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-

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

PIN

NAME

FUNCTION

1

USB

USB Port Charger Supply Input. USB draws up to 100mA to charge the battery. Decouple USB with a 1µF

ceramic capacitor to GND.

2

GND

Ground

POK

Power-OK Active-Low Open-Drain Charger Status Indicator.

POK pulls low when either charger source is

present (MAX1551 only).

3

CHG

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).

4DC

DC Charger Supply Input for an AC Adapter. DC draws 280mA to charge the battery. Decouple DC with a

1µF ceramic capacitor to GND.

5

BAT

Battery Connection. Decouple BAT with a 1µF ceramic capacitor to GND.

Exceeds operating input range. Not allowed. See the

Absolute Maximum Ratings section.

280mA (typ)

charging from DC

100mA (max)

charging from USB

Undervoltage lockout

Table 1. USB and DC Input Selection

MAX1551/MAX1555

Maxim Integrated

5