Rainbow Electronics MAX8903A - Manual (Page 15)

MAX8903A
2A 1-Cell Li+ DC-DC Charger for USB*
and Adapter Power
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step-down regulator is turned off and a low-dropout lin-
ear regulator is connected between DC and SYS.
USB Input—Linear Regulator
If a valid USB input is present with no valid DC input,
current for SYS and battery charging is supplied by a
low-dropout linear regulator connected from USB to
SYS. The SYS regulation voltage shows the same char-
acteristic as when powering from the DC input (see
Figure 4). The battery charger operates from SYS with
any extra available current, while not exceeding the
maximum-allowed USB current. If both USB and DC
inputs are valid, power is only taken from the DC input.
The maximum USB input current is set by the logic
state of the IUSB input to either 100mA or 500mA.
Power Monitor Outputs (UOK, DOK)
DOK is an open-drain, active-low output that indicates
the DC input power status. With no source at the USB
pin, the source at DC is considered valid and DOK is
driven low when: 4.15V < V
DC < 16V. When the USB
voltage is also valid, the DC source is considered valid
and DOK is driven low when: 4.45V < V
DC < 16V. The
higher minimum DC voltage with USB present helps
guarantee cleaner transitions between input supplies. If
the DC power-OK output feature is not required, con-
nect DOK to ground.
UOK is an open-drain, active-low output that indicates
the USB input power status. UOK is low when a valid
source is connected at USB. The source at USB is valid
when 4.1V < V
USB < 6.6V. If the USB power-OK output
feature is not required, connect UOK to ground.
Both the UOK and the DOK circuitry remain active in
thermal overload, USB suspend, and when the charger
is disabled. DOK and UOK can also be wire-ORed
together to generate a single power-OK (POK) output.
Thermal Limiting
When the die temperature exceeds +100°C, a thermal
limiting circuit reduces the input current limit by 5%/°C,
bringing the charge current to 0mA at +120°C. Since
the system load gets priority over battery charging, the
battery charge current is reduced to 0mA before the
input limiter drops the load voltage at SYS. To avoid
false charge termination, the charge termination detect
function is disabled in this mode. If the junction temper-
ature rises beyond +120°C, no current is drawn from
DC or USB, and V
SYS regulates at 50mV below V BAT .
System Voltage Switching
DC Input
When charging from the DC input, if the battery is
above the minimum system voltage, SYS is connected
to the battery. Current is provided to both SYS and the
battery, up to the maximum program value. The step-
down output current sense and the charger current
sense provide feedback to ensure the current loop
demanding the lower input current is satisfied. The
advantage of this approach when powering from DC is
that power dissipation is dominated by the step-down
regulator efficiency, since there is only a small voltage
drop from SYS to BAT. Also, load transients can be
absorbed by the battery while minimizing the voltage
disturbance on SYS. If both the DC and USB inputs are
valid, the DC input takes priority and delivers the input
current, while the USB input is off.
After the battery is done charging, the charger is turned
off and the SYS load current is supplied from the DC
input. The SYS voltage is regulated to 4.4V. The charg-
er turns on again after the battery drops to the restart
threshold. If the load current exceeds the input limiter,
SYS drops down to the battery voltage and the 50m
Ω
SYS-to-BAT PMOS switch turns on to supply the extra
load current. The SYS-to-BAT switch turns off again
once the load is below the input current limit. The 50m
Ω
PMOS also turns on if valid DC input power is removed.
USB Input
When charging from the USB input, the DC input step-
down regulator turns off and a linear regulator from
USB to SYS powers the system and charges the bat-
tery. If the battery is greater than the minimum system
voltage, the SYS voltage is connected to the battery.
The USB input then supplies the SYS load and charges
the battery with any extra available current, while not
exceeding the maximum-allowed USB current. Load
transients can be absorbed by the battery while mini-
mizing the voltage disturbance on SYS. When battery
charging is completed, or the charger is disabled, SYS
is regulated to 4.4V. If both USB and DC inputs are
valid, power is only taken from the DC input.
USB Suspend
Driving USUS high turns off charging as well as the SYS
output and reduces input current to 170µA to accom-
modate USB suspend mode.
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 MAX8903A smart power selector
circuitry independently manages charging and
adapter/battery power hand-off. In these situations,
CEN may be connected to ground.