REV. 0

ADP3408

–13–

Applying an external charger can also turn the handset on. This

will turn on all the LDOs, except the SIM LDO, again starting

up the baseband processor. Note that if the battery voltage is

below the undervoltage lockout threshold, applying the adapter

will not start up the LDOs.

Deep Discharge Lockout (DDLO)

The DDLO block in the ADP3408 has two functions:

• To shut off the VRTC LDO in the event that the main battery

discharges to below the RTC LDO’s output voltage. This will

force the real-time clock to run off the backup coin cell or

double layer capacitor.

• To shut down the handset in the event that the software fails

to turn off the phone when the battery drops below 2.9 V to

3.0 V. The DDLO will shut down the handset when the

battery falls below 2.4 V to prevent further discharge and

damage to the cells.

Undervoltage Lockout (UVLO)

The UVLO function in the ADP3408 prevents startup when the

initial voltage of the battery is below the 3.2 V threshold. If the

battery voltage is this low with no load, there is insufficient

capacity left to run the handset. When the battery is greater than

3.2 V, such as inserting a fresh battery, the UVLO comparator

trips, and the threshold is reduced to 3.0 V. This allows the

handset to start normally until the battery decays to below

3.0 V. Note that the DDLO has enabled the RTC LDO under

this condition.

Once the system is started, and the core and memory LDOs are

up and running, the UVLO function is disabled. The ADP3408

is then allowed to run until the battery voltage reaches the

DDLO threshold, typically 2.4 V. Normally, the battery voltage

is monitored by the baseband processor and usually shuts off the

phone at around 3.0 V.

If the handset is off, and the battery voltage drops below 3.0 V,

the UVLO circuit disables startup and puts the ADP3408 into

UVLO shutdown mode. In this mode the ADP3408 draws very

low quiescent current, typically 30

µA. The RTC LDO is still

running until the DDLO disables it. In this mode the ADP3408

draws 5

µA of quiescent current. NiMH batteries can reverse

polarity if the three-cell battery voltage drops below 3.0 V which

will degrade the batteries’ performance. Lithium ion batteries

will lose their capacity if repeatedly overdischarged, so minimizing

the quiescent currents helps prevent battery damage.

RESET

The ADP3408 contains a reset circuit that is active at both

power-up and power-down. The RESET pin is held low at

initial power-up. An internal power good signal is generated by

the core LDO when its output is up, which starts the reset delay

timer. The delay is set by an external capacitor on RESCAP:

t

ms

nF

C

RESET

RESCAP

=×

12

.

(1)

At power-off, RESET will be kept low to prevent any baseband

processor starts.

Over-Temperature Protection

The maximum die temperature for the ADP3408 is 125

°C. If

the die temperature exceeds 160

°C, the ADP3408 will disable

all the LDOs except the RTC LDO. The LDOs will not be

This ensures that the handset will always power-off before the

ADP3408 exceeds its absolute maximum thermal ratings.

Battery Charging

The ADP3408 battery charger can be used with Lithium Ion

(Li+) and Nickel Metal Hydride (NiMH) batteries. The charger

initialization, trickle charging, and Li+ charging are imple-

mented in hardware. Battery type determination and NiMH

charging must be implemented in software.

The charger block works in three different modes:

• Low Current (Trickle) Charging

• Lithium Ion Charging

• Nickel Metal Hydride Charging

Charge Detection

The ADP3408 charger block has a detection circuit that deter-

mines if an adapter has been applied to the CHRIN pin. If the

adapter voltage exceeds the battery voltage by 90 mV, the

CHRDET output will go high. If the adapter is then removed

and the voltage at the CHRIN pin drops to only 45 mV above

the BATSNS pin, CHRDET goes low.

Trickle Charging

When the battery voltage is below the UVLO threshold, the

charge current is set to the Low Current Limit, or about 10% of

the full charge current. The low current limit is determined by

the voltage developed across the current sense resistor. There-

fore, the trickle charge current can be calculated by:

I

mV

R

CHR TRICKLE

SENSE

() =

20

(2)

Trickle charging is performed for deeply discharged batteries

to prevent undue stress on either the battery or the charger.

Trickle charging will continue until the battery voltage exceeds

the UVLO threshold.

Once the UVLO threshold has been exceeded the charger will

switch to the high current limit, the LDOs will start up, and the

baseband processor will start to run. The processor must then

poll the battery to determine which chemistry is present and set

the charger to the proper mode.

Lithium Ion Charging

For lithium ion charging, the CHGEN input must be low. This

allows the ADP3408 to continue charging the battery at the full

current. The full charge current can be calculated by using:

I

mV

R

CHR FULL

SENSE

() =

160

(3)

If the voltage at BATSNS is below the charger’s output voltage

of 4.2 V, the battery will continue to charge in the constant

current mode. If the battery has reached the final charge voltage,

a constant voltage is applied to the battery until the charge

current has reduced to the charge termination threshold. The

charge termination threshold is determined by the voltage across

the sense resistor. If the battery voltage is above 4.0 V and the

voltage across the sense resistor has dropped to 14 mV, an End-

of-Charge signal is generated and the EOC output goes high. See

Figure 6.