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

(

)

<

)

t

L

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

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

5

X

>70mV

+1.25 to +5.5