ISL6256 Datasheet, PDF (17/26 Pages) Intersil Corporation – Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

ISL6256, ISL6256A
Overvoltage Protection
ISL6256 has an Overvoltage Protection circuit that limits the
output voltage when the battery is removed or disconnected
by a pulse charging circuit. If CSON exceeds the output
voltage set point by more than V_OVP an internal comparator
pulls VCOMP down and turns off both upper and lower FETs
of the buck as in Figure 17. The trip point for Overvoltage
Protection is always above the nominal output voltage and
can be calculated from Equation 17:
V_OVP
=
V_OUT,
N
O
M
+
N_C
E
LL
S
×
⎛
⎝
42.2 m
V
–
22.2
m
V
×
2^-V---.^-A-3^---9^D----V^{J--⎠⎞}
(EQ. 17)
For example, if the CELLS pin is connected to ground
(N_CELLS = 3) and V_ADJ is floating (V_ADJ = 1.195V) then
V_OUT,NOM = 12.6V and VOVP = 12.693V or
V_OUT,NOM + 93mV.
There is a delay of approximately 400ns between V_OUT
exceeding the OVP trip point and pulling VCOMP, LGATE
and UGATE low.
VCOMP
ICOMP
BATTERY
REMOVAL
PHASE
V_OUT
WHEN V_OUT EXCEEDS
THE OVP THRESHOLD
VCOMP IS PULLED LOW
AND FETS TURN OFF
CURRENT FLOWS IN THE
LOWER FET BODY DIODE
UNTIL INDUCTOR CURRENT
REACHES ZERO
FIGURE 17. OVERVOLTAGE PROTECTION IN ISL6256
During normal operation with cells installed, the CSON pin
voltage will be the cell stack voltage. When EN is low and the
cells are removed, this voltage may drop below 100mV. Due
to non-linearities in the OVP comparator at this low input
level, the VCOMP pin may be held low even after EN is
commanded high. If regulation is required in the absence of
cells then a series resistor and diode need to be installed
which inject current into the CSON pin from the VDD pin.
See R23 and D3 in Figure 3. This will maintain the CSON pin
voltage well within its linear range in the absence of cells,
and will be effectively out of the circuit when the diode is
reversed biased by the cell stack. Resistor values from 10k
to 100k have been found to be effective.
Application Information
The following battery charger design refers to the typical
application circuit in Figure 2, 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,
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 18:
L
=
-----------^V----^I--^N----^,--^M-----^A----^X-------------
4 ⋅ f_SW ⋅ I_RIPPLE
(EQ. 18)
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 19:
I_RIPPLE = 0.3 ⋅ I_{L, MAX}
(EQ. 19)
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 20:
I_PEAK
=
I_L,
MA
X
+
1_--
2
⋅
I_R
IPP
L
E
(EQ. 20)
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
17
FN6499.3
September 14, 2010