Step-Up/Down DC-DC Converter

in QSOP Package

8

_______________________________________________________________________________________

__________________Design Procedure

Output Voltage Selection

For fixed output voltages of 3.3V or 5V, connect 3/5 to

PS or GND and connect FB to GND (Table 2).

Alternatively, adjust the output voltage from 1.25V to

5.5V by connecting two resistors, R1 and R2 (Figure 2),

which form a voltage divider between OUT and FB.

Choose resistor values as follows:

has a maximum value of 50nA, R1 and R2 can be large

with no significant accuracy loss. Choose R2 in the

100k

Ω to 270kΩ range and calculate R1 using the

above formula. For 1% error, the current through R1

should be at least 100 times FB’s bias current.

Whenever the voltage at FB exceeds 70mV above

GND, the state of the 3/5 pin is ignored. Connect 3/5 to

leave 3/5 unconnected.

Low-Battery Detection

The MAX1672 contains a comparator for low-battery

ly 1.25V), the open-drain comparator output (PGO)

goes high. Hysteresis is typically 30mV. Set the low-

battery detector’s threshold with resistors R3 and R4

(Figure 2) using the following equation:

at PGI has a maximum value of 50nA, R3 and R4 can

be large to minimize input loading with no significant

accuracy loss. Choose R4 in the 100k

Ω to 270kΩ range

and calculate R3 using the above formula. For 1%

error, the current through R3 should be at least 100

times PGI’s bias current.

The PGO output is open-drain and should be pulled

high with external resistor R5 for normal operation. If the

low-battery comparator is not used, connect PGI and

PGO to GND.

Inductor Selection

A 10µH inductor performs well in most MAX1672 appli-

cations. Smaller inductor values typically offer a smaller

physical size for a given series resistance, but may

increase switching losses. Larger inductor values

exhibit higher output current capability and larger phys-

ical dimensions for a given series resistance. For opti-

mum performance, choose an inductor value from

Table 3 or by using the following equation:

The inductor’s incremental saturation current rating

should also be greater than the peak switch-current

limit. However, it is generally acceptable to bias most

inductors into saturation by as much as 20% with slight-

ly reduced efficiency. The inductor’s DC resistance sig-

nificantly affects efficiency. See Tables 4 and 5 for a list

of suggested inductors and suppliers.

V

+ V

I

V

+ V

I

OUT

DIODE

LIM

IN(min)

SWITCH

LIM

()

<

tL

t

OFF

ON

2

(max)

MAX1672

IN

ONA

PGI

ONB

OFF

ON

C3

0.1

µF

C1

100

µF

C2

100

µF

R2

R1

R5

1M

Ω

C4

4.7

µF

R3

R4

L1

10

µH

PGND

GND

PS

1.25V TO

5.5V OUTPUT

INPUT

1.8V TO 11V

LOW-BATTERY-

DETECTOR

OUTPUT

OUT

PG0

FB

REF

LX

OFF

3/5

ILIM

0.8A

ON

0.5A

Figure 2. Adjustable Output Voltage Configuration

Table 2. Output Voltage Control

FB

0

GND

+5

1

GND

+3.3

3/

5

X

>70mV

+1.25 to +5.5