ADP3806 Analog Devices, Inc., ADP3806 Datasheet - Page 10

• Manufacturer Part Number: ADP3806
• Description: High-frequency Switch Mode Li-ion Battery Charger
• Manufacturer: Analog Devices, Inc.

ADP3806
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
voltage of 12.6 V or 16.8 V. The ADP3806 is available in three
versions, a selectable 12.525 V or 16.7 V output, a selectable
12.6 V or 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
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
ADP3806 provides flexibility in programming the charge
current over a wide range. An external resistor is used to sense
the charge current and this voltage is compared to a dc input
voltage. 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
BSTREG
0.1µF
100kΩ
7.0V
C10
R7
SD
2
V
LC
IN
2.2µF
C14
AGND
ADP3806
V
V
REF
CONTROL
R13
10Ω
CC
LOGIC
UVLO
BIAS
+ V
BST
REG
22µF
100nF
C15
C17
REG
6.0V
100nF
DRV
C9
FD56990A
+
–
V
REF
1/2 Q1
SD
REF
2.5V
BOOTSTRAPPED
SYNCHRONOUS
SW
NOTES
1
2
ADP3806, R11 = 412kΩ, R12 = 102kΩ, R14 = OPEN.
R7, OPEN IF LC FUNCTION IS NOT USED.
ADP3806-12.6, ADP3806-12.5: R11 = SHORT, R12 = OPEN;
DRIVER
IN
DRVL
C7
200pF
SYNC
DRVLSD
OSCILLATOR
PGND
180pF
CT
1/2 Q1
FD56990A
C6
+
–
DRVLSD
Figure 18. Typical Application
0.22µF
COMP
Rev. C | Page 10 of 16
–
+
22µH
L1
22µF
C8
C16
V
TH
–
R8
56Ω
CS+
+
g
g
m
m
R3
249Ω
+
2
+
–
1
–
40mΩ
22nF
AMP1
+
R
C13
V
CS
REF
CS–
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 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 18 has a differential input to amplify the
voltage drop across an external sense resistor R
common-mode range is from ground to V
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 resistor. During constant current (CC) mode, g
the voltage at the output of AMP1 to be equal to the external
voltage at the ISET pin. By choosing R
a wide range of charge currents can be programmed.
–
470nF
R4
249Ω
C1
I
SYS+
CHARGE
R1
2.2Ω
10mΩ
AMP2
412kΩ
–
R
SYS–
R12
0.1%
SS
+
1
=
R2
2.2Ω
12.6V/16.8V
25
SELECT
2.5V
C2
470nF
V
ISYS
R14
0Ω
×
BATSEL
REF
–
+
1
R
SYSTEM
DC/DC
CS
6.81kΩ
R5
7.5kΩ
ISET
BAT
LIMIT
R6
CS
R11
412kΩ
0.1%
and V
1
BATTERY
12.6V/16.8V
CC
, allowing current
ISET
CS
. The input
appropriately,
m
1 forces
(1)