Datasheet

20
Maxim Integrated
Dual-Input Linear Charger, Smart Power Selector
with Advanced Battery Temperature Monitoring
MAX8934G
Soft-Start
To prevent input transients that can cause instability in
the USB or AC adapter power source, the rate of change
of input current and charge current is limited. When a
valid DC or USB input is connected, the input current
limit is ramped from zero to the set current-limit value (as
shown in Table 2). If DC is connected with no USB power
present, input current ramps in 1.5ms. If DC is con-
nected with USB already present, input current ramps in
50Fs. When USB is connected with no DC present, input
current also ramps in 50Fs. If USB is connected with DC
already present, the USB input is ignored.
If an adapter is plugged into DC while USB is already
powered, the input current limit reramps from zero back
up to the DC current limit so that the AC adapter does
not see a load step. During this transition, if the input
current limit is below the SYS load current, the battery
supplies the additional current needed to support the
load. Additionally, capacitance can be added to SYS to
support the load during input power transitions. When
the charger is turned on, charge current ramps from zero
to the ISET current value in 1.5ms. Charge current also
ramps when transitioning to fast-charge from prequal
and when changing the USB charge current from 100mA
to 500mA with PEN2. There is no dI/dt limiting, however,
if ISET is changed suddenly using a switch at R
ISET
.
Battery Charger
The battery charger state diagram is illustrated in Figure
7. With a valid DC or USB input, the battery charger
initiates a charge cycle when the charger is enabled. It
first detects the battery voltage. If the battery voltage is
less than the BATT prequal threshold (3.0V), the charger
enters prequal mode and charges the battery at 20% of
the maximum fast-charge current. This reduced charge
rate ensures that the maximum fast-charge current set-
ting does not damage a deeply discharged battery.
Once the battery voltage rises to 3.0V, the charger tran-
sitions to fast-charge mode and applies the maximum
charge current. As charging continues, the battery volt-
age rises until it approaches the battery regulation volt-
age where charge current starts tapering down. When
charge current decreases to 20% of the fast-charge
current, the charger enters a brief 15s top-off state, then
DONE pulls low and charging stops. If the battery volt-
age subsequently drops below the recharge threshold,
charging restarts and the timers reset.
Charge Enable (CEN)
When CEN is low, the charger is on. When CEN is high,
the charger turns off. CEN does not affect the SYS out-
put. In many systems, there is no need for the system
controller (typically a microprocessor) to disable the
charger, because the MAX8934G Smart Power Selector
circuitry independently manages charging and adapter/
battery power hand-off. In these situations, CEN can be
connected to ground.
Setting the Charge Current
ISET adjusts charge current to match the capacity of the
battery. A resistor from ISET to ground sets the maximum
fast-charge current:
I
CHGMAX
= 2000 x 1.5V/R
ISET
= 3000V/R
ISET
Determine the I
CHGMAX
value by considering the char-
acteristics of the battery. It is not necessary to limit the
charge current based on the capabilities of the expected
AC adapter/USB charging input, the system load, or
thermal limitations of the PCB. The MAX8934G automati-
cally adjusts the charging algorithm to accommodate
these factors.
Monitoring the Charge Current
In addition to setting the charge current, ISET can also
be used to monitor the actual current charging the bat-
tery. See Figure 4. The ISET output voltage is:
V
ISET
= I
CHG
x 1.5V/I
CHGMAX
= I
CHG
x R
ISET
/2000
where I
CHGMAX
is the set fast-charge current and I
CHG
is the actual battery charge current. A 1.5V output indi-
cates the battery is being charged at the maximum set
fast charge current; 0V indicates no charging. This volt-
age is also used by the charger control circuitry to set
and monitor the battery current. Avoid adding more than
10pF capacitance directly to the ISET pin. If filtering of
the charge-current monitor is necessary, add a resistor
of 100kI or more between ISET and the filter capacitor
to preserve charger stability.
Note that the actual charge current can be less than the
set fast-charge current when the charger enters voltage
mode or when the input current limiter or thermal limiter
reduces charge current. This prevents the charger from
overloading the input source or overheating the system.
Charge Termination
When the charge current falls to the termination threshold
and the charger is in voltage mode, charging is com-
plete. Charging continues for a brief 15s top-off period
and then enters the DONE state where charging stops.
The DONE current threshold (I
DONE
) is set to 20% of the