REV. B

ADP3806

–9–

THEORY OF OPERATION

The ADP3806 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 volt-

age of 12.6 V or 16.8 V. The ADP3806 is available in three

versions, a selectable 12.525 V/16.7 V output, a selectable

12.6 V/16.8 V output, and an adjustable output. The adjustable

output can be programmed for a wide range of battery voltages

using two external precision resistors.

Another requirement for safely charging Li-Ion batteries is

accurate control of the charge current. The actual charge cur-

rent 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

ADP3806 provides flexibility in programming the charge cur-

rent over a wide range. An external resistor is used to sense the

charge current and this voltage is compared to a dc input volt-

age. 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 ADP3806 drives two external power NMOS

transistors for a simple, lower cost power stage.

The ADP3806 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.

Charge Current Control

AMP1 in Figure 1 has a differential input to amplify the voltage

drop across an external sense resistor RCS. The input common-

mode range is from ground to VCC, allowing current control in

short circuit and low dropout conditions. The gain of AMP1 is

internally set to 25 V/V for low voltage drop across the sense

of AMP1 to be equal to the external voltage at the ISET pin.

charge currents can be programmed.

I

V

R

CHARGE

REF

CS

=

¥

25

(1)

–

+

SELECT

12.6/16.8

BOOTSTRAPPED

SYNCHRONOUS

DRIVER

VIN

CS+

–

+

OSCILLATOR

L1

VREF + VREG

UVLO

BIAS

+

CS–

R3

249

COMP

REF

2.5V

AGND

BAT

SD

+

CT

SW

REG

6.0V

PGND

VCC

BST

BATTERY

12.6V/16.8V

7.0V

C10

0.1 F

DRVH

DRVL

SYS+

SYS–

ISYS

ISET

LOGIC

CONTROL

LC

SYNC

LIMIT

VREF

2.5V

SYSTEM

DC/DC

VREF

ADP3806

1/2 Q1

FD56990A

C15

22 F

C14

2.2 F

**R7

100k

*ADP3806-12.6, ADP3806-12.5: R11 = SHORT, R12 = OPEN;

ADP3806, R11 = 412k , R12 = 102k , R14 = OPEN.

**R7, OPEN IF LC FUNCTION IS NOT USED.

C17

100nF

C7

200pF

C6

180pF

R8

56

C8

0.22 F

*R11

412k

0.1%

*R12

102k

0.1%

R1

2.2

C13

22nF

C16

22 F

C9

100nF

1/2 Q1

FD56990A

22 H

40m

R4

249

R5

6.81k

R6

7.5k

R13

10

C2

470nF

C1

470nF

BSTREG

BATSEL

SD

IN DRVLSD

VTH

DRVLSD

*R14

0

R2

2.2

–

–

+

AMP2

–

+

–

–

+

gm1

gm2

10m

AMP1

Figure 1. Typical Application