ADP3801 datasheet(9/20 Pages) AD

9 / 20 page

ADP3801/ADP3802

–9–

REV. 0

Adjusting the Final Battery Voltage

In addition to the PROG input, the ADP3801/ADP3802 pro-

vides an input (ADJ) for fine adjustment of the final battery

voltage. For example, the ADJ amplifier allows the nominal

4.2 V per cell setting for Li-Ion battery cells to be adjusted to

4.1 V for certain chemistries. An internal amplifier buffers the

ADJ pin and adjusts the internal reference voltage on the input

to GM3. Figure 5 shows a graph of the percent change in final

battery voltage vs. the ADJ voltage. The linear portion between

0.6 VREF and 1.4 VREF follows the formula below:

∆V

BAT

REF

() =

−

100

The factor of four in the denominator is due to internal scaling.

When VADJ is above 2.5 V, an internal comparator switches off

the ADJ amplifier, giving a 0% change in VBAT. Whenever the

ADJ function is not used it should be connected to VL.

The total range of adjustment is

±10%. For example, the 4.2 V

final battery voltage setting can be adjusted from 3.78 V to

4.62 V. Of course, care must be taken not to adjust the final

battery voltage to an unsafe charging level for Li-Ion batteries.

Follow the battery manufacturers specifications for the appro-

priate final battery voltage. Never charge a Li-Ion battery above

the manufacturers rated maximum!

Voltage Loop Accuracy

The ADP3801/ADP3802 guarantees that the battery voltage be

within

±0.75% of the setpoint over the specified temperature

range and the specified charge current range. This inclusive

specification saves the designer the time and expense of having

to design-in additional high accuracy components such as a

reference and precision resistors.

To maintain the

±0.75% specification, the layout and design of

the external circuitry must be considered. The input impedance

of BATA and BATB is typically 265 k

Ω, so any additional im-

pedance on these inputs will cause an error. As a result, do not

add external resistors to the battery inputs. Furthermore, if the

output voltage is being used for other purposes, such as to sup-

ply additional circuitry, the current to this circuitry should be

routed separately from the sense lines to prevent voltage drops

due to impedance of the PC-board traces. In general, route the

sense lines as Kelvin connections as close to the positive termi-

nals of the battery as possible.

The same care must be given to the ground connection for the

ADP3801/ADP3802. Any voltage difference between the bat-

tery ground and the GND pin will cause an error in the charge

voltage. This error includes the voltage drop due to the ground

current of the part. Thus, the GND pin should have a thick

trace or ground plane connected as close as possible to the

battery’s negative terminal. Any current from additional cir-

cuitry should be routed separately to the supply return and not

share a trace with the GND pin.

Dual Battery Operation

The ADP3801/ADP3802 is designed to charge two separate

battery packs. These batteries can be of different chemistries

and have a different number of cells. At any given time, only

one of the two batteries is being charged. To select which bat-

tery is being monitored, and therefore which battery is being

charged, the ADP3801/ADP3802 includes a battery selector

mux. This two-channel mux is designed to be “break-before-

make” to ensure that the two batteries are not shorted together

momentarily when switching from one to the other. The A/B

input is a standard logic input, with a logic low selecting BATA

and a logic high selecting BATB. See the application in Figure

31 for more information.

Overvoltage Comparator

GM3 includes an overvoltage comparator. Its output bypasses

the COMP node to quickly reduce the duty cycle of the PWM

to 0% when an overvoltage event occurs. A second output is

connected to the COMP node and, with slower response, re-

duces the voltage on the COMP cap to provide a soft start re-

covery. The threshold of the comparator is typically 8% above

the final battery voltage. This comparator protects external

circuitry from any condition that causes the output voltage to

quickly increase. The most likely reason is if the battery is

suddenly removed while it is being charged with high current.

Figure 27 shows the transient response when the battery is

removed. Notice that the output voltage increases to the com-

parator trip point, but it is quickly brought under control and

held at the final battery voltage.

ADP3801 датащи(PDF) 9 Page - Analog Devices

ADP3801 датащи(HTML) 9 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

датащи скачать

ссылки URL