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MAX863EEE

Dual, High-Efficiency, PFM, Step-Up DC-DC Controller

Maxim Integrated 

Dual, High-Efficiency, PFM, Step-Up

DC-DC Controller

Select the Inductor Component

Two essential parameters are required for selecting the

inductor: inductance and current rating.

Inductance should be low enough to allow the MAX863

to reach the peak current limit during each cycle before

the 17.5µs maximum on-time. Conversely, if the induc-

tance is too low, the current will ramp up to a high level

before the current-sense comparator can turn the

switch off. A practical minimum on-time (tON(MIN)) is

1.5µs.

VIN(MAX) x tON(MIN)

LMIN ≥

IPEAK

and:

VIN(MIN) x tON(MAX)

LMAX ≥

IPEAK

When selecting IPEAK using the graphs in Figure 5,

choose inductance values between 1.3 and 1.7 times

the minimum inductance value to provide a good trade-

off between switching frequency and efficiency.

The lower of the inductor saturation current rating or

heating current rating should be greater than IPEAK:

ISATURATION and IHEATING > IPEAK

The saturation current limit is the current level where

the magnetic field in the inductor has reached the max-

imum the core can sustain, and inductance starts to

fall. The heating current rating is the maximum DC cur-

rent the inductor can sustain without overheating.

Disregarding the inductor’s saturation current rating is

a common mistake that results in poor efficiency, bad

regulation, component overheating, or other problems.

The resistance of the inductor windings should be com-

parable to or less than that of the current-sense

resistor. To minimize radiated noise in sensitive

applications, use a toroid, pot core, or shielded bobbin

core inductor.

Choose the MOSFET Power Transistor

Use N-channel MOSFETs with the MAX863. When

selecting an N-channel MOSFET, five important para-

meters are gate-drive voltage, drain-to-source break-

down voltage, current rating, on-resistance (RDS(ON)),

and total gate charge (Qg).

The MAX863’s EXT1 and EXT2 outputs swing from

GND to VDD. To ensure the external N-channel MOS-

FET is turned on sufficiently, use logic-level MOSFETs

when VDD is less than 8V and low-threshold logic-level

MOSFETs when starting from input voltages below 4V.

This also applies in bootstrapped mode to ensure

start-up.

The MOSFET in a simple boost converter must with-

stand the output voltage plus the diode forward volt-

age. Voltage ratings in SEPIC, flyback, and

autotransformer-boost circuits are more stringent.

Choose a MOSFET with a maximum continuous drain-

current rating higher than the current limit set by CS.

The two most significant losses contributing to the

MOSFET’s power dissipation are I2R losses and switch-

ing losses. Reduce I2R losses by choosing a MOSFET

with low RDS(ON), preferably near the current-sense

resistor value or lower.

A MOSFET with a gate charge (Qg) of 50nC or smaller

is recommended for rise and fall times less than 100ns

on the EXT pins. Exceeding this limit results in slower

MOSFET switching speeds and higher switching loss-

es, due to a longer transition time through the linear

region as the MOSFET turns on and off.

Select the Output Diode

Schottky diodes, such as the 1N5817–1N5822 family or

surface-mount equivalents, are recommended. Ultra-

fast silicon rectifiers with reverse recovery times around

60ns or faster, such as the MUR series, are acceptable

but have greater forward voltage drop. Make sure that

the diode’s peak current rating exceeds the current

limit set by RSENSE, and that its breakdown voltage

exceeds VOUT. Schottky diodes are preferred for heavy

loads, especially in low-voltage applications, due to

their low forward voltage. For high-temperature applica-

tions, some Schottky diodes may be inadequate due to

high leakage currents. In such cases, ultra-fast silicon

rectifiers are recommended, although acceptable per-

formance can often be achieved by using a Schottky

diode with a higher reverse voltage rating.

Determine Input and Output Filter

Capacitors

Low-ESR capacitors are recommended for both input

bypassing and output filtering. Capacitor equivalent

series resistance (ESR) is a major contributor to output

ripple—typically 60% to 90%. Low-ESR tantalum

capacitors offer a good tradeoff between price and

performance. Ceramic and Sanyo OS-CON capacitors

have the lowest ESR. Ceramic capacitors are often a

good choice in high-output-voltage applications where

large capacitor values may not be needed. Low-ESR

aluminum-electrolytic capacitors are tolerable and can

be used when cost is the primary consideration; how-

ever, standard aluminum-electrolytic capacitors should

be avoided.

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