ISL6251 Datasheet, PDF (14/20 Pages) Intersil Corporation – Low Cost Multi-Chemistry Battery Charger Controller

ISL6251, ISL6251A
Current Measurement
Use ICM to monitor the input current being sensed across
CSIP and CSIN. The output voltage range is 0 to 2.5V. The
voltage of ICM is proportional to the voltage drop across
CSIP and CSIN, and is given by the following equation:
ICM = 19.9 • I_INPUT • R₂
where I_INPUT is the DC current drawn from the AC adapter.
ICM has ±3% accuracy.
A low pass filter connected to ICM output is used to filter the
switching noise.
LDO Regulator
VDD provides a 5.075V 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 12. VDDP connects
to VDD through an external resistor. Bypass VDDP and VDD
with a 1µF capacitor.
Shutdown
The ISL6251, ISL6251A features a low-power shutdown
mode. Driving EN low shuts down the charger. In shutdown,
the DC/DC converter is disabled, and VCOMP and ICOMP
are pulled to ground. The ICM, ACPRN outputs continue to
function.
EN can be driven by a thermistor to allow automatic
shutdown when the battery pack is hot. Often a 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 1.06V 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 charger when the battery pack is above a critical
temperature.
Another method for inhibiting charging is to force CHLIM
below 88mV (Typ.).
Short Circuit Protection and 0V Battery Charging
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 temp exceeds 150°C, it stops charging. Once the
die temp drops below 125°C, charging will start up again.
Application Information
The following battery charger design refers to the typical
application circuit in Figure 12, where typical battery
configuration of 4S2P is used. This section describes how to
select the external components including the inductor, input
and output capacitors, switching MOSFETs, and current
sensing resistors.
Inductor Selection
The inductor selection has trade-offs between cost, size and
efficiency. For example, the lower the inductance, the
smaller the size, but ripple current is higher. This also results
in higher AC losses in the magnetic core and the windings,
which decrease the system efficiency. On the other hand,
the higher inductance results in lower ripple current and
smaller output filter capacitors, but it has higher DCR (DC
resistance of the inductor) loss, and has slower transient
response. So, the practical inductor design is based on the
inductor ripple current being ±(15-20)% of the maximum
operating DC current at maximum input voltage. The
required inductance can be calculated from:
L = ^{VIN,MAX − VBAT VBAT}
∆ I_L
V_IN,MAX f_s
Where V_IN,MAX, V_BAT, and f_s are the maximum input
voltage, battery voltage and switching frequency,
respectively. The inductor ripple current ∆I is found from:
∆ I_L = 30% ⋅ I_BAT,MAX
where the maximum peak-to-peak ripple current is 30% of
the maximum charge current is used.
For V_IN,MAX=19V, V_BAT=16.8V, I_BAT,MAX=2.6A, and
f_s=300kHz, the calculated inductance is 8.3µH. Choosing
the closest standard value gives L=10µH. Ferrite cores are
often the best choice since they are optimized at 300kHz to
600kHz operation with low core loss. The core must be large
enough not to saturate at the peak inductor current I_Peak:
I _Peak
= I_{BAT ,MAX}
+
1
2
∆ I_L
Output Capacitor Selection
The output capacitor in parallel with the battery is used to
absorb the high frequency switching ripple current and
smooth the output voltage. The RMS value of the output
ripple current I_rms is given by:
I_RMS
=
V_{IN ,MAX}
12 L f_s
D
(1
−
D)
where the duty cycle D is the ratio of the output voltage
(battery voltage) over the input voltage for continuous
conduction mode which is typical operation for the battery
charger. During the battery charge period, the output voltage
varies from its initial battery voltage to the rated battery
voltage. So, the duty cycle change can be in the range of
14
FN9202.1
June 17, 2005