ADP3408

REV. 0

–14–

ICHG

VBAT

EOC

TIME

Figure 6. End of Charge

The baseband processor can either let the charger continue to

charge the battery for an additional amount of time or terminate

the charging. To terminate the charging, the processor must pull

the GATEIN and CHGEN pins high.

NiMH Charging

For NiMH charging, the processor must pull the CHGEN pin

high. This disables the internal Li+ mode control of the gate

drive pin. The gate drive must now be controlled by the base-

band processor. By pulling GATEIN high, the GATEDR pin is

driven high, turning the PMOS off. By pulling the GATEIN pin

low, the GATEDR pin is driven low, and the PMOS is turned

on. So, by pulsing the GATEIN input, the processor can charge

a NiMH battery. Note that when charging NiMH cells, a cur-

rent-limited adapter is required.

During the PMOS off periods, the battery voltage needs to be

monitored through the MVBAT pin. The battery voltage is

continually polled until the final battery voltage is reached, at

which time the charge can either be terminated or the frequency

of the pulsing reduced. An alternative method of determining

the end of charge is to monitor the temperature of the cells and

terminate the charging when a rapid rise in temperature is detected.

Battery Voltage Monitoring

The battery voltage can be monitored at MVBAT during

charging and discharging to determine the condition of the

battery. An internal resistor divider can be connected to BATSNS

when both the digital and analog baseband sections are pow-

ered up. To enable MVBAT both PWRONIN and TCXOEN

must be high.

The ratio of the voltage divider is selected so that the 2.4 V

maximum input of the AD6521’s auxiliary ADC will correspond

with the maximum battery voltage of 5.5 V. The divider will be

disconnected from the battery when the baseband sections are

powered down.

APPLICATION INFORMATION

Input Capacitor Selection

For the input (VBAT, VBAT2, and VRTCIN) of the ADP3408,

a local bypass capacitor is recommended. Use a 10

µF, low

ESR capacitor. Multilayer ceramic chip (MLCC) capacitors

provide the best combination of low ESR and small size but

may not be cost effective. A lower cost alternative may be to use

a 10

µF tantalum capacitor with a small (1 µF to 2 µF) ceramic

in parallel.

Separate inputs for the SIM LDO and the RTC LDO are supplied

for additional bypassing or filtering. The SIM LDO has VBAT2

as its input and the RTC LDO has VRTCIN.

LDO Capacitor Selection

The performance of any LDO is a function of the output capacitor.

The core, memory, SIM, and analog LDOs require a 2.2

µF

capacitor and the TCXO LDO requires a 0.22

µF capacitor.

Larger values may be used, but the overshoot at startup will

increase slightly. If a larger output capacitor is desired, be sure

to check that the overshoot and settling time are acceptable for

the application.

All the LDOs are stable with a wide range of capacitor types and

ESR (anyCAP

low ESR capacitors (ESR ~ 0), such as Multilayer Ceramic

Capacitors (MLCC), but care should be taken in their selection.

Note that the capacitance of some capacitor types show wide

variations over temperature or with dc voltage. A good quality

dielectric, X7R or better, capacitor is recommended.

The RTC LDO can have a rechargeable coin cell or an electric

double-layer capacitor as a load, but an additional 0.1

µF ceramic

capacitor is recommended for stability and best performance.

RESET Capacitor Selection

RESET is held low at power-up. An internal power-good signal

starts the reset delay when the core LDO is up. The delay is set

by an external capacitor on RESCAP:

t

ms

nF

C

RESET

RESCAP

=×

12

.

(4)

A 100 nF capacitor will produce a 120 ms reset delay. The

current capability of RESET is minimal (a few hundred nA)

when VCORE is off to minimize power consumption. When

VCORE is on, RESET is capable of driving 500

µA.

Setting the Charge Current

The ADP3408 is capable of charging both Lithium Ion and

NiMH batteries. For NiMH batteries, the charge current is

limited by the adapter. For Lithium Ion batteries, the charge

current is programmed by selecting the sense resistor, R1.

The Lithium Ion charge current is calculated using:

I

V

R

mV

R

CHR

SENSE

==

1

160

1

(5)

the charge current is known, R1 can be found.

R

V

I

mV

I

SENSE

CHR

CHR

1

160

==

(6)

Similarly the trickle charge current and the end of charge cur-

rent can be calculated:

II

V

R

mV

R

TRICKLE

EOC

SENSE

==

=

1

20

1

(7)

Example: Assume an 800mA-H capacity Lithium Ion battery

and an 1C charge rate. R1 = 200 m

Ω, I

anyCAP is a registered trademark of Analog Devices Inc.