RT9643 datasheet(14/17 Pages) RICHTEK

14 / 17 page

RT9643

DS9643-03 August 2007

www.richtek.com

Front-side bus GTL termination. The driving voltage on

the gate drive pin can be pull up to within 0.5V of VCC.

Use low Vth MOSFET to assure RDS(ON) is small enough

for full load operation. The soft start for the LDO is

accomplished by clamping the input voltage to a smooth

up-going ramp. The final input reference voltage after soft

start is 0.9V.

Component Selection

Components should be appropriately selected to ensure

stable operation, fast transient response, high efficiency,

minimum BOM cost and maximum reliability.

Output Inductor Selection

The selection of output inductor is based on the

considerations of efficiency, output power and operating

frequency. For a synchronous buck converter, the ripple

current of inductor (

ΔIL) can be calculated as follows :

Generally, an inductor that limits the ripple current between

20% and 50% of output current is appropriate. Make sure

that the output inductor could handle the maximum output

current and would not saturate over the operation

temperature range.

Output Capacitor Selection

The output capacitors determine the output ripple voltage

(

ΔVOUT) and the initial voltage drop after a high slew-rate

load transient. The selection of output capacitor depends

on the output ripple requirement. The output ripple voltage

is described as follows :

For electrolytic capacitor application, typically 90~95%

of the output voltage ripple is contributed by the ESR of

output capacitors. Paralleling lower ESR ceramic capacitor

with the bulk capacitors could dramatically reduce the

equivalent ESR and consequently the ripple voltage.

Input Capacitor Selection

Use mixed types of input bypass capacitors to control

the input voltage ripple and switching voltage spike across

the MOSFETs. The buck converter draws pulsewise

current from the input capacitor during the on time of upper

MOSFET. The RMS value of ripple current flowing through

the input capacitor is described as :

The input bulk capacitor must be cable of handling this

ripple current. Sometime, for higher efficiency the low ESR

capacitor is necessarily. Appropriate high frequency

ceramic capacitors physically near the MOSFETs

effectively reduce the switching voltage spikes.

MOSFET Selection

The selection of MOSFETs is based upon the

considerations of RDS(ON), gate driving requirements, and

thermal management requirements. The power loss of

upper MOSFET consists of conduction loss and switching

loss and is expressed as :

where TRISE and TFALL are rising and falling time of VDS of

upper MOSFET respectively. RDS(ON) and QG should be

simultaneously considered to minimize power loss of upper

MOSFET.

The power loss of lower MOSFET consists of conduction

loss, reverse recovery loss of body diode, and conduction

loss of body diode and is express as :

where TDIODE is the conducting time of lower body diode.

Special control scheme is adopted to minimize body diode

conducting time. As a result, the RDS(ON) loss dominates

the power loss of lower MOSFET. Use MOSFET with

adequate RDS(ON) to minimize power loss and satisfy

thermal requirements.

Bypass Capacitor Notes

Input capacitor C1 is typically chosen based on the ripple

current requirements. COUT is typically selected based

on both current ripple rating and ESR requirement. C17

ESR

ΔI

ΔV

OUT

OSC

OUT

−

OUT

IN(RMS)

−

OSC

FALL

RISE

OUT

DS(ON)

OUT

SW_UPPER

_UPPER

COND

UPPER

)

x V

OSC

DIODE

OUT

OSC

DS(ON)

OUT

DIODE

_LOWER

COND

LOWER

x V

−

)

ΔI

OSC

OUT

−

RT9643 Datasheet(PDF) 14 Page - Richtek Technology Corporation

RT9643 Datasheet(HTML) 14 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL