Step-Down Controllers with Prebias Startup,

Lossless Sensing, Synchronization, and OVP

______________________________________________________________________________________

23

The aluminum electrolytic capacitor is the least expen-

sive; however, it has higher ESR. To compensate for this,

use a ceramic capacitor in parallel to reduce the switch-

ing ripple and noise. For reliable and safe operation,

ensure that the capacitor’s voltage and ripple-current rat-

ings exceed the calculated values.

The response to a load transient depends on the

selected output capacitors. After a load transient, the

output voltage instantly changes by ESR x

Before the controller can respond, the output voltage

deviates further depending on the inductor and output

capacitor values. After a short period of time (see the

Typical Operating Characteristics), the controller

responds by regulating the output voltage back to its

nominal state. The controller response time depends on

its closed-loop bandwidth. With a higher bandwidth,

the response time is faster, thus preventing the output

voltage from further deviation from its regulation value.

Compensation Design

The MAX8543/MAX8544 use an internal transconduc-

tance error amplifier whose output compensates the

control loop. The external inductor, output capacitor,

compensation resistor, and compensation capacitors

determine the loop stability. The inductor and output

capacitor are chosen based on performance, size, and

cost. Additionally, the compensation resistor and capaci-

tors are selected to optimize control-loop stability. The

component values, shown in the Typical Application

Circuits (Figures 1 and 2), yield stable operation over the

given range of input-to-output voltages.

The controller uses a current-mode control scheme that

regulates the output voltage by forcing the required cur-

rent through the external inductor, so the MAX8543/

MAX8544 use the voltage drop across the DC resistance

of the inductor or the alternate series current-sense resis-

tor to measure the inductor current. Current-mode control

eliminates the double pole in the feedback loop caused

by the inductor and output capacitor resulting in a smaller

phase shift and requiring a less elaborate error-amplifier

compensation than voltage-mode control. A simple single

high-bandwidth loop in applications where ceramic

capacitors are used for output filtering. For other types of

capacitors, due to the higher capacitance and ESR, the

frequency of the zero created by the capacitance and

ESR is lower than the desired closed-loop crossover fre-

quency. To stabilize a nonceramic output-capacitor loop,

GND to cancel this ESR zero.

The basic regulator loop is modeled as a power modu-

lator, output feedback divider, and an error amplifier.

that define the power modulator:

dependent on the current-limit selection at ILIM, and

ranges from 3 to 11 (see Current-Sense Amplifier

The frequencies at which the pole and zero created by

the power modulator are determined as follows:

allel combination of like capacitors is the same as for an

individual capacitor.

The transconductance error amplifier has a DC gain,

output resistance of the error amplifier, which is 10M

Ω.

A dominant pole is set by the compensation capacitor

f

CR

R

pdEA

CO

C

=

××

+

1

2

π

()

f

C

ESR

zMOD

OUT

=

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1

2

π

f

C

Rf

L

Rf

L

ESR

pMOD

OUT

LOAD

S

LOAD

S

=

××

××

+×

+

⎛

⎝⎜

⎞

⎠⎟

1

2

π

()

Gg

Rf

L

Rf

L

MOD dc

mc

LOAD

S

LOAD

S

()

()

=×

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+×