MAX8903NETI+T datasheet(17/30 Pages) MAXIM

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MAX8903A–E/G/H/J/N/Y

2A 1-Cell Li+ DC-DC Chargers

for USB and Adapter Power

Maxim Integrated

circuit of Figure 3 shows a simple MOSFET and resistor

on DCM to prevent any current from flowing from DCM

through the internal diode to DC. This circuit of Figure 3

allows a microprocessor to drive the gate of the MOS-

FET to any state at any time.

An alternative to the simple MOSFET and resistor on

DCM as shown in Figure 3 is to place a 1M

Ω resistor in

series with the DCM input to the microprocessor. The

microprocessor can then monitor the

DOK output and

make sure that whenever

DOK is high DCM is also low.

In the event that DCM is driven to a higher voltage than

DC, the 1M

Ω series resistance limits the current from

DCM through the internal diode to DC to a few µA.

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 < VDC < 16V. When the USB

voltage is also valid, the DC source is considered valid

and

DOK is driven low when: 4.45V < VDC < 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 < VUSB < 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 VSYS regulates at 50mV below VBAT.

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 VSYSREG. The

charger 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

IBAT x RON

VBATREG

VSYSREG

VBAT

VSYSMIN

VSYS

VCEN = 0V

VDC AND/OR VUSB = 5.0V

MAX8903_

Figure 4. SYS Tracking VBAT to the Minimum System Voltage

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