ADP3801/ADP3802

–13–

REV. 0

Current Sense

The maximum charging current is specified by the battery

manufacturer as 4.0 A. To avoid losing excessive power on the

current-sense resistor, it is advisable to keep the voltage drop

across the resistor at maximum current to 160 mV or below.

power rating can be calculated using the data sheet specification

for the Overcurrent Comparator. The overcurrent protection is

specified at 4.9 A when using a 40 m

Ω resistor; therefore, the

2

× 0.04 = 0.96 W. Thus a

1.0 W or higher power rated resistor should be used. Two

2.2 nF capacitors are connected from the CS+ and CS– inputs

to ground to filter out high frequency switching noise.

ISET Programming Voltage

This voltage programs the charge current based on the above

programming at the ISET input of 0.1 V/V, we need:

V

RI

VV

A

VV

V

ISET

CS

CHARGE

=

×

=

×

=

01

004

4 0

01

16

./

..

./

.

Ω

The 1.6 V can be obtained from the 3.3 V LDO by a resistor

divider of 20 k

Ω and 22 kΩ.

PROG Voltage

Next, the PROG voltage has to be determined to set the proper

batteries in series (12.6 V) is between 2.05 V and 2.3 V. A 2.2 V

input can be obtained from the 3.3 V LDO by a resistor divider

of 66.5 k

Ω and 33.2 kΩ.

ADJ Voltage

Since no further adjustment of the final battery voltage is re-

quired, this pin is tied to the VL pin, which disables the internal

amplifier.

Output Voltage and Duty Cycle

as follows:

V

VD

V

T

T

O

IN

IN

ON

=

×

=

×

×

100

100

In the above equation,

D is the maximum duty cycle of the

converter in percentage, and

for external voltage drops and the common-mode input range of

the current sense amplifier.

For

For

Buck Inductor

The inductor value can be calculated after determining the

allowable amount of inductor ripple current. For continuous

buck operation, and considering low cost inductor core materi-

als and acceptable core losses at 200 kHz, the usual peak-to-

the maximum input voltage of 20 V:

T

D

f

kHz

s

OFFMAX

MAX

OSC

=

−

×

=

−

×

=

100

100

100

63

200

100

19

.

µ

This gives an inductor value of:

L

VT

I

Vs

A

H

OMAX

OFFMAX

RPP

>

×

=

×

=

12 6

1 9

10

24

..

.

µ

µ

The max inductor peak current is calculated as follows:

The max inductor rms current is calculated (where 0.577 is the

conversion factor for a peak to RMS value):

I

VT

L

s

H

A

LRMS

O

OFF

=

××

×

=

××

×

=

0 577

0 5

0 577

0 5 12 6 1 9

24

03

..

..

.

.

.

µ

µ

An appropriate inductor is the Coiltronix UP4B330, which is

specified at 33

µH and can carry the 4.5 A current with about a

20

°C temperature rise. For the ADP3802, the above formulas

give:

PFET Selection and Thermal Design

We have to consult the available P-channel MOSFET (PFET)

transistor selection charts for switch-mode power supply appli-

cations to find a PFET in the desired package whose Safe

requirements with acceptable margin. For this application, the

Temic Si4463 was selected in an SO-8 package. This transistor

margin.

Since the switching losses are negligible for properly driven

PFETs compared to conduction losses, the worst-case conduc-

tion losses can be estimated from the worst case ON resistance

current at the minimum input voltage and close to 100% duty

worst case value we can use is 0.023

Ω. The maximum PFET

dissipation is calculated as follows:

2

× R

2

× 0.023 Ω = 0.47W

can be calculated:

where

transistor’s data sheet for a 1 inch square PCB-pad. The calcu-

lated

temperature of the transistor with adequate margin. The

than adequate margin.