The top-off charge is done at 1/8 the fast-charge rate.

For the MAX2003, the MOD pin is activated in every 34

second period to supply current to the battery for 4

seconds (MOD oscillates for 4 seconds and stays low

for 30 seconds) (Figure 7). If external regulation is used

(SNS tied to ground), MOD stays high for 4 seconds

and low for 30 seconds (Figure 8). This top-off process

continues until the fast-charge timeout (Table 4) is

exceeded, or if a maximum temperature or maximum

voltage condition is detected. The MAX2003A is slightly

modified to turn the MOD pin on for 0.5sec in every 4

second period. This shorter on-time reduces battery

heat and increases charge acceptance. During the top-

off charge, the CHG pin will cycle low (LED on) for

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

Trickle-Charge

A trickle-charge is applied to the battery after fast-

charge and top-off charge have terminated to compen-

sate for self discharge. There are two methods of trickle

charge: constant and pulsed.

Pulsed Trickle-Charge (MAX2003A)

The MAX2003A provides a pulsed trickle-charge to the

battery by turning on the MOD pin briefly during a fixed

period of time. The duty cycle of the pulse is a function of

the programmable inputs TM1 and TM2 (Table 4 ). The

MAX2003A does not use the trickle resistor to provide the

trickle charge. However, the trickle resistor cannot be

entirely omitted because it is also used for the battery-

detect circuitry.

Constant Trickle-Charge (MAX2003)

The MAX2003 provides a steady trickle-charge to the

battery by connecting a resistor from the DC supply to

the positive battery terminal. This resistor has a dual

purpose, in that it provides a trickle-charge and pulls

the BAT pin above the MCV when the battery is absent.

The trickle-charge rate depends on the type of battery

used. For NiCd batteries, a nominal trickle-charge rate

would be C/16, and NiMH batteries could use a rate of

C/40. The resistor value used depends on the maxi-

mum DC voltage and the typical battery voltage. For

example, a six-cell 800mAh NiCd pack with a nominal

voltage of 1.2V per cell would have a total voltage of

1.2V x 6V = 7.2V. If the DC supply voltage used is 14V,

the voltage across the trickle resistor would be 14.0V -

7.2V = 6.8V. The trickle current needed would be C/16

= 800 / 16 = 50mA. The trickle resistor would therefore

should be made for NiMH batteries using C/40 as the

trickle-charge rate.

If a trickle-charge is not needed, a higher value of trick-

le resistor (like 100k

Ω) can be selected to sense the

battery insertion.

Charge Status

The CHG pin is connected to a LED that indicates the

operating mode. Table 5 summarizes the different

charge conditions.

_______________________Design Guide

Using the circuit of Figure 1 as an example, the follow-

ing nine steps show how to design a 1.7A switch-mode

fast-charger that can charge a Duracell DR17 (NiMH

six-cell battery pack with a 1700mAh capacity).

1) Select DC Power Supply. The first step is to select

the DC power supply (such as a wall cube). The mini-

mum supply voltage should have a supply equal to

about 2V per cell, plus 1V headroom for external cir-

cuitry ((2V/cell) + 1V). The minimum supply voltage

must be greater than 6V. If, as in our example, there

are six cells, a minimum supply of about 13V is needed

((6 cells x 2V) + 1V).

2) Determine Charge Rate. The charge rate, or fast-

the capacity of the battery, and the time in which the

user wants the battery to be charged. The battery man-

ufacturer recommends a maximum fast-charge rate,

which must not be exceeded.

Capacity of Battery (mAh)

Charge Time (h)

For example, if a 1700mAh battery needs to be charged

in two hours (C/2), a fast-charge current of at least

850mA is needed. A charge rate of C/2 will ideally charge

a battery in two hours but, because of inefficiencies in a

battery’s chemical processes, the time could be 30% to

40% more. Our example circuit (Figure 1) charges the

Duracell battery pack at a C rate of 1.7A, which should fully

charge a discharged battery in approximately 80 minutes.

NiCd/NiMH Battery

Fast-Charge Controllers

12

______________________________________________________________________________________

Table 5. Charge Status

LED on for 0.125sec, off for

0.125sec

Charge Complete and Top-

Off

LED on

Fast-Charge

LED on for 1.375sec, off for

0.125sec

Discharge-Before-Charge

LED on for 0.125sec, off for

1.375sec

Charge Pending

Charge State

LED off

Battery Absent

CHG LED Status