ISL6257

FN9288 Rev 2.00

Page 13 of 22

Jan 17, 2007

regulation and current-regulation loops allows for optimal

compensation.

Setting the Battery Charge Current Limit

The CHLIM input sets the maximum charging current. The

current set by the current sense-resistor connects between

CSOP and CSON. The full-scale differential voltage between

CSOP and CSON is 165mV for CHLIM = 3.3V, so the

maximum charging current is 4.125A for a 40m

 sensing

resistor. Other battery charge current-sense threshold values

can be set by connecting a resistor divider from VREF or 3.3V

to ground, or by connecting a low impedance voltage source

like a D/A converter in the micro-controller. Unlike VADJ and

ACLIM, CHLIM does not have an internal resistor divider

network. The charge current limit threshold is given by

Equation 3:

To set the trickle charge current for the dumb charger, an A/D

output controlled by the micro-controller is connected to

CHLIM pin. The trickle charge current is determined by

Equation 4:

When the CHLIM voltage is below 88mV (typical), it will disable

the battery charge. When choosing the current sensing

resistor, note that the voltage drop across the sensing resistor

causes further power dissipation, reducing efficiency. However,

adjusting CHLIM voltage to reduce the voltage across the

current sense resistor R11 will degrade accuracy due to the

smaller signal to the input of the current sense amplifier. There

is a trade-off between accuracy and power dissipation. A low

pass filter is recommended to eliminate switching noise.

Connect the resistor to the CSOP pin instead of the CSON pin,

as the CSOP pin has lower bias current and less influence on

current-sense accuracy and voltage regulation accuracy.

Setting the Input Current Limit

The total input current from an AC adapter, or other DC source,

is a function of the system supply current and the battery-

charging current. The input current regulator limits the input

current by reducing the charging current, when the input

current exceeds the input current limit set by ACLIM. System

current normally fluctuates as portions of the system are

powered up or down. Without input current regulation, the

source must be able to supply the maximum system current

and the maximum charger input current simultaneously. By

using the input current limiter, the current capability of the AC

adapter can be lowered, reducing system cost.

The ISL6257 limits the battery charge current when the input

current-limit threshold is exceeded, ensuring the battery

charger does not load down the AC adapter voltage. This

constant input current regulation allows the adapter to fully

power the system and prevent the AC adapter from

overloading and crashing the system bus.

An internal amplifier gm3 compares the voltage between CSIP

and CSIN to the input current limit threshold voltage set by

ACLIM. Connect ACLIM to REF, Float and GND for the full-

scale input current limit threshold voltage of 100mV, 75mV and

50mV, respectively, or use a resistor divider from VREF to

ground to set the input current limit as Equation 5:

When choosing the current sense resistor, note that the

voltage drop across this resistor causes further power

dissipation, reducing efficiency. The AC adapter current sense

accuracy is very important. Use a 1% tolerance current-sense

resistor. The highest accuracy of ±1.5% is achieved with

100mV current-sense threshold voltage for ACLIM = VREF, but

it has the highest power dissipation. For example, it has

400mW power dissipation for rated 4A AC adapter and 1W

sensing resistor may have to be used. ±2.5% and ±4.5%

accuracy can be achieved with 75mV and 50mV current-sense

threshold voltage for ACLIM = Floating and ACLIM = GND,

respectively.

A low pass filter is suggested to eliminate the switching noise.

Connect the resistor to CSIN pin instead of CSIP pin because

CSIN pin has lower bias current and less influence on the

current-sense accuracy.

TABLE 1. CELL NUMBER PROGRAMMING

CELLS

CELL NUMBER

VDD

4

GND

3

Float

2

I

CHG

165mV

R

1

-----------------



3.3V

---------------------





=

(EQ. 3)

I

CHG

165mV

R

1

-----------------



,

3.3V

---------------------------------------





=

(EQ. 4)

I

INPUT

1

R

2

------

0.05

VREF

---------------- V

ACLIM

0.05

+









=

V

ACLIM

VREF

R

bot ACLIM



152k





R

top ACLIM



152k





R

bot ACLIM



152k





+

------------------------------------------------------------------------------------------------------------------









=

(EQ. 5)