ADP3806 Analog Devices, Inc., ADP3806 Datasheet - Page 14

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ADP3806

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

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ADP3806
APPLICATION INFORMATION
DESIGN PROCEDURE
Refer to Figure 18, the typical application circuit, for the
following description. The design follows that of a buck
converter. With Li-Ion cells, it is important to have a regulator
with accurate output voltage control.
BATTERY VOLTAGE SETTINGS
The ADP3806 has three options for voltage selection:
When using the fixed versions, R11 should be a short or 0 Ω
wire jumper and R12 should be an open circuit. When using the
adjustable version, the following equation gives the ratio of the
two resistors:
Often 0.1% resistors are required to maintain the overall
accuracy budget in the design.
Inductor Selection
Usually the inductor is chosen based on the assumption that the
inductor ripple current is ±15% of the maximum output dc
current at maximum input voltage. As long as the inductor has
a value close to this, the system should work fine. The final
choice affects the trade-offs between cost, size, and efficiency.
For example, if the inductance is lower, the size is smaller but
ripple current is higher. This situation, if taken too far, leads to
higher ac losses in the core and the windings. Conversely, a
higher inductance results in lower ripple current and smaller
output filter capacitors, but the transient response isslower.
With these considerations, the required inductance can be
found from
where the maximum input voltage V
minimum duty ratio D
of the output voltage to the input voltage, V
current is found from
L1
12.525 V/16.7 V as selectable fixed voltages
12.6 V/16.8 V as selectable fixed voltages
Adjustable
Δ
R12
R11
=
I
V
=
IN
=
,
0 ×
MAX
3 .
Δ
V
I
2
BAT
I
5 .
V
BAT
BAT
MIN
,
MAX
×
. The duty ratio is defined as the ratio
1
D
MIN
×
T
S
IN, MAX
is used with the
BAT
/V
IN
. The ripple
Rev. C | Page 14 of 16
(6)
(5)
(7)
Where the maximum peak-to-peak ripple is 30%, that is 0.3,
and maximum battery current, I
For example, with V
and T
the closest standard value gives L1 = 22 μH.
Output Capacitor Selection
An output capacitor is needed in the charger circuit to absorb
the switching frequency ripple current and smooth the output
voltage. The rms value of the output ripple current is given by
The maximum value occurs when the duty cycle is 0.5. Thus
For an input voltage of 19 V and a 22 μH inductance, the
maximum rms current is 0.26 A. A typical 10 μF or 22 μF
ceramic capacitor is a good choice to absorb this current.
Input Capacitor Ripple
As is the case with a normal buck converter, the pulse current at
the input has a high rms component. Therefore, since the input
capacitor has to absorb this current ripple, it must have an
appropriate rms current rating. The maximum input rms
current is given by
where:
η is the estimated converter efficiency (approximately 90%, 0.9).
P
This is a worst-case calculation and, depending on total charge
time, the calculated number could be relaxed. Consult the
capacitor manufacturer for further technical information.
Decoupling the VCC Pin
It is a good idea to use an RC filter (R13 and C14) from the
input voltage to the IC to filter out switching noise and to
supply bypass to the chip. During layout, this capacitor should
be placed as close to the IC as possible. Values between 0.1 μF
and 2.2 μF are recommended.
BAT
is the maximum battery power consumed.
Irms
I
Irms
S
rms
= 4 μs, the value of L1 is calculated as 18.9 μH. Choosing
_
=
MAX
=
η
×
f
=
P
D
BAT
×
. 0
×
V
V
IN, MAX
L1
072
IN
IN
,
MAX
×
×
×
= 19 V, V
V
D
12
f
IN
×
×
,
D
(
BAT, MAX
MAX
1
×
L1
D
BAT
D
×
)
, is used.
= 12.6 V, I
(
1
D
)
BAT, MAX
= 3A,
(10)
(8)
(9)