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ADP3808AJPZ

High Efficiency Switch Mode Li-Ion Battery Charger

ON Semiconductor 

ADP3808A

Theory of Operation

The ADP3808A 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 ADP3808A 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 2.0 A to 3.0A.

The ADP3808A 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 bootstrapped high-side driver, the ADP3808A

drives two external power NMOS transistors for a simple,

lower cost power stage.

The ADP3808A 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 ADP3808A 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

(eq. 1)

For example, using a 20 mW 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 in Figure 15 provide high

frequency filtering for the current sense signal.

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