ADP3801/ADP3802

–7–

REV. 0

APPLICATIONS SECTION

PRODUCT DESCRIPTION

The ADP3801 and ADP3802 are complete Li-Ion battery charg-

ing ICs. Combined with a microcontroller, they also function as

voltage limited,

µC programmable constant current source chargers

for NiCad and NiMH chemistries. Utilizing an external PMOS

pass transistor, the devices realize a buck type constant current,

constant voltage (CCCV) charger controller that is capable of

charging two separate battery packs for such applications as

portable computer chargers and cellular phone chargers. The

Functional Block Diagram shows the ICs’ functional blocks,

which are detailed below:

• A/B SELECT MUX—Two-channel multiplexer for charging

two battery stacks.

• FINAL BATTERY VOLTAGE PROGRAM—Multiplexer to

program 4.2 V, 8.4 V, or 12.6 V final battery voltage.

• VOLTAGE LOOP AMP—GM-type amplifier to control

the final battery voltage. It includes a built-in overvoltage

comparator.

• EOC COMPARATOR—End-of-charge detection output to

signal when the battery is fully charged.

• BATTERY VOLTAGE ADJUST—Amplifier to adjust the

final battery voltage up to

±10%.

• CURRENT LOOP AMP—High-side-current-sense amplifier

to sense and control the charge current at a programmable

level. It includes an overcurrent comparator.

• PWM—Pulsewidth modulator and oscillator (ADP3801-

200 kHz, ADP3802-500 kHz).

• GATE DRIVE—Gate drive to control an external pass tran-

sistor. It includes a clamp to limit the drive voltage to protect

the external PMOS.

• LDO + REFERENCE—3.3 V low dropout regulator to sup-

ply an external microcrontroller and for on-chip supply. In-

cludes an internal precision reference ( VREF = VL/2).

• SHUTDOWN—Logic input to shut down the charger. The

LDO remains on.

• UVLO—Undervoltage lockout circuit to shut down the charger

for low supply voltages.

• RESET—Active LOW output to reset external logic on power-

up.

During charging, the ADP3801/ADP3802 maintains a constant,

programmable charge current. The high side current sense

amplifier has low offset allowing the use of a low voltage drop

for current sensing: 165 mV for the maximum charge current.

The input common-mode range extends from ground to

VCC – 2 V ensuring current control over the full charging volt-

age of the battery, including a short circuit condition. A high

impedance dc voltage input (ISET) is provided for program-

ming the charge current over a wide range. When the battery

voltage approaches its final limit, the part automatically trans-

fers to voltage control mode. Both the current control loop and

the voltage control loop share the same compensation pin mini-

mizing the number of external components. An internal com-

parator monitors the charge current to detect the end-of-charge

EOC output pulls low.

A 3.3 V LDO is used to generate a regulated supply for internal

circuitry. Additionally, the LDO can deliver up to 10 mA of

current to power external circuitry such as a microcontroller. An

Undervoltage Lockout (UVLO) circuit is included to safely shut

down the charging circuitry when the input voltage drops below

its minimum rating. A shutdown pin is also provided to turn off

the charger when, for example, the battery has been fully charged.

The LDO remains active during shutdown or UVLO and has a

quiescent current of 110

µA.

Battery Charging Overview

Figure 20 shows a simplified Buck type battery charger applica-

tion circuit for the ADP3801/ADP3802. When a discharged

battery is first placed in the charger, the battery voltage is well

below the final charge voltage, so the current sense amplifier

controls the charge loop in constant current mode. The charge

voltage drop is buffered and amplified by amplifier GM1. Am-

plifier GM2 compares the output of GM1 to an external current

control voltage provided at the ISET pin and servos the charger

loop to make these voltages equal. Thus, the charge current is

programmed using the ISET input voltage.

The output of GM2 is analog “OR’ed” with the output of GM3,

the voltage loop amplifier. Only one or the other amplifier con-

trols the charge loop at any given time. As the battery voltage

approaches its final voltage, GM3 comes into balance. As this

occurs, the charge current decreases, unbalancing GM2, and

control of the feedback loop naturally transfers to GM3.

The ADP3801/ADP3802 can control the charging of two inde-

pendent battery stacks or a single battery stack. The A/B SELECT

MUX has a logic input to choose between the two batteries. See

Figure 31 for more information on dual battery charging. The

output of the multiplexer is applied to a precision thin-film

resistor string to divide down the battery voltage. The final

battery voltage is chosen by selecting the proper resistor divider

tap with the PROG multiplexer. The output of this mux goes

directly to the input of GM3, comparing the divided down

battery voltage to the internal reference. To guarantee

±0.75%

accuracy, a high precision internal reference and high accuracy

thin film resistors are used. Including these components on-

chip saves the significant cost and design effort of adding them

externally.