1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

______________________________________________________________________________________

13

the above equation is used; otherwise, the peak current

is calculated by:

Choose an inductor with a saturation current at least as

high as the peak inductor current. The inductor you

select should exhibit low losses at your chosen operat-

ing frequency.

Capacitor Selection

The input filter capacitor reduces peak currents and

noise at the voltage source. Use a low-ESR and low-

ESL capacitor located no further than 5mm from IN.

Select the input capacitor according to the RMS input

ripple-current requirements and voltage rating:

The output filter capacitor affects the output voltage rip-

ple, output load-transient response, and feedback loop

stability. For stable operation, the MAX1742/MAX1842

≥

The minimum ESR of the output capacitor should be:

Stable operation requires the correct output filter capaci-

tor. When choosing the output capacitor, ensure that:

Integrator Amplifier

An internal transconductance amplifier fine tunes the

A large capacitor value maintains a constant average

output voltage but slows the loop response to changes

in output voltage. A small capacitor value speeds up

the loop response to changes in output voltage but

decreases stability. Choose the capacitor values that

result in optimal performance.

Soft-Start

Soft-start allows a gradual increase of the internal cur-

rent limit to reduce input surge currents at startup and

from SS to GND sets the rate at which the internal cur-

rent limit is changed. Upon power-up, when the device

comes out of undervoltage lockout (2.6V typ) or after

the SHDN pin is pulled high, a 4µA constant-current

source charges the soft-start capacitor and the voltage

on SS increases. When the voltage on SS is less than

approximately 0.7V, the current limit is set to zero. As

the voltage increases from 0.7V to approximately 1.8V,

the current limit is adjusted from 0 to the current-limit

threshold (see the Electrical Characteristics).The volt-

age across the soft-start capacitor changes with time

according to the equation:

The soft-start current limit varies with the voltage on the

soft-start pin, SS, according to the equation:

Characteristics.

SSI

VV

V

I

LIMIT

SS

LIMIT

=

−

×

.

.

07

11

V

At

C

SS

SS

=

×

4

µ

C

t

V

FV

s for the MAX

OUT

OFF

OUT

≥µ

µ

/

79

1842

C

t

V

FV

s for the MAX

OUT

OFF

OUT

≥µ

µ

/

33

1742

ESR

L

%

>×

1

II

VV

V

V

RIPPLE

LOAD

OUT

IN

OUT

IN

=

−

()

II

Vt

L

PEAK

OUT

OUT

OFF

=+

×

×

2

0.7V

1.8V

t

SHDN

0

0

0

Figure 5. Soft-Start Current Limit over Time