current value must be sufficiently guardbanded to

ensure the limiting factor is the SNS resistor, and not

the transistor. In our example, the maximum current

supplied by the Darlington will be guardbanded to

1.8A. Since the beta of the Darlington is typically 1000,

the base current needed will be:

The emitter of the TIP115 will see 14V, so the base will

see about 12.6V. When the MOD pin is high, the

2N2222 transistor is on and the base resistor will be:

≈ 6.8kΩ

This 1.8A current will never be reached because MOD

will be off when the SNS voltage reaches 0.25V (0.9A).

Current-Limited Supply

The circuit in Figure 12 is set up to charge a Duracell

DR35 battery pack (nine cells, 2.6Ah) using a 19V, 2.6A

current-limited power supply provided by Advanced

Power Solutions. Since many power supplies have

built-in current limiting, very few external components

are required for this charging method.

signals the MOD pin to stay high until a termination

condition is met. When MOD is high, the NPN transistor

is turned on, hence pulling the gate of the MOSFET low.

This turns the MOSFET on and supplies current to the

battery at the current limit of the source (2.6A). The 12V

zener diode is placed between the source and gate of

the FET to ensure the FET’s maximum source-drain

voltage is not exceeded.

When a termination condition is reached, the MOD pin

goes low to turn off the FET and terminate the fast-

charge current.

NiCd/NiMH Battery

Fast-Charge Controllers

18

______________________________________________________________________________________

Table 7. Operation Summary

Discharge initiated with temperature

and voltage within set limits.

Discharge

Fast-charge initiated and tempera-

ture or voltage outside the set limits.

Charge Pending

a) Power applied and voltage at

CCMD = DCMD

b ) DCMD = Low, CCMD = Rising

Edge (power already present)

c ) DCMD = High, CCMD = Falling

Edge (power already present)

Initiate Fast-Charge

Rising edge on DCMD

Initiate Discharge

Charge Status

Battery Absent

Conditions

Low

Low

Low

Low

Low

MOD Status

High

Low

Low

Low

Low

DIS

Status

1.375

0.125

—

—

—

CHG LED Status

0.125

1.375

Continuous

Continuous

Continuous

Pulse current provided by pulsing

MOD pin after fast-charge/top-off.

Pulsed Trickle-

Charge (MAX2003A)

Trickle current provided by external

resistor after fast-charge/top-off.

Constant Trickle-

Charge (MAX2003)

Charge complete and top-off

enabled without exceeding temper-

ature and voltage limits.

Top-Off Charge

Exceed one of the five termination

conditions.

Charge Complete

Fast-charge initiated with tempera-

ture and voltage within set limits.

Fast-Charge

Pulsed according to charge rate

(Table 4).

Low

MAX2003A: Activate for 0.5sec in

every 4sec period.

MAX2003: Active for 4sec in

every 34sec period.

Low

Low

Low

Low

Low

Low

0.125

0.125

0.125

0.125

Continuous

0.125

0.125

0.125

0.125

—

LED On

(Low)

(sec)

LED Off

(High)

(sec)