3.3V/5V or Adjustable,

Step-Up/Down DC-DC Converters

_______________________________________________________________________________________

9

In this configuration, N/E is driven high by LBO when

boosts, and the linear regulator operates as a switch,

the linear regulator forces a minimum forward drop of

This transition is not seen at the output, since the linear

regulator already has an input-output voltage difference

tion 3 is most suitable when the battery voltage may be

compared with configuration 2, except that two addi-

tional resistors (R5 and R6) are needed.

Configuration 4: Low Noise

With N/E connected to PS, when the IC is boosting, the

cally 10% lower than with the high-efficiency configura-

tions.

ILIM

The current-limit-select input, ILIM, selects between the

two peak current limits: 1.5A (ILIM = GND) and 0.8A

(ILIM = PS). If the application requires 200mA or less

from the MAX710/MAX711, select 0.8A. The lower peak

current limit permits the use of smaller, low-cost induc-

tors. The ILIM input is internally diode clamped to GND

and PS, and should not be connected to signals out-

side this range.

Shutdown and Standby Modes

Grounding SHDN turns off the MAX710/MAX711 com-

pletely, disconnecting the input from the output. Tie

SHDN to PS for normal operation.

The MAX710/MAX711 have a standby mode that shuts

down the step-up converter. The linear regulator

remains on with a 7µA (typ) LDO quiescent current.

Connect STBY to ground to enter standby mode; other-

wise, connect STBY to PS.

__________________Design Procedure

Output Voltage Selection

For the MAX710, you can obtain a 3.3V or 5V output

voltage by tying 3/5 to GND or PS. Efficiency is typically

85% over a 2mA to 250mA load range. The device is

bootstrapped, with power derived from the step-up

voltage output (at PS). Under all load conditions, the

MAX710/MAX711 typically start up with a 1V input. If

the battery voltage exceeds the programmed output

voltage, the output will linear regulate down to the

selected output voltage.

The MAX711’s adjustable output voltage is set by two

resistors, R1 and R2 (Figure 3), which form a voltage

divider between the output and FB. Use the following

equation to determine the resistor values:

Since the input bias current at FB has a maximum value

of 50nA, R1 and R2 can be large with no significant

accuracy loss. Choose R2 in the 100k

Ω to 1MΩ range

and calculate R1 using the formula above. For 1%

error, the current through R1 should be at least 100

times FB’s bias current.

Low-Battery Comparator

The MAX710/MAX711 contain a comparator for low-

battery detection. If the voltage at LBI+ falls below that

at LBI- (typically connected to REF), LBO goes low.

Hysteresis is typically 50mV. Set the low-battery moni-

tor’s threshold with two resistors, R3 and R4 (Figure 2),

using the following equation:

MAX711

SHDN

LX

STBY

N/E

LBO

LBI+

LBI-

REF

PGND

GND

ILIM

PS

OUT

R4

R3

R2

R1

C4

C1

IN

C2

L1

FB

OFF

ON

Figure 3. MAX711 Adjustable Output Voltage