Detailed Description
The MAX1551/MAX1555 charge a single-cell Li+ bat-
tery from both USB and AC adapter sources, enabling
portable users to forgo carrying a wall cube. These
devices operate with no external FETs or diodes, and
accept operating input voltages up to 7V.
An internal thermal control loop simplifies PC board lay-
out and allows optimum charging rate without the ther-
mal limits imposed by worst-case battery and input
voltage. When the MAX1551/MAX1555 thermal limits
are reached, the chargers do not shut down, but simply
reduce charging current by 17mA/°C above a die tem-
perature of +110°C.
With USB connected, but without DC power, the charge
current is set to 100mA (max). This allows charging
from both powered and unpowered USB hubs with no
port communication required. When DC power is con-
nected, charging current is set at 280mA (typ). The
MAX1551/MAX1555 do not feature an enable input.
Once power is connected to USB and/or DC, the
charger is on.
When input power is removed, battery leakage current
is less than 5µA. No input-blocking diodes are required
to prevent battery drain. Insert a diode at DC (the
adapter input) if protection from negative voltage inputs
(reversed-polarity adapter plugs) is required.
USB to Adapter Power Handoff
The MAX1551/MAX1555 can charge from either the
USB input or the DC input. The battery does not charge
from both sources at the same time. The MAX1551/
MAX1555 automatically detect the active input and
charge from that. If both power sources are active, the
DC input takes precedence. The switchover between
DC and USB is detailed in Table 1.
MAX1551 Power-OK (
POK
)
The MAX1551’s POK is an active-low, open-drain out-
put that goes low when V
DC
or V
USB
is above 3.95V.
POK can be used as a logic output or can drive an
LED. POK indicates the charger is connected to input
power and is charging.
MAX1555 Charge Status (
CHG
)
The MAX1555’s CHG is an active-low, open-drain
charge status indicator. CHG pulls low when the battery
is charging (whenever USB or DC are powered) and
charge current is greater than 50mA. CHG indicates
when the battery is fully charged by going high imped-
ance when the charger is in voltage mode and charge
current falls below 50mA. Charging does not stop when
CHG goes high. CHG is low in precharge mode.
SOT23 Dual-Input USB/AC Adapter 1-Cell Li+
Battery Chargers
Pin Description
USB Port Charger Supply Input. USB draws up to 100mA to charge the battery. Decouple USB with a 1µF
ceramic capacitor to GND.
Power-OK Active-Low Open-Drain Charger Status Indicator. POK pulls low when either charger source is
present (MAX1551 only).
Active-Low Open-Drain Charge Status Indicator. CHG pulls low when the battery is charging. CHG goes to a
high-impedance state, indicating the battery is fully charged, when the charger is in voltage mode and
charge current falls below 50mA. CHG is high impedance when both input sources are low (MAX1555 only).
DC Charger Supply Input for an AC Adapter. DC draws 280mA to charge the battery. Decouple DC with a
1µF ceramic capacitor to GND.
Exceeds operating input range. Not allowed. See the
