LTC4100 datasheet(36/48 Pages) LINER

36 / 48 page

LTC1760

1760fa

APPLICATIONS INFORMATION

Figure 9.

100mV

–

5kΩ

CLP

DCIN

1760 F09

0.1μF

RCL*

CIN

VIN

CL1

AC ADAPTER

INPUT

*RCL =

100mV

ADAPTER CURRENT LIMIT

Automatic Current Sharing

In a dual parallel charge configuration, the LTC1760 does

not actually control the current flowing into each individual

battery. The capacity, or Amp-Hour rating, of each battery

determines how the charger current is shared. This auto-

matic steering of current is what allows both batteries to

reach their full capacity points at the same time. In other

words, given all other things equal, charge termination

will happen simultaneously.

A battery can be modeled as a huge capacitor and hence

governed by the same laws.

I = C • (dV/dt) where:

I = The current flowing through the capacitor

C = Capacity rating of battery (using amp-hour values

instead of capacitance)

dV = Change in voltage

dt = Change in time

The equivalent model of a set or parallel batteries is a

set of parallel capacitors. Since they are in parallel, the

change in voltage over change in time is the same for both

batteries 1 and 2.

dV/dtBAT1 = dV/dtBAT2

From here we can simplify.

IBAT1/CBAT1 = dV/dt = IBAT2/CBAT2

IBAT2 = IBAT1 CBAT2/CBAT1

At this point you can see that the current divides as the

ratio of the two batteries capacity ratings. The sum of the

current into both batteries is the same as the current being

supply by the charger. This is independent of the mode of

the charger (CC or CV).

ICHRG = IBAT1 + IBAT2

From here we solve for the actual current for each battery.

IBAT2 = ICHRG CBAT2/(CBAT1 + CBAT2)

IBAT1 = ICHRG CBAT1/(CBAT1 + CBAT2)

Please note that the actual observed current sharing will

vary from manufacturer’s claimed capacity ratings since

it is actual physical capacity rating at the time of charge.

Capacity rating will change with age and use and hence

the current sharing ratios can change over time.

Adapter Limiting

An important feature of the LTC1760 is the ability to auto-

matically adjust charging current to a level which avoids

overloading the wall adapter. This allows the product to

operate at the same time that batteries are being charged

without complex load management algorithms. Addition-

ally, batteries will automatically be charged at the maximum

possible rate of which the adapter is capable.

This feature is created by sensing total adapter output cur-

rent and adjusting charging current downward if a preset

adapter current limit is exceeded. True analog control is

used, with closed loop feedback ensuring that adapter load

current remains within limits. Amplifier CL1 in Figure 9

senses the voltage across RCL, connected between the

CLP and DCIN pins. When this voltage exceeds 100mV,

the amplifier will override programmed charging current

to limit adapter current to 100mV/RCL. A lowpass filter

formed by 5kΩ and 0.1μF is required to eliminate switch-

ing noise. If the current limit is not used, CLP should be

connected to DCIN.

Setting Input Current Limit

To set the input current limit, you need to know the mini-

mum wall adapter current rating. Subtract 5% for the input

current limit tolerance and use that current to determine

the resistor value.

RCL = 100mV/ILIM

ILIM = Adapter Min Current

– (Adapter Min Current • 5%)

LTC4100 数据表(PDF) 36 Page - Linear Technology

LTC4100 数据表(HTML) 36 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

数据表下载