ISL6251HAZ Просмотр технического описания (PDF) - Renesas Electronics
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ISL6251HAZ Datasheet PDF : 20 Pages
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ISL6251, ISL6251A
ICM is proportional to the voltage drop across CSIP and CSIN, and
is given by Equation 9:
ICM = 19.9 IINPUT R2
(EQ. 9)
where IINPUT 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
FN9202 Rev 3.00
March 13, 2014
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
Equation 10:
L VIN,MAX VBAT VBAT
IL
VIN,MAX fs
(EQ. 10)
Where VIN,MAX, VBAT, and fs are the maximum input voltage,
battery voltage and switching frequency, respectively. The
inductor ripple current I is found from Equation 11:
IL 30% IBAT,MAX
(EQ. 11)
where the maximum peak-to-peak ripple current is 30% of the
maximum charge current is used.
For VIN,MAX = 19V, VBAT = 16.8V, IBAT,MAX = 2.6A, and
fs = 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 IPeak:
I Peak
IBAT ,MAX
1
2
IL
(EQ. 12)
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 Irms is
given by Equation 13:
IRMS
VIN ,MAX
12 L fs
D
1
D
(EQ. 13)
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 between 0.53 and 0.88 for the
minimum battery voltage of 10V (2.5V/Cell) and the maximum
battery voltage of 16.8V.
For VIN,MAX = 19V, VBAT = 16.8V, L = 10µH, and fs = 300kHz, the
maximum RMS current is 0.19A. A typical 10F ceramic capacitor
is a good choice to absorb this current and also has very small
size. The tantalum capacitor has a known failure mechanism
when subjected to high surge current.
EMI considerations usually make it desirable to minimize ripple
current in the battery leads. Beads may be added in series with
the battery pack to increase the battery impedance at 300kHz
switching frequency. Switching ripple current splits between the
battery and the output capacitor depending on the ESR of the
output capacitor and battery impedance. If the ESR of the output
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