SC2434SWTR datasheet(10/19 Pages) SEMTECH

10 / 19 page

 2005 Semtech Corp.

www.semtech.com

PRELIMINARY

POWER MANAGEMENT

SC2434

Applications Information (Cont.)

The compensator transfer function has two poles and one

zero:

H c s

()

R drp

R FB

To optimize the transient responses, it is recommended

that:

To use the first compensator pole to cancel the power

stage ESR zero;

To place the compensator zero at one half of the

switching frequency;

And to place the second compensator pole at high

frequency.

The Bode plots based this model and those obtained from

experiment are depicted in Fig. 5 and Fig. 6, respectively.

It can be seen that the model agrees well with the

experiment. The control model provides us physical

insight of the loop dynamics and helps the designer to

achieve good transient responses and system stability. Here

are few comments:

The loop crossover frequency (0dB frequency) should

be lower than one fifth (20%) of the switch frequency

to avoid noise pick up and the phase lag introduced

by the complex pole located at one half of the switching

frequency;

A >20KHz crossover frequency is adequate to assure

good transient response when the VR output

impedance, or droop impedance, is programmed to

be equal to the output capacitor ESR. The ESR

frequency for the output bulk capacitor is usually less

than 20KHz, and beyond that frequency the capacitor

behaves like a resistor up to few hundred KHz, which

is desired for dynamic droop. There is no point to

demand the control loop to have much higher

crossover frequency beyond the ESR zero frequency.

Fig. 5 - Loop gain Bode plot based on control loop model.

Fig. 6 - Measured loop gain Bode plots.

100

1 .10

Loop-Gain (dB)

mag_Loop

i R

()

100

1 .10

1 . 10

1 .10

180

LoopGain (Degree)

phase_Loop

i R

()

100

1 .10

Loop-Gain (dB)

mag_Loop

i R

()

100

1 .10

1 . 10

1 .10

100

1 .10

Loop-Gain (dB)

mag_Loop

i R

()

100

1 .10

1 . 10

1 .10

180

LoopGain (Degree)

phase_Loop

i R

()

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