ISL6252 Datasheet, PDF (16/25 Pages) Intersil Corporation – Highly Integrated Battery Charger Controller for Notebook Computers

ISL6252, ISL6252A
Inductor Selection
The inductor selection has trade-offs between cost, size,
crossover frequency 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,
lower saturation current and has slower transient response.
So, the practical inductor design is based on the inductor
ripple current being ±15% to ±20% of the maximum
operating DC current at maximum input voltage. Maximum
ripple is at 50% duty cycle or V_BAT = V_IN,MAX/2. The
required inductance can be calculated from Equation 16:
L
=
-----------^V----^I--^N----^,--^M-----^A----^X-------------
4 ⋅ f_SW ⋅ I_RIPPLE
(EQ. 16)
Where V_IN,MAX and f_SW are the maximum input voltage,
and switching frequency, respectively.
The inductor ripple current ΔI is found from Equation 17:
I_RIPPLE = 0.3 ⋅ I_{L, MAX}
(EQ. 17)
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 IPeak in
Equation 18:
I_PEAK
=
I_L,
M
A
X
+
1_--
2
⋅
I_R
IPP
L
E
(EQ. 18)
Inductor saturation can lead to cascade failures due to very
high currents. Conservative design limits the peak and RMS
current in the inductor to less than 90% of the rated
saturation current.
Crossover frequency is heavily dependent on the inductor
value. f_CO should be less than 20% of the switching
frequency and a conservative design has f_CO less than 10%
of the switching frequency. The highest f_CO is in voltage
control mode with the battery removed and may be
calculated (approximately) from Equation 19:
f_CO
=
5_-----⋅_---1_----1_-----⋅_--R_{-----S----E----N----S----E--}
2π ⋅ L
(EQ. 19)
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 Equation 20:
I_RMS
=
------^V----^I--^N----^,---^M----^A----^X-------- ⋅ D ⋅ (1 – D)
12 ⋅ L ⋅ F_SW
(EQ. 20)
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.
The maximum RMS value of the output ripple current occurs
at the duty cycle of 0.5 and is expressed as Equation 21:
I_RMS
=
---------^V----^I--^N----^,---^M----^A----^X-----------
4 ⋅ 12 ⋅ L ⋅ f_SW
(EQ. 21)
For V_IN,MAX = 19V, VBAT = 16.8V, L = 10µH, and
f_s = 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. Organic polymer
capacitors have high capacitance with small size and have a
significant equivalent series resistance (ESR). Although
ESR adds to ripple voltage, it also creates a high frequency
zero that helps the closed loop operation of the buck
regulator.
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. Switching
losses in the low-side FET are very small. The choice of
low-side FET is a trade-off between conduction losses
(r_DS(ON)) and cost. A good rule of thumb for the r_DS(ON) of
the low-side FET is 2x the r_DS(ON) of the high-side FET.
16
FN6498.3
August 25, 2010