MAX8939/MAX8939A/MAX8939B
System Power Management
for Mobile Handset
done. The top-off current threshold is a percentage of
the fast-charge current, the threshold is programmable.
When the top-off current threshold is set to 0% and
restart is disabled, the top-off mode continues until the
top-off timer expires. The top-off timer is programmable
and can also be disabled. With the op-off threshold set
to 0% and top-off timer disabled, the charger continu-
ously charges the battery with a constant voltage and
decreasing charge current. This makes it possible to
control the charge algorithm through software, without
influence of automatic maintaining charge.
To qualify charge as done, the current has to be below top-
off current threshold or a timeout has occurred. To main-
tain the battery voltage, the charger can be programmed
to restart once the battery voltage drops below a program-
mable threshold. When restart is enabled and the battery
voltage drops below the restart threshold, the charger
starts a new charging cycle by entering fast-charge.
If restart is disabled, the charger stops charging when
done and does not maintain the battery voltage. When
charge done occurs, an IRQ is sent to the host and a
flag is set in register 0x03. Reading the register disables
the charger. The charger can be enabled by writing to
register 0x09 bit 0 (CHG_EN). If one of the safety timers
(fast-charge or top-off) expires, an interrupt is sent to
the host and a flag is set in register 0x03. The charger is
disabled 5s after the safety times out.
If, at any point while charging the battery, the die tem-
perature approaches the thermal regulation threshold
(+100°C default), the ICs reduce the charging current
so that the die temperature does not increase. This fea-
ture not only protects the ICs from overheating, but also
allows the higher charge current without risking damage
to the system.
Note all charger registers are reset to their default set-
tings by power-on reset (POR) or RESET .
Charge On/Off Control
CHG is a logic hardware control input. Logic-high disables
the charger and logic-low enables the charger.
1. CHG = logic-high, the charger is disabled when
power pluck is asserted on CHG_IN and register
0x09 has not been affected. When CHG changes
logic state, a flag is set in the event register 0x03,
and an interrupt occurs.
2. CHG = Logic-low, the charger is enabled and starts
charging if charging conditions are within operating
limits.
Maxim Integrated
Once the CHG_CONTROL_A register 0x09 is accessed
either by reading or writing, the CHG is ignored. When CHG
changes status after register 0x09 has been accessed,
only STATUS and EVENT_A register is updated and an
interrupt occurs. The CHG_EN bit in CHG_CONTROL_A
register 0x09 is always [1] by default. The CHG_EN does
not follow the status of CHG , and the charger is enabled
just by reading the CHG_CONTROL_A register 0x09 and
CHG is ignored. To avoid the charger enabling just by
accessing the CHG_CONTROL_A register 0x09, write [0]
in the CHG_EN bit.
For the MAX8939 and MAX8939A, if the CHG_IN is
reconnected, the CHG is reset and the status of the
charger is following the logic level on CHG , as long
CHG_CONTROL_A register 0x09 is not affected. For the
MAX8939B, the CHG is reset only by reasserting CHG_IN
if RESET or UVLO is low.
SAFE_OUT
SAFE_OUT is an LDO powered from the CHG_IN input.
SAFE_OUT is enabled when a charger is detected (4.1V
< V CHG_IN < 10V (MAX8939) or 6.25V (MAX8939A/
MAX8939B)) and provides a protected output regulated to
4.9V (5V max). Typically, SAFE_OUT is used to power low-
voltage USB systems and the precharge indicator.
Indicator LED
The LED3 output sinks 3mA (typ) to drive an indica-
tor LED. LED3 is on by default and can be controlled
by the host by I 2 C (bit 7 of the REG_CONTROL
register). Typically, this LED indicates charge status
and SAFE_OUT powers the LED as shown in Figure 1.
Charge Current Monitor (CHG_MON)
CHG_MON is an analog output used to monitor the
charge current. CHG_MON outputs a voltage propor-
tional to the charge current with 1.2V corresponding to
the programmed fast-charge current.
The CHG_MON output includes ripple from loads on the
battery. If this is not desired, connect a small 0.01 F F to
0.1 F F capacitor at the input of the ADC to filter the ripple.
Charger Watchdog Timer
During battery fast-charge, a watchdog monitoring func-
tion can be activated to ensure that the host processor
has control of the charge algorithm. The watchdog timer
is enabled through register REG_CONTROL bit WD_EN.
When the charger is enabled by CHG_EN or CHG_IN,
the watchdog timer starts counting. Within 5s of enabling
the charger, the host must read or write register 0x09
or 0x0A to indicate it is alive. This resets the watchdog
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