RT9212PC datasheet(10/14 Pages) RICHTEK

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RT9212

Preliminary

DS9212-05 March 2007

www.richtek.com

Applications Information

Inductor

The inductor is required to supply constant current to the

output load. The inductor is selected to meet the output

voltage ripple requirements and minimize the converter's

response time to the load transient.

A larger value of inductance reduces ripple current and

voltage. However, the larger value of inductance has a larger

physical size, lower output capacitor and slower transient

response time.

A good rule for determining the inductance is to allow the

peak-to-peak ripple current in the inductor to be

approximately 30% of the maximum output current. The

inductance value can be calculated by the following

equation :

Where

VIN is the input voltage,

VOUT is the output voltage,

FS is the switching frequency,

IOUT is the peak-to-peak inductor ripple current.

The inductance value determines the converter's ripple

current and the ripple voltage. The ripple current is

calculated by the following equations :

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values raise

the converter's response time to a load transient.

One of the parameters limiting the converter's response to

a load transient is the time required to change the inductor

current. Given a sufficiently fast control loop design, the

RT9212 will provide 0% to 100% duty cycle in response to

a load transient.

The response time is the time required to slew the inductor

current from an initial current value to the transient current

level. The inductor limit input current slew rate during

the load transient. Minimizing the transient response time

can minimize the output capacitance required. The response

time is different for application of load and removal of load

to a transient. The following equations give the approximate

response time for application and removal of a transient

load :

Where

TRise is the response time to the application of load,

TFall is the response time to the removal of load,

IOUT is the transient load current step.

Input Capacitor

The input capacitor is required to supply the AC current to

the Buck converter while maintaining the DC input voltage.

The capacitor should be chosen to provide acceptable ripple

on the input supply lines. Use a mix of input bypass

capacitors to control the voltage overshoot across the

MOSFETs. Use small ceramic capacitors for high frequency

decoupling and bulk capacitors to supply the current. Place

the small ceramic capacitors close to the MOSFETs and

between the drain of Q1/Q3 and the source of Q2/Q4.

The key specifications for input capacitor are the voltage

rating and the RMS current rating. For reliable operation,

select the bulk capacitor with voltage and current ratings

above the maximum input voltage and largest RMS current.

The capacitor voltage rating should be at least 1.25 times

greater than the maximum input voltage and voltage rating

of 1.5 times is a conservative guideline. The RMS current

rating for the input capacitor of a buck regulator should be

greater than approximately 0.5 the DC load current.

−×

××

OUT

) V

ΔI

VFs L

OUT

Rise

ΔI

−

OUT

Fall

ΔI

−×

××

OUT

) V

ΔI

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