ISL6255 INTERSIL [Intersil Corporation], ISL6255 Datasheet - Page 16

• Manufacturer Part Number: ISL6255
• Description: Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers
• Manufacturer: INTERSIL [Intersil Corporation]

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:
Where V
voltage, battery voltage and switching frequency,
respectively. The inductor ripple current ∆I is found from:
where the maximum peak-to-peak ripple current is 30% of
the maximum charge current is used.
For V
f
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
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
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
L
I
I
∆
s
RMS
Peak
=300kHz, the calculated inductance is 8.3µH. Choosing
=
I
L
V
=
IN
IN,MAX
=
=
30%
,
MAX
V
I
IN,MAX
BAT
12
IN
∆
,
MAX
⋅
I
f L
−
L
=19V, V
,
I
MAX
BAT,
V
rms
s
BAT
, V
D
MAX
+
is given by:
BAT
(
1
1
2
V
BAT
−
IN
∆
, and f
V
D
,
BAT
MAX
I
=16.8V, I
)
L
f
s
16
s
are the maximum input
BAT,MAX
=2.6A, and
ISL6255, ISL6255A
Peak
:
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:
For V
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 10m Ω and
battery impedance is raised to 2 Ω 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
gate current to prevent it from conduction, which is due to
the injected current into the drain-to-source parasitic
capacitor (Miller capacitor C
rising rate at phase node at the time instant of the high-side
MOSFET turning on; otherwise, cross-conduction problems
may occur. Reasonably slowing turn-on speed of the high-
side MOSFET by connecting a resistor between the BOOT
pin and gate drive supply source, and the high sink current
capability of the low-side MOSFET gate driver help reduce
the possibility of cross-conduction.
For the high-side MOSFET, the worst-case conduction
losses occur at the minimum input voltage:
The optimum efficiency occurs when the switching losses
equal the conduction losses. However, it is difficult to
calculate the switching losses in the high-side MOSFET
since it must allow for difficult-to-quantify factors that
influence the turn-on and turn-off times. These factors
I
P
RMS
Q
, 1
Conduction
IN,MAX
=
4
V
IN
12
,
=19V, L=10H, and f
MAX
f L
=
s
V
V
OUT
IN
I
BAT
2
gd
R
), and caused by the voltage
DSON
s
=300kHz, the maximum
June 17, 2005
FN9203.1