MAX866 データシートの表示（PDF） - Maxim Integrated

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MAX866

3.3V/5V or Adjustable-Output, Single-Cell DC-DC Converters

Maxim Integrated 

3.3V/5V or Adjustable-Output,

Single-Cell DC-DC Converters

FOR VTH > 1.25V

MAX866

VIN

R3

5

LBI

R4

FOR VTH < 1.25V VIN

R5

MAX866 LBI 5

R6

OUT 6

( ) R3 = R4

VTH

VREF

-1

WHERE VTH = THE VIN TRIP THRESHOLD

( ) R5 = R6

VREF - VTH

VOUT - VREF

WHERE VTH = THE VIN TRIP THRESHOLD

Figure 3. Low-Battery Detector Circuits

6

OUT

Q1

MMDFZP02E VOUT (3.3V/5V)

MAX866

4

LBO

R9 R11

1M 1M

R7 R10

1M

5

LBI (1.25V)

R8

1M

Q2

2N3904

LOAD

Figure 4. Low-Voltage Start-Up Circuit

Since the input bias current at FB has a maximum value

of 100nA, large values (10kΩ 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.

Low-Battery Detection, VTH > 1.25V

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 (_V_T_H_ - 1)

VREF

where VTH is the desired threshold of the low-battery

detector and VREF is the internal 1.25V reference.

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

values (typically 10kΩ 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

100kΩ 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,

connect LBI to VIN and leave LBO open.

Low-Battery Detection, VTH < 1.25V

When the low-battery detection threshold voltage is

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

Figure 3. This circuit uses VOUT (3.3V or 5.0V in the

MAX866, adjustable in MAX867) as a reference. The

voltage divider formed by R5 and R6 allows the effec-

tive trip point of VIN to be set below 1.25V. R6 is usually

set to approximately 100kΩ, and R5 is given by the

formula:

R5 = [R6 x (VREF - VTH)] / (VOUT - VREF)

Note that LBI drops below the 1.25V LBI threshold trip

point when either VIN or VOUT is low.

Since VOUT regulation and the LBI threshold are derived

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.

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