ADP3810/ADP3811

–7–

REV. 0

converter and causes the battery voltage to fall, balancing the

feedback loop.

Each GM stage is designed to be asymmetrical so that each am-

plifier can only source current. The outputs are tied together at

the COMP node and loaded with an internal constant current

sink of approximately 100

µA. Whichever amplifier sources

more current controls the voltage at the COMP node and there-

fore controls the feedback. This scheme is a realization of an

analog “OR” function where GM1 or GM2 has control of the

dc-dc converter and the charging circuitry. Whenever the cir-

cuit is in full current limiting or full voltage limiting, the respec-

tive GM stage sources an identical amount of current to the

fixed current sink. The other GM stage sources zero current

and is out of the loop. In the transition region, both GM stages

source some of the current to comprise the full amount of the

current sink. The high gains of GM1 and GM2 ensure a

smooth but sharp transition from current control to voltage con-

trol. Figure 24 shows a graph of the transition from current to

voltage mode, that was measured on the circuit in Figure 23 as

detailed below. Notice that the current stays at its full pro-

grammed level until the battery is within 200 mV of the final pro-

grammed voltage (10 V in this case), which maintains fast

charging through almost all of the battery voltage range. This

improves the speed of charging compared to a scheme that re-

duces the current at lower battery voltages.

The second element in a battery charging system is some form

of a dc-dc converter. To achieve high efficiency, the dc-dc con-

verter can be an isolated off-line switching power supply, or it

can be an isolated or nonisolated Buck or other type of switch-

ing power supply. For lower efficiency requirements, a linear

regulator fed from a wall adapter can be used. In the above dis-

pass transistor drive. Examples of these topologies are shown

later in this data sheet. If an off-line supply such as a flyback

converter is used, and isolation between the control logic and

the ADP3810/ADP3811 is required, an optocoupler can be in-

serted between the ADP3810/ADP3811 output and the control

input of the primary side PWM.

Charge Termination

If the system is charging a LiIon battery, the main criteria to de-

termine charge termination is the absolute battery voltage. The

ADP3810, with its accurate reference and internal resistors, ac-

complishes this task. The ADP3810’s guaranteed accuracy

specification of

±1% of the final battery voltage ensures that a

LiIon battery will not be overcharged. This is especially impor-

tant with LiIon batteries because overcharging can lead to cata-

strophic failure. It is also important to insure that the battery be

charged to a voltage equal to its optimal final voltage (typically

4.2 V per cell). Stopping at less than 1% of full-scale results in

a battery that has not been charged to its full mAh capacity,

reducing the battery’s run time and the end equipment’s operat-

ing time.

The ADP3810/ADP3811 does not include circuitry to detect

charge termination criteria such as –

∆V/∆t or ∆T/∆t, which are

common for NiCad and NiMH batteries. If such charge termi-

nation schemes are required, a low cost microcontroller can be

added to the system to monitor the battery voltage and tempera-

ture. A PWM output from the microcontroller can subsequently

tegrate a PWM output into a dc control voltage.

Compensation

The voltage and current loops have significantly different natu-

ral and crossover frequencies in a battery charger application, so

the two loops most likely need different pole/zero feedback com-

pensation. Figure 1 shows a single RC network from the

COMP node to ground. This is primarily for low frequency

COMP node is shared by both GM stages, this compensation

also affects the current loop. The internal 200

Ω resistor does

change the zero location of the compensation for the current

loop with respect to the voltage loop. To provide a separate

series RC may be needed. A detailed calculation of the com-

pensation values is given later in this data sheet.

ADP3810 and ADP3811 Differences

The main difference between the ADP3810 and the ADP3811

is illustrated in Figure 1. The resistors R1 and R2 are external

for the ADP3811 and internal for the ADP3810. The ADP3810

is specifically designed for LiIon battery charging, and thus, the

internal resistors are precision thin-film resistors laser trimmed

for LiIon cell voltages. Four different final voltage options are

available in the ADP3810: 4.2 V, 8.4 V, 12.6 V, and 16.8 V.

For slightly different voltages to accommodate different LiIon

chemistries, please contact the factory. The ADP3811 does not in-

clude the internal resistors, allowing the designer to choose any

final battery voltage by appropriately selecting the external resis-

tors. Because the ADP3810 is specifically for LiIon batteries,

the reference is trimmed to a tighter accuracy specification of

±1% instead of ±2% for the ADP3811.

level. This input is buffered by an internal single supply ampli-

resistor divider from the reference output. If a microcontroller

the voltage to be set by a PWM output from the micro. Of

course, a digital-to-analog converter could also be used, but the

high impedance input makes a PWM output the economical

flows out of the pin.

The guaranteed input voltage range of the buffer is from 0.0 V

put of the internal amplifier is fixed at 0.1 V. This corresponds

Ω, R3 = 20 kΩ.

relationship. Figure 1 shows a diode in series with the buffer’s

output and a 1.5 M

Ω resistor from V

diode prevents the amplifier from sinking current, so for small

input voltages the buffer has an open output. The 1.5 M

Ω

resistor forms a divider with the internal 80 k

Ω resistor to fix the

output at 0.1 V, i.e., about 10% of the maximum current. This

corresponds to the typical trickle charge current level for NiCad

current and the output follows the input. The total range of

Ω, R3 = 20 kΩ). Larger