V

1uF

PWM

1N4148

BOOT

Please note that the VCC trigger point to RT9618/A is at

3V, and the clamped level on PHASE pin is at about 2.4V.

Obviously since the PHASE pin voltage increases during

initial start-up, the VCORE increases correspondingly, but

it would quickly drop-off followed by LGATE and VCC

decreased.

Layout Consideration

Figure 6 shows the schematic circuit of a two-phase

synchronous buck converter to implement the RT9618/A.

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

Figure 5. Waveforms at High Side MOSFET Shorted

VCC

PHASE

LGATE

VCORE

(2V/Div)

Time (50ms/Div)

Figure 3. Test Circuit

This dissipation is a function of switching frequency and

total gate charge of the selected MOSFET. Figure 3 shows

the power dissipation test circuit. CL and CU are the UGATE

and LGATE load capacitors, respectively. The bootstrap

capacitor value is 0.01

μF.

Figure 4 shows the power dissipation of the RT9618/A as

a function of frequency and load capacitance. The value of

the CU and CL are the same and the frequency is varied

from 100kHz to 1MHz.

Figure 4. Power Dissipation vs. Frequency

TJ = (160

°C/W x 100mW) + 25°C = 41°C

(11)

where the ambient temperature is 25

°C.

The method to improve the thermal transfer is to increase

the PCB copper area around the RT9618/A first. Then,

adding a ground pad under IC to transfer the heat to the

peripheral of the board.

Over-Voltage Protection Function at Power-On

An unique feature of the RT9618/A driver is the addition of

over-voltage protection in the event of upper MOSFET direct

shorted before power-on. The RT9618/A detects the fault

condition during initial start-up, the internal power-on OVP

sense circuitry will rapidly drive the output lower MOSFET

on before the multi-phase PWM controller takes control.

Figure 5 shows the measured waveforms with the high

side MOSFET directly shorted to 12V.

RT9618GS Datasheet(PDF) 10 Page - Richtek Technology Corporation

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