MAX16909 Datasheet, PDF(14/18 Page) Maxim Integrated Products – 36V, 220kHz to 1MHz Step-Down Converter with Low Operating Current

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MAX16909

36V, 220kHz to 1MHz Step-Down Converter

with Low Operating Current

When using low-capacity filter capacitors, such as

ceramic capacitors, size is usually determined by the

capacity needed to prevent voltage droop and volt-

age 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.

Rectifier Selection

The device requires an external Schottky diode recti-

fier as a freewheeling diode. Connect this rectifier close

to the device using short leads and short PCB traces.

Choose a rectifier with a voltage rating greater than the

maximum expected input voltage, VSUPSW. Use a low

forward-voltage-drop Schottky rectifier to limit the nega-

tive voltage at LX. Avoid higher than necessary reverse-

voltage Schottky rectifiers that have higher forward-

voltage drops.

Compensation Network

The device uses 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 capaci-

tor are chosen based on performance, size, and cost.

Additionally, the compensation network optimizes the

control-loop stability.

The controller uses a current-mode control scheme that

regulates the output voltage by forcing the required current

through the external inductor. The device uses the volt-

age drop across the high-side MOSFET to sense inductor

VOUT

VREF

COMP

Figure 3. Compensation Network

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 (Figure 3). 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,

add another compensation capacitor (CF) from COMP to

GND 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 gmc x RLOAD,

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

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

allow 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 device.

GAINMOD(DC)

gmc

RLOAD Ã fSW

RLOAD + (fSW

ÃL

Ã L)

where RLOAD = VOUT/ILOUT(MAX) in I, fSW is the switch-

ing frequency in MHz, L is the output inductance in H,

and gmc = 3S.

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

capacitor, its ESR, and the load resistance introduce a

pole at the following frequency:

fpMOD

2Ï

C OUT

ï£«

ï£¬

ï£

RLOAD Ã fSW

RLOAD + (fSW

ÃL

Ã L)

ESRï£¶ï£·

ï£¸

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

fzMOD

2Ï

Ã ESR Ã

C OUT

When COUT is composed of ânâ identical capacitors

in parallel, the resulting COUT = n x 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.

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