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ADP3808

High Efficiency Switch Mode Li-Ion Battery Charger

Analog Devices 

ADP3808

THEORY OF OPERATION

The ADP3808 combines a bootstrapped synchronous switching

driver with programmable current control and accurate final

battery voltage control in a constant-current, constant-voltage

(CCCV) Li-Ion battery charger. High accuracy voltage control

is needed to safely charge Li-Ion batteries, which are typically

specified at 4.2 V ± 1% per cell. For a typical notebook computer

battery pack, three or four cells are in series, giving a total

voltage of 12.6 V or 16.8 V. The ADP3808 allows the final

battery voltage to be programmed. The programmable range is

4.0 V to 4.5 V per cell. The total number of cells to be charged

can be set to either 3 or 4 via a control pin.

Another requirement for safely charging Li-Ion batteries is

accurate control of the charge current. The actual charge current

depends on the number of cells in parallel within the battery

pack. Typically, this is in the range of 2 A to 3 A. The ADP3808

provides flexibility in programming the charge current over a

wide range. An external resistor is used to sense the charge

current. The charge current can be set by programming the

sense resistor voltage drop. The voltage drop can be set to a

maximum of 96 mV. This programmability allows the current

to be changed during charging. For example, the charge current

can be reduced for trickle charging.

The synchronous driver provides high efficiency when charging

at high currents. Efficiency is important mainly to reduce the

amount of heat generated in the charger, but also to stay within

the power limits of the ac adapter. With the addition of a boot-

strapped high-side driver, the ADP3808 drives two external

power NMOS transistors for a simple, lower cost power stage.

The ADP3808 also provides an uncommitted current sense

amplifier. This amplifier provides an analog output pin for

monitoring the current through an external sense resistor. The

amplifier can be used anywhere in the system that high-side

current sensing is needed. The sense amplifier output is compared

to a programmable voltage limit. If the limit is exceeded, the

LIMIT pin is asserted. The system sense amplifier is also used

to detect the presence of an ac adaptor. If the adaptor is detected,

the ADP3808 asserts a logic pin to signal the detection.

SETTING THE CHARGE CURRENT

The charge current is measured across an external sense

resistor, RCS, between the CSP and CSM pins. The input

common-mode range is from ground to VCC, allowing current

control in short-circuit and low dropout conditions. The voltage

between CSP and CSM is programmed by a ratio of the voltages

at CSADJ and REFIN according to Equation 1.

VCSP

− VCSM

= 96 mV CSADJ

REFIN

(1)

For example, using a 20 mΩ sense resistor gives a range from

150 mA with CSADJ = REFIN/32 to 4.8 A maximum when

CSADJ = REFIN.

The power dissipation in RCS should be kept below 500 mW.

Components R4 and C13 provide high frequency filtering for

the current sense signal.

Rev. 0 | Page 10 of 16

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