6A, 2MHz Step-Down Regulator

with Integrated Switches

______________________________________________________________________________________

11

Shutdown Mode

Drive EN to GND to shut down the IC and reduce quies-

cent current to less than 12µA. During shutdown, the LX

is high impedance. Drive EN high to enable the

MAX8646.

Thermal Protection

Thermal-overload protection limits total power dissipation

= +165°C, a thermal sensor forces the device into shut-

down, allowing the die to cool. The thermal sensor turns

the device on again after the junction temperature cools

by 20°C, causing a pulsed output during continuous

overload conditions. The soft-start sequence begins after

recovery from a thermal-shutdown condition.

Applications Information

To decrease the noise effects due to the high switching

frequency and maximize the output accuracy of

to GND. Place these capacitors as close to the IC

as possible.

Inductor Selection

Choose an inductor with the following equation:

where LIR is the ratio of the inductor ripple current to full

load current at the minimum duty cycle. Choose LIR

between 20% to 40% for best performance and stability.

Use an inductor with the lowest possible DC resistance

that fits in the allotted dimensions. Powdered iron ferrite

core types are often the best choice for performance.

With any core material, the core must be large enough

not to saturate at the current limit of the MAX8646.

Output-Capacitor Selection

The key selection parameters for the output capacitor are

capacitance, ESR, ESL, and voltage-rating requirements.

These affect the overall stability, output ripple voltage,

and transient response of the DC-DC converter. The out-

put ripple occurs due to variations in the charge stored

in the output capacitor, the voltage drop due to the

capacitor’s ESR, and the voltage drop due to the

capacitor’s ESL. Calculate the output voltage ripple

due to the output capacitance, ESR, and ESL:

where the output ripple due to output capacitance,

ESR, and ESL is:

or:

or whichever is larger.

Use these equations for initial capacitor selection.

Determine final values by testing a prototype or an

evaluation circuit. A smaller ripple current results in less

output-voltage ripple. Since the inductor ripple current

is a factor of the inductor value, the output voltage rip-

ple decreases with larger inductance. Use ceramic

capacitors for low ESR and low ESL at the switching

frequency of the converter. The ripple voltage due to

ESL is negligible when using ceramic capacitors.

Load-transient response depends on the selected out-

put capacitance. During a load transient, the output

troller can respond, the output deviates further,

depending on the inductor and output capacitor val-

ues. After a short time, the controller responds by regu-

lating the output voltage back to its predetermined

I

VV

fL

x

V

V

PP

IN

OUT

S

OUT

IN

−

×

V

I

t

x ESL

RIPPLE ESL

PP

OFF

−

V

I

t

x ESL

RIPPLE ESL

PP

ON

−

V

I

x ESR

RIPPLE ESR

P P

−

V

I

xC

xf

RIPPLE C

PP

OUT

S

−

8

VV

VV

RIPPLE

RIPPLE C

RIPPLE ESR

RIPPLE ESL

=+

+

()

()

(

)

L

VV

V

f

V

LIR I

OUT

IN

OUT

S

IN

OUT MAX

=

×

−

××

×

()

()

CTL1

CTL2

GND

GND

0.6

0.7

GND

Unconnected

0.8

GND

1.0

Unconnected

GND

1.2

Unconnected

Unconnected

1.5

Unconnected

1.8

GND

2.0

Unconnected

2.5

Table 1. CTL1 and CTL2 Output Voltage

Selection