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MAX16936RATEA/V

36V, 220kHz to 2.2MHz Step-Down Converter with 28µA Quiescent Current

Maxim Integrated 

MAX16936

36V, 220kHz to 2.2MHz Step-Down Converter

with 28µA Quiescent Current

voltage ripple. So the size of the output capacitor depends

on the maximum ESR required to meet the output voltage

ripple (VRIPPLE(P-P)) specifications:

VRIPPLE (P−P ) = ESR × ILOAD (MAX ) × LIR

The actual capacitance value required relates to the

physical size needed to achieve low ESR, as well as

to the chemistry of the capacitor technology. Thus, the

capacitor is usually selected by ESR and voltage rating

rather than by capacitance value.

When using low-capacity filter capacitors, such as ceramic

capacitors, size is usually determined by the capacity

needed to prevent voltage droop and voltage rise from

causing problems during load transients. Generally,

once enough capacitance is added to meet the over-

shoot 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 rectifier

as a freewheeling diode when the device is configured

for skip mode operation. Connect this rectifier close to the

device using short leads and short PCB traces. In FPWM

mode, the Schottky diode helps minimize efficiency loss-

es by diverting the inductor current that would otherwise

flow through the low-side MOSFET. 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 negative 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 ampli-

fier 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 controller uses a current-mode control scheme that

regulates the output voltage by forcing the required cur-

rent through the external inductor. The device uses 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

VOUT

R1

gm

R2

VREF

COMP

RC

CF

CC

Figure 4. Compensation Network

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 4). 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

frequency. 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 modulator,

output feedback divider, and an error amplifier. The power

modulator has a DC gain set by gm O 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)), neglect-

ing 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 (d= c) gm × RLOAD

where RLOAD = VOUT/ILOUT(MAX) in I and gm = 35FS.

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

capacitor, its ESR, and the load resistance introduce a

pole at the following frequency:

fpMOD =

1π

2

×

COUT

× RLOAD

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

fzMOD

=

1

2 π × ESR × COUT

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