MAX20002CATPC datasheet(15/19 Pages) MAXIM

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When using low-capacity filter capacitors, such as ceramic

capacitors, size is usually determined by the capacity need-

ed to prevent voltage droop and voltage rise from causing

problems during load transients. Generally, once enough

capacitance is added to meet the overshoot requirement,

undershoot at the rising load edge is no longer a problem.

However, low-capacity filter capacitors typically have high-

ESR zeros that can affect the overall stability.

Compensation Network

The devices use an internal transconductance error

amplifier with its inverting input and its output available to

the user for external frequency compensation. The output

capacitor and compensation network determine the loop

stability. The inductor and the output capacitor are chosen

based on performance, size, and cost. Additionally, the

compensation network optimizes the control-loop stability.

The converter uses a current-mode control scheme

that regulates the output voltage by forcing the required

current through the external inductor. The devices use

the voltage drop across the high-side MOSFET to sense

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 less elaborate error-amplifier compensation

than voltage-mode control. Only a simple single series

resistor (RC) and capacitor (CC) are required to have a

stable, high-bandwidth loop in applications where ceramic

capacitors are used for output filtering (see Figure 3). For

other types of capacitors, due to the higher capacitance

and ESR, the frequency of the zero created by the capaci-

tance and ESR is lower than the desired closed-loop

crossover frequency. To stabilize a nonceramic output-

capacitor loop, add another compensation capacitor (CF)

from COMP to ground to cancel this ESR zero.

The basic regulator loop is modeled as a power modula-

tor, output feedback divider, and an error amplifier. The

power modulator has a DC gain set by gm × RLOAD,

with a pole and zero pair set by RLOAD, the output

capacitor (COUT), and its ESR. The following equations

help to approximate the value for the gain of the power

modulator (GAINMOD(dc)), neglecting the effect of the

ramp stabilization. Ramp stabilization is necessary when

the duty cycle is above 50% and is internally done for the

devices:

MOD(dc)

LOAD

GAIN

where RLOAD = VOUT/IOUT(MAX) in Ω and gmc = 3S.

In a current-mode step-down converter, the output capaci-

tor, its ESR, and the load resistance introduce a pole at

the following frequency:

pMOD

OUT

LOAD

π×

The output capacitor and its ESR also introduce a zero at:

zMOD

OUT

ESR C

π×

When COUT is composed of “n” identical capacitors

in parallel, the resulting COUT = n × COUT(EACH), and

ESR = ESR(EACH)/n. Note that the capacitor zero for a

parallel combination of alike capacitors is the same as

for an individual capacitor.

The feedback voltage-divider has a gain of GAINFB =

VFB/VOUT, where VFB is 1V (typ).

The transconductance error amplifier has a DC gain

of GAINEA(DC) = gm_EA × ROUT_EA, where gm_EA is

the error amplifier transconductance, which is 700µS

(typ), and ROUT_EA is the output resistance of the error

amplifier (50MΩ).

Figure 3. Compensation Network

VOUT

COMP

REF

www.maximintegrated.com

Maxim Integrated │ 15

MAX20002/MAX20003

36V, 220kHz to 2.2MHz, 2A/3A Fully

Integrated Step-Down Converters

with 15μA Operating Current

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