MAX1612 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – Bridge-Battery Backup Controllers for Notebooks

Bridge-Battery Backup Controllers
for Notebooks
BRIDGE
BATTERY
V_CC
470k
MICROCONTROLLER
I/O
I/O
I/O
I/O
I/O
I/O
470k
250k
2N7002*
MAIN
47µF
BATTERY
15µH
BBATT
LX
LRO
1µF
LBO
PGND
MAX1612
MAX1613
ISET
FULL
FB
BBON
MBR0530
20µF
2.4k
0.33µF
DCMD
CCMD
CC
0.01µF
LBI
CD
GND
0.1µF
SYSTEM
DC-DC
(MAX1630)
750k
35.2k
479.1k
*OPTIONAL, TO RESET COUNTER
Figure 4. Microcontroller-Based Application
Low-Battery Comparator (LBI, LBO)
The MAX1612/MAX1613 feature a low-battery com-
parator with a factory-preset 1.8V threshold. This com-
parator is intended to monitor the main high-voltage
battery. As the voltage falls below 1.8V, the open-drain
LBO output sinks current. With 200mV of hysteresis, the
output will not go high until V_LBI exceeds 2.0V. LBO
can easily be connected to BBON to start the DC-DC
converter when V_LBI < 1.8V (stand-alone application,
Figure 3). Figure 4 shows an application using a micro-
controller, where LBO alerts the microcontroller to the
falling voltage and pulls BBON low through an external
resistor to start the DC-DC converter while also pulling
DCMD low to start the counter.
BBON Control Input
The BBON input serves two functions: setting the peak
LX switch current, and enabling the DC-DC converter.
The control signal is normally applied to R_BBON rather
than at the pin itself. The peak LX switch current is
directly proportional to and 42,000 times greater than
the current through R_BBON (see Typical Operating
Characteristics). The BBON pin is internally regulated
to 2V, so that when the control input is forced low, the
voltage across R_BBON is 2V.
When driving BBON from external logic, ensure the low
state has minimal noise. Otherwise, drive R_BBON with
an N-channel FET whose source is returned directly to
GND (Figure 2).
Applications Information
Design Procedure
The following section refers to the Functional Diagram
of Figure 1.
Step 1: Select the output voltage and maximum output
current for the boost DC-DC converter. Generally,
choose an output voltage high enough to run the main
system’s buck DC-DC converters. Assuming the maxi-
mum battery capacity is 50mAh (Sanyo 1.2V N-50AAA),
the following equations can help the design process:
I_PEAK = 2 · I_OUT · (V_OUT + V_D) / (V_BBATT - V_RDSON)
I_IN = 0.5 · I_PEAK
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