ADP3806JRUZ-REEL7 View Datasheet(PDF) - Analog Devices

Part Name

Description

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ADP3806JRUZ-REEL7

High Frequency Switch Mode Li-Ion Battery Charger

Analog Devices 

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:

• 12.525 V/16.7 V as selectable fixed voltages

• 12.6 V/16.8 V as selectable fixed voltages

• Adjustable

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:

R11 = ⎜⎛ VBAT ⎟⎞ − 1

(5)

R12 ⎝ 2.5 ⎠

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

L1 =

VIN,MAX −

ΔI

VBAT

× DMIN

× TS

(6)

where the maximum input voltage VIN, MAX is used with the

minimum duty ratio DMIN. The duty ratio is defined as the ratio

of the output voltage to the input voltage, VBAT/VIN. The ripple

current is found from

ΔI = 0.3× I BAT ,MAX

(7)

Where the maximum peak-to-peak ripple is 30%, that is 0.3,

and maximum battery current, IBAT, MAX, is used.

For example, with VIN, MAX = 19 V, VBAT = 12.6 V, IBAT, MAX = 3A,

and TS = 4 μs, the value of L1 is calculated as 18.9 μH. Choosing

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

Irms = VIN ,MAX × D × (1 − D)

(8)

f × L1 × 12

The maximum value occurs when the duty cycle is 0.5. Thus

I rms _ MAX

= 0.072×VIN, MAX

f × L1

(9)

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

( ) Irms =

PBAT

D× 1−D

×

(10)

η × D ×VIN

D

where:

η is the estimated converter efficiency (approximately 90%, 0.9).

PBAT is the maximum battery power consumed.

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.

Rev. C | Page 14 of 16

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