3.3V/5V or Adjustable-Output,

Single-Cell DC-DC Converters

_______________________________________________________________________________________

9

Since the input bias current at FB has a maximum value

of 100nA, large values (10k

Ω to 300kΩ) can be used

for R1 and R2 with no significant accuracy loss. For 1%

error, the current through R1 should be at least 100

times FB’s bias current.

The MAX866 series contains an on-chip comparator for

low-battery detection. If the voltage at LBI falls below

the regulator’s internal reference voltage (1.25V), LBO

(an open-drain output) sinks current to GND. The low-

battery monitor’s threshold is set by two resistors, R3

and R4 (Figure 3). Set the threshold voltage using the

following equation:

R3 = R4

Since the LBI current is less than 100nA, large resistor

values (typically 10k

Ω to 300kΩ) can be used for R3

and R4 to minimize loading of the input supply.

When the voltage at LBI is below the internal threshold,

LBO sinks current to GND. Connect a pull-up resistor of

100k

Ω or more from LBO to OUT when driving CMOS

circuits. When LBI is above the threshold, the LBO out-

put is off. If the low-battery comparator is not used,

When the low-battery detection threshold voltage is

below 1.25V, use the circuit shown on the right in

MAX866, adjustable in MAX867) as a reference. The

voltage divider formed by R5 and R6 allows the effec-

set to approximately 100k

Ω, and R5 is given by the

formula:

Note that LBI drops below the 1.25V LBI threshold trip

from the same internal voltage reference, they track

together over temperature.

Low-Battery Start-Up

The MAX866/MAX867 are bootstrapped circuits; they

can start under no-load conditions at much lower bat-

tery voltages than under full load. Once started, the out-

put can maintain a moderate load as the battery volt-

age decreases below the start-up voltage (see

Typical

Operating Characteristics). The circuit shown in Figure

4 allows the circuit to start with no load, then uses the

LBI circuit and an external low-threshold P-channel

MOSFET switch to apply the load after the output has

started.

Resistors R7 and R8 are selected to trip the LBI detec-

tor at about 90% of the output voltage. On start-up, LBI

and LBO are low, Q2 is off, and transistor Q1’s gate is

held high by R11. This disconnects the load, allowing

the MAX866 to bootstrap itself at the lowest possible

voltage. When the output reaches its final output volt-

age, LBI and LBO go high, turning on Q2, Q1, and the

load.

Figure 3. Low-Battery Detector Circuits

Figure 4. Low-Voltage Start-Up Circuit

MAX866

LBI

OUT

5

6

R6

R5

MAX866

LBI

5

R4

R3

R3 = R4

-1

()

R5 = R6

()

MAX866

OUT

LBO

LBI

6

4

5

R11

1M

R9

1M

R10

1M

R8

1M

R7

Q1

MMDFZP02E

Q2

2N3904

(1.25V)

LOAD