In order to provide high currents with minimum ripple,

the device must function in the continuous-conduction

mode. Figure 10 shows a current waveform of an

inductor in the continuous-conduction mode (where the

coil current never falls to zero).

give a fairly low ripple while keeping the inductor size

minimal. This means that the total current change (Figure

10) across the inductor is

δi = 2 (1.9 - 1.7) = 0.4A.

The maximum voltage across the inductor is present

when the battery voltage is at its minimum. The mini-

mum cell voltage at the start of fast-charge will be 1V

per cell, giving a battery voltage of 6V for 6 cells. The

The input voltage for this application is 13V, so the

maximum voltage is:

The minimum on-time

δt of the switch is given by:

maximum input voltage, and PERIOD is the period of

the switching signal.

The maximum input voltage for this application will be

14V, and the maximum allowed switching frequency of

100kHz gives a period of 10µs. The minimum on-time

will therefore be:

The inductor value can be calculated from:

L = V

δt / δi = (7V x 4.62µs) / 0.4A = 81µH.

If this inductor value is used, the actual switching fre-

quency will be lower than the 100kHz expected, due to

comparator delays and variations in the duty cycle. The

inductor value selected for our application will be

100µH—a preferred value just above the calculated

value. It is important to choose the saturation current

rating of the inductor to be a little higher than the peak

currents, to prevent the inductor from saturating during

operation. The inductor must be selected to ensure that

the switching frequency of the MOD pin will not exceed

the 100kHz maximum.

Additional Applications

_________________________Information

The MAX2003/MAX2003A can use several other cir-

cuits to charge batteries. Figure 9 shows a circuit that

uses a Darlington transistor to regulate the current a

six-cell NiCd battery pack receives. Figure 10 shows a

gated current-limited supply being used to charge a

Duracell NiMH battery pack. Table 6 lists the external

components used in these two application configura-

tions.

Linear Regulation of Charge Current

The circuit in Figure 11 uses an inexpensive transistor

to provide the charge current. Since the input for the

MAX667 can tolerate up to 16V, this circuit can charge

up to 7 cells. The MAX667 can be replaced with a dif-

ferent regulator if more cells need to be charged. The

DC source must supply a voltage equal to 2x the num-

ber of cells, plus 2V overhead to accommodate the

drop across external components.

When fast-charge is initiated, the voltage at the SNS pin

is sampled and compared to the trip levels (220mV low

and 250mV high). If the voltage at SNS is below

220mV, the MOD pin will switch high, and the 10k/1µF

RC lowpass filter will pull high, turning on the NPN tran-

sistor. This will pull the base of the Darlington TIP115

low, turning it on and allowing current to flow into the

battery. When the current through the battery and SNS

resistor are high enough, the voltage at SNS will

exceed 250mV and the MOD pin will turn off.

The amount of current the battery receives depends on

cuit, the average current through the SNS resistor will

be:

The maximum current the resistor will receive is:

The Darlington transistor must be biased to ensure that

a minimum of 0.90A will be supplied. This minimum

NiCd/NiMH Battery

Fast-Charge Controllers

______________________________________________________________________________________

15

Table 6. External Component Sources

(602) 994-6430

(602) 303-5454

Motorola

Power

MOSFET &

Darlington

Transistor

(904) 462-4726

(203) 791-3273

(904) 462-3911

Energizer

Power

Systems

(800) 431-2658

Duracell

Battery

(619) 549-4791

(510) 460-5498

Fax Number

(800) 235-5445

Alpha

Thermistor

Thermistor

Device

(510) 734-3060

Advanced

Power

Solutions

Power

Supply

Phone Number

Manufacturer