MAX8643 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX8643

3A, 2MHz Step-Down Regulator with Integrated Switches

Maxim Integrated 

3A, 2MHz Step-Down Regulator

with Integrated Switches

The logic states of CTL1 and CTL2 should be pro-

grammed only before power-up. Once the part is

enabled, CTL1 and CTL2 should not be changed. If the

output voltage needs to be reprogrammed, cycle

power or EN and reprogram before enabling.

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

MAX8643.

Thermal Protection

Thermal-overload protection limits total power dissipation

in the device. When the junction temperature exceeds TJ

= +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

IN and VDD Decoupling

To decrease the noise effects due to the high switching

frequency and maximize the output accuracy of

the MAX8643, decouple VIN with a 22µF capacitor from

VIN to PGND. Also decouple VDD with a 1µF from VDD

to GND. Place these capacitors as close to the IC

as possible.

Inductor Selection

Choose an inductor with the following equation:

L = VOUT × (VIN − VOUT)

fS × VIN × LIR × IOUT(MAX)

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 MAX8643.

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:

VRIPPLE = VRIPPLE(C) +

VRIPPLE(ESR) + VRIPPLE(ESL)

where the output ripple due to output capacitance,

ESR, and ESL is:

VRIPPLE(C) =

IP−P

8 x COUT x fS

VRIPPLE(ESR) = IP−P x ESR

VRIPPLE(ESL) = IP−P x ESL

tON

VRIPPLE(ESL) = IP−P x ESL

tOFF

or whichever is larger.

The peak inductor current (IP-P) is:

IP−P

=

VIN − VOUT

fS × L

x

VOUT

VIN

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

instantly changes by ESR x ΔILOAD. Before the con-

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

value. The controller response time depends on the

closed-loop bandwidth. A higher bandwidth yields a

faster response time, preventing the output from deviat-

ing further from its regulating value. See the Compen-

sation Design section for more details.

______________________________________________________________________________________ 11

Share Link: