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RT9238

VRM8.5 PWM and Triple Linear Power System Controller

Richtek Technology 

Preliminary

RT9238

response time to a load transient. The inductor value

determines the converter’s ripple current and the

ripple voltage is a function of the ripple current. The

ripple voltage and current are approximated by the

following equations:

∆I =

VIN − VOUT

FS × L

×

VOUT

VIN

∆VOUT = ∆I × ESR

Increasing the value of inductance reduces the ripple

current and voltage. However, large inductance

values increase 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 RT9238 will provide either

0% or 100% duty cycle in response to a load

transient. The response time is the time interval

required to slew the inductor current from an initial

current value to the post-transient current level.

During this interval the difference between the

inductor current and the transient current level must

be supplied by the output capacitor(s). Minimizing the

response time can minimize the output capacitance

required.

The response time to a transient is different for the

application of load and the removal of load. The

following equations give the approximate response

time interval for application and removal of a transient

load:

tRISE

=

LO × ITRAN

VIN − VOUT

tFALL

=

LO × ITRAN

VOUT

where: ITRAN is the transient load current step, tRISE

is the response time to the application of load, and

tFALL is the response time to the removal of load. Be

sure to check both of these equations at the minimum

and maximum output levels for the worst case

response time.

Input Capacitor Selection

The important parameters for the bulk input capacitor

are the voltage rating and the RMS current rating. For

reliable operation, select bulk input capacitors with

voltage and current ratings above the maximum input

voltage and largest RMS current required by the

circuit. The capacitor voltage rating should be at least

1.25 times greater than the maximum input voltage.

The maximum RMS current rating requirement for the

input capacitors of a buck regulator is approximately

1/2 of the DC output load current. Worst-case RMS

current draw in a circuit employing the RT9238

amounts to the largest RMS current draw of the

switching regulator.

Use a mix of input bypass capacitors to control the

voltage overshoot across the MOSFETs. Use

ceramic capacitance for the high frequency

decoupling and bulk capacitors to supply the RMS

current. Small ceramic capacitors can be placed very

close to the upper MOSFET to suppress the voltage

induced in the parasitic circuit impedances.

For a through-hole design, several electrolytic

capacitors (Panasonic HFQ series or Nichicon PL

series or Sanyo MV-GX or equivalent) may be

needed. For surface mount designs, solid tantalum

capacitors can be used, but caution must be

exercised with regard to the capacitor surge current

rating. These capacitors must be capable of handling

the surge current at power-up. The TPS series

available from AVX, and the 593D series from

Sprague are both surge current tested.

MOSFET Selection/Considerations

The RT9238 requires 5 external transistors. Two N-

channel MOSFETs are employed by the PWM

converter. The GTL, AGP, and memory linear

controllers can each drive a MOSFET or a NPN

bipolar as a pass transistor. All these transistors

should be selected based upon RDS(ON) , current

gain, saturation voltages, gate supply requirements,

and thermal management considerations.

DS9238-01 July 2001

www.richtek-ic.com.tw

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