Low-Cost Voltage-Mode PWM

Step-Down Controllers

10

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compromise between efficiency and economy. Choose

a low-loss inductor having the lowest possible DC

resistance. Ferrite-core-type inductors are often the

best choice for performance, however; the MAX1966/

MAX1967s’ 100kHz switching rate also allows the use

of powdered-iron cores in ultra-low-cost applications

where efficiency is not critical. With any core material,

the core must be large enough not to saturate at the

Setting the Current Limit

The MAX1966/MAX1967 provide current limit by sens-

ing the voltage across the external low-side MOSFET.

The current-limit threshold voltage is nominally -305mV.

The MOSFET on-resistance required to allow a given

peak inductor current is:

or

in terms of actual output current.

A limitation of sensing current across MOSFET resis-

tance is that current-limit threshold is not accurate

This type of current limit provides a coarse level of fault

protection. It is especially suited when the input source

is already current limited or otherwise protected.

However, since current-limit tolerance may be ±45%,

this method may not be suitable in applications where

this device’s current limit is the primary safety mecha-

nism, or where accurate current limit is required.

Output Capacitor Selection

The output filter capacitor must have low enough equiv-

alent series resistance (ESR) to meet output ripple and

load transient requirements, yet have high enough ESR

to satisfy stability requirements. In addition, the capaci-

tance value must be high enough to absorb the induc-

tor energy going from a full-load to no-load condition if

such load changes are anticipated in the system.

In applications where the output is subject to large load

transients, the output capacitor’s size depends primari-

ly on how low an ESR is needed to prevent the output

from dipping too low under load transients. Ignoring the

sag due to finite capacitance:

In applications with less severe load steps, the output

capacitor’s size may then primarily depend on how low

an ESR is required to maintain acceptable output ripple:

The actual capacitance value required relates to the

physical size and technology needed to achieve low

ESR. Thus, the capacitor is usually selected by physi-

cal size, ESR, and voltage rating rather than by capaci-

tance value. With current capacitor technology, once

the ESR requirement is satisfied, the capacitance is

usually also sufficient. When using a low-capacity filter

capacitor such as ceramic or polymer types, capacitor

size is usually determined by the capacitance needed

to prevent undershoot and overshoot voltages during

load transients. The overshoot voltage is given by:

Generally, once enough capacitance is added to meet

the overshoot requirement, undershoot at the rising

load edge is no longer a problem.

Stability and Compensation

To ensure stable operation, use the following compen-

sation procedure:

1) Determine accaptable output ripple and select the

inductor and output capacitor values as outlined in

the Inductor Selection and Output Capacitor

Selection sections.

2) Check to make sure that output capacitor ESR zero

π. Otherwise, increase capaci-

tance until this condition is satisfied.

3) Select R3 value to set high-frequency error-amplifi-

er gain so that the unity-gain frequency of the loop

occurs at the output ESR zero:

Ω. Do not exceed 100kΩ.

R

V

VR

L

C

OUT

VIN

ESR

OUT

3

6

80 10

=

××

×

Ω

−

()

V

LI

VC

SOAR

PEAK

OUT

OUT

=

×

××

2

2

R

V

LIR

I

ESR

RIPPLE

LOAD MAX

≤

×

()

R

V

I

ESR

DIP

LOAD MAX

≤

()

R

mV

I

LIR

DS ON MAX

LOAD MAX

()

()

≤

×+









305

1

2

RmV I

DS ON MAX

PEAK

()

/

≤ 305

II

LIR

I

PEAK

LOAD MAX

LOAD MAX

=+ 







×

()

()

2