RT9259CGS datasheet(10/15 Pages) RICHTEK

10 / 15 page

RT9259C

DS9259C-03

August 2007

www.richtek.com

1) Modulator Frequency Equations

The modulator transfer function is the small-signal transfer

function of VOUT/VCOMP (output voltage over the error

amplifier output. This transfer function is dominated by a

DC gain, a double pole, and a zero as shown in Figure 10.

The DC gain of the modulator is the input voltage (VIN)

divided by the peak to peak oscillator voltage VOSC. The

output LC filter introduces a double pole, 40dB/decade

gain slope above its corner resonant frequency, and a total

phase lag of 180 degrees. The resonant frequency of the

LC filter expressed as :

OUT

If MOSFET with RDS(ON) = 16m

Ω is used, the OCP

threshold current is about 25A. Once OCP is triggered,

the RT9259C enters hiccup mode and re-soft starts again.

The RT9259C shuts down after 4 time OCP hiccups.

Figure 7. Shorted then Start Up (Extended Figure 3)

Feedback Compensation

The RT9259C is a voltage mode controller. The control

loop is a single voltage feedback path including a

compensator and modulator as shown Figure 8. The

modulator consists of the PWM comparator and power

stage. The PWM comparator compares error amplifier EA

output (COMP) with oscillator (OSC) sawtooth wave to

provide a pulse-width modulated (PWM) with an amplitude

of VIN at the PHASE node. The PWM wave is smoothed

by the output filter LOUT and COUT. The output voltage (VOUT)

is sensed and fed to the inverting input of the error amplifier.

Figure 8. Closed Loop

Figure 6. Shorted then Start Up

Time (5

μs/Div)

UGATE

(5V/Div)

LGATE

(5V/Div)

(20A/Div)

Time (2.5ms/Div)

Inductor Current

(20A/Div)

A well-designed compensator regulates the output voltage

to the reference voltage VREF with fast transient response

and good stability.

In order to achieve fast transient response and accurate

output regulation, an adequate compensator design is

necessary. The goal of the compensation network is to

provide adequate phase margin (greater than 45 degrees)

and the highest 0dB crossing frequency. It is also

recommended to manipulate loop frequency response that

its gain crosses over 0dB at a slope of

−20dB/dec.

OSC

ΔV

OSC

Driver

REF

PWM

Comparator

COMP

REF

OUT

COMP

ESR

PHASE

OUT

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