ISL6257 Datasheet, PDF (14/22 Pages) Intersil Corporation – Highly Integrated Narrow VDC Battery Charger for Notebook Computers

ISL6257
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 20. ACPRN is an open-drain output and is high when
ACSET is less than V_th,fall, and active low when ACSET is
above V_th,rise. V_th,rise and V_th,fall are given by Equation 6
and Equation 7:
V_{th, rise}
=
⎛
⎜
⎝
R_----8-
R₉
+
⎞
1⎟
⎠
⋅ V_ACSET
(EQ. 6)
V_{th, fall}
=
⎛
⎜
⎝
R_----8-
R₉
+
⎞
1⎟
⎠
⋅ V_ACSET – I_hys ⋅ R₈
(EQ. 7)
where:
• I_hys is the ACSET input bias current hysteresis, and
• V_ACSET = 1.24V (min), 1.26V (typ) and 1.28V (max).
The hysteresis is I_hysR₈, where I_hys = 2.2µA (min),
3.4µA (typ) and 4.4µA (max).
DC Adapter Detection
Connect the DC input through a resistor divider to DCSET to
detect when lower voltage (i.e. aircraft) DC power is
available. DCPRN is an open-drain output and is high when
DCSET is less than V_th,fall, and active low when DCSET is
above V_th,rise. V_th,rise and V_th,fall are given by Equation 8
and Equation 9:
V_{th, rise}
=
⎛
⎜
⎝
R----_2---4-
R₂₅
+
⎞
1⎟
⎠
• V_DCSET
(EQ. 8)
V_{th, fall}
=
⎛
⎜
⎝
R----_2---4-
R₂₅
⎞
+ 1⎟
⎠
• V_DCSET – I_hysR₂₄
where:
(EQ. 9)
• I_hys is the DCSET input bias current hysteresis, and
• V_DCSET = 1.24V (min), 1.26V (typ) and 1.28V (max).
The hysteresis is I_hysR₁₄, where I_hys = 2.2µA (min),
3.4µA (typ) and 4.4µA (max).
LDO Regulator
VDD provides a 5.0V supply voltage from the internal LDO
regulator from DCIN and can deliver up to 30mA of current.
The MOSFET drivers are powered by VDDP, which must be
connected to VDDP as shown in Figure 20. VDDP connects
to VDD through an external low pass filter. Bypass VDDP
and VDD with a 1µF capacitor.
Shutdown
The ISL6257 features a low-power shutdown mode. Driving
EN low shuts down the ISL6257. In shutdown, the DC/DC
converter is disabled, and VCOMP and ICOMP are pulled to
ground. The ACPRN and DCPRN outputs continue to
function.
EN can be driven by a thermistor to allow automatic
shutdown of the ISL6257 when the battery pack is hot. Often
an NTC thermistor is included inside the battery pack to
measure its temperature. When connected to the charger,
the thermistor forms a voltage divider with a resistive pull-up
to the VREF. The threshold voltage of EN is 1.0V with 60mV
hysteresis. The thermistor can be selected to have a
resistance vs temperature characteristic that abruptly
decreases above a critical temperature. This arrangement
automatically shuts down the ISL6257 when the battery pack
is above a critical temperature.
Another method for inhibiting charging is to force CHLIM
below 85mV (typ).
Supply Isolation
If the voltage across the adapter sense resistor R₂ is
typically greater than 8mV, the P-channel MOSFET
controlled by SGATE is turned on reducing the power
dissipation. If the voltage across the adapter sense resistor
R₂ is less than 3mV, SGATE turns off the P-channel
MOSFET isolating the adapter from the system bus.
Battery Power Source Selection and Aircraft
Power Application
The battery voltage is monitored by CSON. If the battery
voltage measured on CSON is less than the adapter voltage
measured on DCIN, then the P-channel MOSFET controlled
by SGATE is allowed to turn on when the adapter current is
high enough. If it is greater, then the P-channel MOSFET
controlled by SGATE turns off.
When operating on aircraft power it is desirable to disable
charging to minimize loading of the aircraft power systems.
DCIN is usually lower when connected to aircraft power
(15V) than it is when connected AC power (20V). The
DCSET pin provides means of detecting this lower DC input
voltage. If the DC input voltage is below the ACSET
threshold and above the DCET threshold, ACPRN will be
high and DCPRN will be low, and the host may turn off Q₅
(Figure 20) to stop charging the battery.
Short Circuit Protection
Since the battery charger will regulate the charge current to
the limit set by CHLIM, it automatically has short circuit
protection and is able to provide the charge current to wake
up an extremely discharged battery.
Over Temperature Protection
If the die temperature exceeds +150°C, it stops charging.
Once the die temperature drops below +125°C, charging will
start up again.
14
FN9288.2
January 17, 2007