ISL6255_06 Datasheet, PDF (14/22 Pages) Intersil Corporation – Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

ISL6255, ISL6255A
An external resistor divider from VREF sets the voltage at
VADJ according to:
V_VADJ
=
VREF × -------------------------^R-----^b---^o---^t--^_---^V----^A---^D----^J----^|-^|---⁵---¹----⁴---^k--------------------------
R_{top_VADJ || 514k + Rbot_VADJ || 514k}
where R_{bot_VADJ} and R_{top_VADJ} are external resistors at
VADJ.
To minimize accuracy loss due to interaction with VADJ's
internal resistor divider, ensure the AC resistance looking
back into the resistor divider is less than 25k.
Connect CELLS as shown in Table 1 to charge 2, 3 or 4 Li+
cells. When charging other cell chemistries, use CELLS to
select an output voltage range for the charger. The internal
error amplifier gm1 maintains voltage regulation. The voltage
error amplifier is compensated at VCOMP. The component
values shown in Figure 16 provide suitable performance for
most applications. Individual compensation of the voltage
regulation and current-regulation loops allows for optimal
compensation.
TABLE 1. CELL NUMBER PROGRAMMING
CELLS
CELL NUMBER
VDD
4
GND
3
Float
2
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 40mΩ 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:
I_CHG
=
1_----6_---5_----m_-----V_{--- -}^V_----C_----H_----L_---I_--M_---
R₁ 3.3V
To set the trickle charge current for the dumb charger, a
resistor in series with a switch Q6 (Figure 15) controlled by
the micro-controller is connected from CHLIM pin to ground.
The trickle charge current is determined by:
I_CHG
=
1_----6_---5_----m_-----V_{--- -}^V_----C_----H_----L_---I_--M_-----,_-t_--r_--i_--c_---k_---l_--e_-
R₁
3.3 V
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 R1 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 point.
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 ISL6255, ISL6255A 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 the following
equation
I_INPUT
=
1
R₂
⎜⎛ ^0.05
⎝ ^VREF
V_ACLIM
+ 0.050 ⎟⎞
⎠
An external resistor divider from VREF sets the voltage at
ACLIM according to:
V_ACLIM
=
VREF × --------------------------^R-----^b---^o----^t-^_---^A----^C----^L---^I-^M------^|-^|---¹---⁵----²---^k----------------------------
R_{top_ACLIM || 152k + Rbot_ACLIM || 152k}
where R_{bot_ACLIM} and R_{top_ACLIM} are external resistors at
ACLIM.
To minimize accuracy loss due to interaction with ACLIM's
internal resistor divider, ensure the AC resistance looking
back into the resistor divider is less than 25k.
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
14
FN9203.2
May 23, 2006