MAX8903A–E/G/H/J/N/Y

2A 1-Cell Li+ DC-DC Chargers

for USB and Adapter Power

18

Maxim Integrated

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

valid, power is only taken from the DC input.

USB Suspend

Driving USUS high and DCM low turns off charging as

well as the SYS output and reduces input current to

170µA to accommodate USB suspend mode. See

Table 2 for settings.

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 MAX8903_ smart power selector

circuitry independently manages charging and

adapter/battery power hand-off. In these situations,

CEN

may be connected to ground.

Soft-Start

To prevent input transients that can cause instability in

the USB or AC adapter power source, the rate of change

of the input current and charge current is limited. When

an input source is valid, SYS current is ramped from

zero to the set current-limit value in typically 50µs. This

also means that if DC becomes valid after USB, the

SYS current limit is ramped down to zero before switch-

ing from the USB to DC input. At some point, SYS is no

longer able to support the load and may switch over to

This threshold is a function of the SYS capacitor size

and SYS load. The SYS current limit then ramps from

zero to the set current level and SYS supports the load

again as long as the SYS load current is less than the

set current limit.

POWER SOURCE

DOK

UOK

DCM***

IUSB

USUS

DC STEP-DOWN

OUTPUT

CURRENT LIMIT

USB INPUT

CURRENT LIMIT

MAXIMUM

CHARGE

CURRENT**

AC Adapter at DC Input

L

X

H

X

X

Lesser of

L

X

L

L

L

100mA

Lesser of

100mA

L

X

L

H

L

500mA

Lesser of

500mA

USB Power at DC Input

L

X

L

X

H

USB suspend

USB input off. DC

input has priority.

0

H

L

X

L

L

100mA

Lesser of

100mA

H

L

X

H

L

500mA

Lesser of

500mA

USB Power at USB Input,

DC Unconnected

H

L

X

X

H

USB suspend

0

DC and USB Unconnected

H

H

X

X

X

No DC input

No USB input

0

Table 2. Input Limiter Control Logic

**

Charge current cannot exceed the input current limit. Charge may be less than the maximum charge current if the total SYS load

exceeds the input current limit.

***

There is an internal diode from DCM (anode) to DC (cathode) as shown in Figure 1. If the DCM level needs to be set by a µP, use

a MOSFET for isolation as shown in FIgure 3.

X = Don’t care.