Battery Replacement

Before a battery is inserted, the BAT pin is pulled high-

er than the maximum cell voltage (MCV) by the resistor

When the battery is inserted, the voltage per cell at BAT

falls from the default voltage to the battery voltage.

Fast-charge is initiated on a falling edge when the BAT

voltage crosses the voltage on MCV.

Applying Power to the MAX2003/MAX2003A

(battery already present)

There may be some cases where a battery is connect-

ed before power is applied to the MAX2003/

MAX2003A. When power is applied, the device goes

into reset mode for approximately 1.5sec and then

samples the CCMD and DCMD pins. Its charge status

is determined by the voltage at both the CCMD and

DCMD pins. Table 2 summarizes the various conditions

the MAX2003/MAX2003A might see on power-up.

Table 2 shows that the MAX2003/MAX2003A can be

set-up for fast-charge on power-up by making sure

CCMD and DCMD are at the same potential. If fast-

charge on power-up is not desired, make sure CCMD

and DCMD are at different logic levels during power-

up, and use a digital signal to control fast-charge (see

Digital Control section).

Digital Control

The CCMD pin can be used to initiate fast-charge. This is

useful when neither the power supply nor the battery can

be removed from the charger. The CCMD signal needed

to initiate fast-charge depends on the potential at DCMD.

If DCMD is low, a rising edge on CCMD initiates fast-

charge. If DCMD is high, a falling edge on CCMD pro-

vides the fast-charge signal. Table 3 summarizes the

conditions used to start fast-charge.

Discharge-Before-Charge (optional)

The discharge-before-charge function is optional and

can be used to condition old batteries. It is especially

useful in NiCd batteries, since it alleviates the voltage

depression problems associated with partially dis-

charged NiCd cells. The discharge-before-charge

function is initiated by a rising edge into DCMD.

When the digital signal is applied, the DIS pin will be

pulled high, turning on the attached circuit and dis-

charging its battery. The discharge process continues

the discharge phase, the CHG pin will be low (LED on)

for 1.375sec and high (LED off) for 0.125sec.

The MAX2003/MAX2003A does not control the current

during discharge-before-charge. If the discharge rate

is too great, the battery could overheat and be dam-

aged. The battery manufacturer will be able to specify

a safe discharge rate, but a rate of C or slower is typi-

cally acceptable. It is also important to choose compo-

discharge rate. Since the gate-source drive for Q2 can

be as low as 4.5V, use a logic-level MOSFET.

Fast-Charge Current

The fast-charge current can be generated using two

categories of circuits:

Circuits with SNS Resistor

The standard application circuit of Figure 1 uses an

inductor and a switched mode of operation to supply

the current. The charge current is determined by the

sense resistor placed between the negative terminal of

The SNS pin is the input to a comparator with hystere-

MOD pin is turned on. If the SNS voltage is above

NiCd/NiMH Battery

Fast-Charge Controllers

_______________________________________________________________________________________

9

Table 2. Device Status on Power-Up if

Battery is Already Present

Table 3. Digital Control of Fast-Charge

• The device does not enter fast-charge

immediately.

• Fast-charge is initiated by the rising

edge of a pulse on CCMD.

Low

High

• Fast-charge is initiated on power-up.

High

High

• The device does not enter fast-charge

immediately.

• Fast-charge is initiated by the falling

edge of a pulse on CCMD.

High

Low

CCMD

• Fast-charge is initiated on power-up.

Low

Low

MAX2003/MAX2003A Status when

Power is Applied

DCMD

• Fast-charge is initiated by a falling

edge on CCMD.

High

CCMD

• Fast-charge is initiated by a rising

edge on CCMD.

Low

CCMD Status to Initiate Fast-Charge

DCMD