ISL6257 Datasheet, PDF (15/22 Pages) Intersil Corporation – Highly Integrated Narrow VDC Battery Charger for Notebook Computers

ISL6257
Application Information
The following battery charger design refers to the typical
application circuit in Figure 20, where typical battery
configuration of 3S2P 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% 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 10:
L = -₄----_â‹---_I--_S^R---_W-^I--^N---_â‹-^,--_I^M--_R---^A-_I--_P-^X---_P---_L----_E-
(EQ. 10)
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 11:
I_RIPPLE = 0.3 ⋅ I_{L, MAX}
(EQ. 11)
where the maximum peak-to-peak ripple current is 30% of
the maximum charge current is used.
For V_IN,MAX = 19V, V_BAT = 12.6V, I_L,MAX = 10A, and
f_s = 300kHz, the calculated inductance is 4.7µH. 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 in Equation 12:
I_PEAK
=
I_L,
MA
X
+
1_--
2
⋅
I_R
I
P
P
LE
(EQ. 12)
Output Capacitor Selection
The output capacitor in parallel with the battery is used to
absorb the high frequency switching ripple current and
supply very high di/dt load transients. In a Narrow VDC
system the output capacitance is also the bypass
capacitance on the input of the CORE regulator and may be
several hundred µF. The following examples use 330µF with
ESR = 6mΩ.
The RMS value of the output ripple current I_rms is given by
Equation 13:
I_RMS
=
------^V---^I---^N---^,---^M-----^A---^X------- ⋅ D ⋅ (1 – D)
12 ⋅ L ⋅ F_SW
(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 0.5 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 Equation 14:
I_RMS
=
---------^V----^I--^N----^,--^M-----^A----^X----------
4 ⋅ 12 ⋅ L ⋅ F_SW
(EQ. 14)
For V_IN,MAX = 19V, L = 4.7µH, and f_s = 300kHz, the
maximum RMS current is 0.98A. Ceramic capacitors are
good choices 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, MOSFETs should be used that are rated for 30V
VDS with low r_DS(ON) at 5V VGS.
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.
The ISL6257 LGATE gate driver can drive sufficient gate
current to switch most MOSFETs efficiently. However, some
FETs may exhibit cross conduction (or shoot through) due to
current injected into the drain-to-source parasitic capacitor
(C_gd) by the high dV/dt rising edge at phase node when the
high-side MOSFET turns on. Although LGATE sink current
(1.8A typical) is more than enough to switch the FET off
15
FN9288.2
January 17, 2007