ISL6255_14 Datasheet, PDF (16/22 Pages) Intersil Corporation – Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

ISL6255, ISL6255A
charging function is disabled. If designing for airplane power,
DCSET is tied to a resistor divider sensing the adapter voltage.
When a user is plugged into the 15V airplane supply and the
battery voltage is lower than 15V, the MOSFET driven by
BGATE (See Figure 16) is turned off which keeps the battery
from supplying the system bus. The comparator looking at
CSON and DCIN has 300mV of hysteresis to avoid chattering.
Only 2S and 3S are supported for DC aircraft power
applications. For 4S battery packs, set DCSET = 0.
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 15, 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}
Δ 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 ΔI is found from:
Δ 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
Δ 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
between 0.5 and 0.88 for the minimum battery voltage of
10V (2.5V/Cell) and the maximum battery voltage of 16.8V.
The maximum RMS value of the output ripple current occurs
at the duty cycle of 0.5 and is expressed as:
I_RMS = ^VIN,MAX
4 12 L f_s
For V_IN,MAX = 19V, L = 10H, and f_s = 300kHz, the maximum
RMS current is 0.46A. A typical 10µF 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 capacitor is 10mΩ and
battery impedance is raised to 2Ω with a bead, then only
0.5% of the ripple current will flow in the battery.
MOSFET Selection
The Notebook battery charger synchronous buck converter
has the input voltage from the AC adapter output. The
maximum AC adapter output voltage does not exceed 25V.
Therefore, 30V logic MOSFET should be used.
The high side MOSFET must be able to dissipate the
conduction losses plus the switching losses. For the battery
charger application, the input voltage of the synchronous
buck converter is equal to the AC adapter output voltage,
which is relatively constant. The maximum efficiency is
achieved by selecting a high side MOSFET that has the
conduction losses equal to the switching losses. Ensure that
ISL6255, ISL6255A LGATE gate driver can supply sufficient
16
FN9203.2
May 23, 2006