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LT1571

Constant-Current/Constant-Voltage Battery Charger with Preset Voltage and Termination Flag

Linear Technology 

LT1571 Series

APPLICATIO S I FOR ATIO

For example, if 10% threshold is needed for the 1A charger

(see Figure 4), then with RPROG = 4.93k:

RCAP

=

1.331• 4.93k

0.20 – 0.10

=

65.6k

Because of low level errors, as the threshold level is

reduced, the accuracy is also reduced. It is not recom-

mended to program a level less than 7.5%.

Preset Battery Voltage Settings

The LT1571-2 operates at 200kHz and is preset for 8.2V

battery voltage with SELECT pin floating and 8.4V with

SELECT pin grounded.

The LT1571-5 operates at 500kHz and is preset for 4.1V

battery voltage with SELECT pin floating and 4.2V with

SELECT pin grounded.

BAT2 pin is for Kelvin sensing the battery voltage and

should be connected to the battery.

Other Battery Voltage Settings

For battery voltages other than the preset voltages, the

LT1571-1 should be used. It operates at 200kHz and the

battery voltage is programmed with R3 and R4 divider at

VFB pin (Figure 6).

VFB

LT1571-1

VBAT

R3

R4

1571 F06

Figure 6. Programming Other Battery Voltages

Current through the R3/R4 divider is set at a compromise

value of 25µA to minimize battery drain when the charger

is off. The VFB pin input current of 3nA contributes very

little output voltage error and can be neglected.

With divider current set at 25µA, R4 = 2.465/25µA = 100k

and,

R3 = (R4)(VBAT − 2.465)

2.465

Lithium-ion batteries typically require float voltage accu-

racy of 1% to 2%. Accuracy of the LT1571-1 VFB voltage

is ±0.5% at 25°C and ±1% over full temperature. This

leads to the possibility that very accurate (0.1%) resistors

might be needed for R3 and R4. Actually, the temperature

of the LT1571-1 rarely exceeds 50°C in float mode

because charge currents have tapered off to a low level, so

0.25% resistors normally provide the required level of

overall accuracy.

External Shutdown

The LT1571 can be externally shut down by pulling the VC

pin low with an open-drain N-FET, such as 2N7002. The VC

pin should be pulled below 0.6V to stop switching. When

VC is pulled below 40mV, LT1571 supply current drops to

typical 150µA.

Removing input power to the charger puts the LT1571 into

a sleep mode and draws only 5µA from the battery.

Nickel-Cadmium and Nickel-Metal-Hydride Charging

The circuit in Figure 7 uses the LT1571-1 to charge NiCd

or NiMH batteries up to 20V with charge currents of 0.5A

when Q1 is on and 50mA when Q1 is off.

For a 2-level charger, R1 and R2 are found from:

IBAT

=

(2000)(2.465)

RPROG

R1= (2.465)(2000)

ILOW

R2 = (2.465)(2000)

IHI − ILOW

All battery chargers with fast-charge rates require some

means to detect full charge in the battery and terminate the

high charge current. NiCd batteries are typically charged at

high current until the battery temperature begins to in-

crease or until the battery voltage reaches a peak and

begins to decrease (– dV/dt). This is an indication of near

full charge. The charge current is then reduced to a much

10

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