ISL6256_14 Datasheet, PDF (15/26 Pages) Intersil Corporation – Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

ISL6256, ISL6256A
Charge Current Limit Accuracy
The “Electrical Specifications” table on page 7 gives
minimum and maximum values for the CSOP-CSON voltage
resulting from IC variations at 3 different CHLIM voltages
(see “CSOP to CSON Full-Scale Current Sense Voltage” on
page 7). It also gives formulae for calculating the minimum
and maximum CSOP-CSON voltage at any CHLIM voltage.
Equation 5 shows the formula for the max full scale
CSOP-CSON voltage (in mV) for the ISL6256A:
I S L 6256 A
(CSOP – CSON)_MAX = CHLIM • 50.28 + 2.4
(EQ. 5)
(CSOP – CSON)_MIN = CHLIM • 49.72 – 2.4
Equation 5 shows the formula for the max full scale CSOP-
CSON voltage (in mV) for the ISL6256:
I S L 6256
MAX(CSOP – CSON) = CHLIM • 50 + 5
(EQ. 6)
MIN(CSOP – CSON) = CHLIM • 50 – 5
With CHLIM = 1.5V, the maximum CSOP-CSON voltage is
78mV and the minimum CSOP-CSON voltage is 72mV.
When ISL6256A is in charge current limiting mode, the
maximum charge current is the maximum CSOP-CSON
voltage divided by the minimum sense resistor. This can be
calculated for ISL6256A with Equation 7:
I S L 6256 A
I_{CHG, MAX} = (CHLIM • 50.28 + 2.4) ⁄ R_1min
I_{CHG, MIN} = (CHLIM • 49.72 – 2.4) ⁄ R_1max
(EQ. 7)
Maximum charge current can be calculated for ISL6256 with
Equation 8:
I S L 6256
I_{CHG, MAX} = (CHLIM • 50 + 5) ⁄ R_1min
(EQ. 8)
I_{CHG, MIN} = (CHLIM • 50 – 5) ⁄ R_1max
With CHLIM = 0.7V and R₁ = 0.02Ω, 1%:
I S L 6256 A
I_{CHG, MAX} = (1.5V • 50.28 + 2.4) ⁄ 0.0198 = 3930mA
I_{CHG, MIN} = (1.5V • 49.72 – 2.4) ⁄ 0.0202 = 3573mA
(EQ. 9)
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 ISL6256 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 10:
I_INPUT
=
---¹---
R₂
⋅
⎛
⎝
---⁰----^.-⁰----⁵----
VREF
⋅
V_ACLIM
+
0.05_⎠⎞
(EQ. 10)
An external resistor divider from VREF sets the voltage at
ACLIM according to Equation 11:
V_ACLIM
=
V
R
E
F
⋅
⎛
⎜
⎝
-_R----_t--_o---_p---_,---_A----_C----_L---_I--_M---^R---_|-_|-^b--₁-^o--₅-^t--^,-₂-^A--_k--^C--_Ω---^L---^I_+--^M---_R---^|-^|_b---¹-_o--⁵-_t--_,²--_A--^k--_C-^Ω---_L----_I-_M------_|--_|--₁----₅---₂----_k---_Ω---_{⎠⎟⎞}
(EQ. 11)
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
current-sense resistor. The highest accuracy of ±3% 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 1Ω sensing resistor may have to be used. ±4%
and ±6% 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.
AC Adapter Detection
Connect the AC adapter voltage through a resistor divider to
ACSET to detect when AC power is available, as shown in
Figure 2. ACPRN is an open-drain output and is high when
ACSET is less than V_th,rise, and active low when ACSET is
above V_th,fall. V_th,rise and V_th,fall are given by Equation 12
and Equation 13:
15
FN6499.3
September 14, 2010