MAX866 Maxim, MAX866 Datasheet - Page 9

• Manufacturer Part Number: MAX866
• Description: 3.3V/5V or Adjustable-Output / Single-Cell DC-DC Converters
• Manufacturer: Maxim

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.
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:
where V
detector and V
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 V
When the low-battery detection threshold voltage is
below 1.25V, use the circuit shown on the right in
Figure 3. Low-Battery Detector Circuits
WHERE V
FOR V
R3 = R4
TH
MAX866
= THE V
TH
Low-Battery Detection, V
R3 = R4
TH
Low-Battery Detection, V
(
V
V
is the desired threshold of the low-battery
> 1.25V
TH
REF
IN
TRIP THRESHOLD
REF
LBI
-1
IN
)
_______________________________________________________________________________________
(
5
is the internal 1.25V reference.
and leave LBO open.
____ - 1
V
V
V
TH
REF
IN
R3
R4
)
WHERE V
FOR V
R5 = R6
MAX866
TH
= THE V
TH
(
V
V
REF
OUT
< 1.25V
OUT
IN
TH
LBI
- V
TH
- V
TRIP THRESHOLD
3.3V/5V or Adjustable-Output,
TH
REF
Single-Cell DC-DC Converters
< 1.25V
> 1.25V
5
6
)
V
IN
R5
R6
Figure 3. This circuit uses V
MAX866, adjustable in MAX867) as a reference. The
voltage divider formed by R5 and R6 allows the effec-
tive trip point of V
set to approximately 100kΩ, and R5 is given by the
formula:
Note that LBI drops below the 1.25V LBI threshold trip
point when either V
Since V
from the same internal voltage reference, they track
together over temperature.
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 4. Low-Voltage Start-Up Circuit
MAX866
OUT
R5 = [R6 x (V
regulation and the LBI threshold are derived
OUT
LBO
LBI
IN
6
4
5
(1.25V)
IN
to be set below 1.25V. R6 is usually
or V
REF
OUT
R7
R8
1M
- V
Low-Battery Start-Up
OUT
is low.
TH
R9
1M
R10
1M
)] / (V
(3.3V or 5.0V in the
MMDFZP02E
R11
1M
OUT
Q1
LOAD
2N3904
- V
Q2
V
REF
OUT
(3.3V/5V)
)
9