ADP3522 datasheet(18/20 Pages) AD

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REV. 0

–18–

ADP3522

current is programmed by selecting the sense resistor, R1 (see

Figure 2).

The lithium ion charge current is calculated using

CHR

SENSE

160

(5)

where VSENSE is the high current limit threshold voltage. Or if

the charge current is known, R1 can be found:

SENSE

CHR

160

(6)

Similarly the trickle charge current and the end of charge cur-

rent can be calculated:

TRICKLE

SENSE

(7)

EOC

SENSE

(8)

Example: Assume an 800 mA-H capacity lithium ion battery

and a 1 C charge rate. R1 = 200 m . Then ITRICKLE = 100 mA

and IEOC = 70 mA.

Appropriate sense resistors are available from the following

vendors:

• Vishay Dale

• IRC

• Panasonic

Charger FET Selection

The type and size of the pass transistor is determined by the

threshold voltage, input-output voltage differential, and charge

current. The selected PMOS must satisfy the physical, electri-

cal, and thermal design requirements.

To ensure proper operation, the minimum VGS the ADP3522

can provide must be enough to turn on the FET. The available

gate drive voltage can be estimated using the following:

ADAPTER MIN

GATEDR

SENSE

()

(9)

where

VADAPTER(MIN) is the minimum adapter voltage.

VGATEDR is the gate drive “low” voltage, 0.5 V.

VSENSE is the maximum high current limit threshold voltage.

The difference between the adapter voltage (VADAPTER) and the

final battery voltage (VBAT) must exceed the voltage drop due to

the blocking diode, the sense resistor, and the on resistance of

the FET at maximum charge current.

ADAPTER

DIODE

SENSE

BAT

(10)

Then the RDS(ON) of the FET can be calculated:

DS ON

CHR MAX

()

(11)

The thermal characteristics of the FET must be considered

next. The worst-case dissipation can be determined using:

UVLO

DISS

ADAPTER MAX

DIODE

SENSE

CHR

=−

−

−×

()

(12)

It should be noted that the adapter voltage can be either

preregulated or nonregulated. In the preregulated case, the

difference between the maximum and minimum adapter voltage

is probably not significant. In the unregulated case, the adapter

voltage can have a wide range specified. However, the maxi-

mum voltage specified is usually with no load applied. So, the

worst-case power dissipation calculation will often lead to an

overspecified pass device. In either case, it is best to determine

the load characteristics of the adapter to optimize the charger

design.

For example:

VADAPTER(MIN) = 5.0 V

VADAPTER(MAX) = 6.5 V

VDIODE = 0.5 V at 800 mA

VGATEDR = 0.5 V

VSENSE = 160 mV

VGS = 5 V – 0.5 V – 0.160 V = 4.3 V. So choose a low

threshold voltage FET.

ADAPTER MIN

DIODE

SENSE

BAT

()

50 50 160

140

(13)

DS ON

CHR MAX

()

140

800

175

Ω

(14)

UVLO

DISS

ADAPTER MAX

DIODE

SENSE

CHR

DISS

=−

−

−×

=−

−

×=

(

)

( ...

)

()

0160

(15)

Appropriate PMOS FETs are available from the following vendors:

• Siliconix

• IR

• Fairchild

Charger Diode Selection

The diode, D1, shown in Figure 2 is used to prevent the battery

from discharging through the PMOS’ body diode into the

charger’s internal bias circuits. A Schottky diode is recom-

mended to minimize the voltage difference from the charger to

the battery and the power dissipation. Choose a diode with a

current rating high enough to handle the battery charging cur-

rent and a voltage rating greater than VBAT. The blocking

diode is required for both lithium and nickel battery types.

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