ADP3810AR-12.6 Просмотр технического описания (PDF) - Analog Devices

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ADP3810AR-12.6

Secondary Side, Off-Line Battery Charger Controllers

Analog Devices 

ADP3810/ADP3811

current loop, the higher louT reduces the duty cycle of the dc-dc

converter and causes the battery voltage to fall, balancing the

feedback loop.

program the VCTRLinput to set the charge current. The high

impedance of VCTRLenables the inclusion of an RC filter to in-

tegrate a PWM output into a dc control voltage.

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

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

the CaMP node and loaded with an internal constant current

The voltage and current loops have significantly different natu-

ral and crossover frequencies in a battery charger application, so

sink of approximately 100 IJA. Whichever amplifier sources

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

more current controls the voltage at the CaMP node and there- pensation. Figure I shows a single RC network from the

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

CaMP node to ground. This is primarily for low frequency

analog "OR" function where GMI or GM2 has control of the

compensation (fc< 100 Hz) of the voltage loop. Since the

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

CaMP node is shared by both GM stages, this compensation

cuit is in full current limiting or full voltage limiting, the respec- also affects the current loop. The internal 200 Q resistor does

tive GM stage sources an identical amount of current to the

change the zero location of the compensation for the current

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 GMI 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

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

B grammed leveluntil the battery is within 200 mV of the final pro-

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

S charging through almost all of the battery voltage range. This

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

O duces the current at lower battery voltages.

L The second element in a battery charging system is some form

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

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

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

TE ing power supply. For lower efficiency requirements, a linear

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

higher frequency compensation (fc I kHz-I 0 kHz), a second

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 ADP381 0 and the ADP3811

is illustrated in Figure I. The resistors RI and R2 are external

for the ADP3811 and internal for the ADP3810. The ADP3810

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

internal resistors are precision thin-film resistors laser trimmed

for Lilon 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 ADP381 0 is specifically for Lilon batteries,

the reference is trimmed to a tighter accuracy specification of

regulator fed from a wall adapter can be used. In the above dis- :tl % instead of:t2% for the ADP3811.

cussion, the current, loUT,controls the duty cycle of a switching

supply; but in the case of the linear regulator, louT controls the

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 ADP38 10/ADP38 I I output and the control

input of the primary side PWM.

VCTRLInput and Charge Current Programming Range

The voltage on the VCTRLinput determines the charge current

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

fier (labeled BUFFER) to allow easy programmability of VCTRL.

For example, for a fixed charge current, VCTRLcan be set by a

resistor divider from the reference output. If a microcontroller

is setting the charge current, a simple RC filter on VCTRLenables

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

Charge Termination

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

If the system is charging a Lilon battery, the main criteria to de- high impedance input makes a PWM output the economical

termine charge termination is the absolute battery voltage. The choice. The bias current of VCTRLis typically 25 nA, which

ADP3810, with its accurate reference and internal resistors, ac- flows out of the pin.

complishes this task. The ADP3810's guaranteed accuracy

specification of :t I % of the final battery voltage ensures that a

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

tant with Lilon 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 I % 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 guaranteed input voltage range of the buffer is from 0.0 V

to 1.2 V. When VCTRLis in the range of 0.0 V to 0.1 V, the out-

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

= to a charge current of 100 mA for Res 0.25Q, R3 = 20kQ.

The graph of charge current versus VCTRLin Figure 7 shows this

relationship. Figure I shows a diode in series with the buffer's

output and a 1.5 MQ resistor from VREFto this output. The

diode prevents the amplifier from sinking current, so for small

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

resistor forms a divider with the internal 80 kQ resistor to fix the

The ADP3810/ADP3811 does not include circuitry to detect

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

charge termination criteria such as -!::'v/!:J.t or !:J.T/!:J.t,which are

corresponds to the typical trickle charge current level for NiCad

common for NiCad and NiMH batteries. If such charge termi- batteries. When VCTRLrises above 0.1 V, the buffer sources

nation schemes are required, a low cost micro controller can be

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

added to the system to monitor the battery voltage and tempera- VCTRLfrom 0.0 V. to 1.2 V results in a charge current range

ture. A PWM output from the microcontroller can subsequently

= from 100 mA to 1.2 A (for Res 0.25 Q, R3 = 20 kQ). Larger

REV. 0

-7-

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