ISL9211A
9
FN6702.3
November 5, 2014
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The size of the power FET Q
1
is 31,250 times the size of the
sensing FET. Therefore, when the current in the power FET is
31,250 times the current in the sensing FET, the drain voltage of
the power FET falls below that of the sensing FET. The
comparator CP2 then outputs a signal to turn off the power FET,
where the 0.8V is the regulated reference voltage at the ILIM pin.
The OCP comparator CP2 has a built-in 180µs delay to prevent
false triggering by transient signals. The OCP function also has a
4-bit binary counter that accumulates during an OCP event.
When the total count reaches 16, the power NFET is turned off
permanently until the input power is recycled or the enable pin is
toggled. Figures 8 and 9 illustrate the waveforms during the
power-up when the output is shorted to ground.
Internal Over-Temperature Protection
The ISL9211A monitors its own internal temperature to prevent
thermal failures. When the internal temperature reaches
+150°C, the IC turns off the N-channel power MOSFET. The IC
does not resume operation until the internal temperature drops
below +110°C.
Fault Indication Output
The FAULT pin is an open-drain output that indicates a LOW
signal when any of the three fault events happens. This provides
a signal to the microprocessor to take further action to enhance
the safety of the charging system.
Applications Information
The ISL9211A is designed to meet the “Lithium-Safe” criteria
when operating together with a qualified Li-ion battery charger.
The “Lithium-Safe” criteria requires the charger output to fall
within the green region shown in Figure 15 under normal
operating conditions and NOT to fall in the red region when there
is a single fault in the charging system. Taking into account the
safety circuit in a Li-ion battery pack, the charging system is
allowed to have two faults without creating hazardous conditions
for the battery cell. The output of the Li-ion charger, such as the
ISL6292C, has a typical I-V curve shown with the blue lines under
normal operation, which is within the green region. The function
of the ISL9211A is to add a redundant protection layer such that,
under any single fault condition, the charging system output does
not exceed the I-V limits shown with the red lines. As a result, the
charging system adopting the ISL9211A and the ISL6292C chip
set can easily pass the “Lithium-Safe” criteria test procedures.
The ISL9211A is a simple device that requires only three external
components, in addition to the ISL6292 charger circuit, to meet
the “Lithium-Safe” criteria, as shown in the “TYPICAL
APPLICATION CIRCUIT” on page 1. The selection of the current
limit resistor R
ILIM
is given in “Overcurrent Protection (OCP)” on
page 8.
R
VB
Selection
The R
VB
prevents a large current from the VB pin to the battery
terminal, in case the ISL9211A fails. The recommended value
should be between 200k to 1M. With 200k resistance, the
worst case current flowing from the VB pin to the charger output
is shown in Equation 1, assuming the VB pin voltage is 24V under
a failure mode and the battery voltage is 4.2V.
Such a small current can be easily absorbed by the bias current
of other components in the handheld system. Increasing the R
VB
value reduces the worst case current, but at the same time
increases the error for the 4.34V battery OVP threshold.
The error of the battery OVP threshold is the original accuracy at
the VB pin given in the “Electrical Specifications” table on page 4
plus the voltage built across the R
VB
by the VB pin leakage
current. The VB pin leakage current is less than 20nA, as given in
the “Electrical Specifications” table on page 4. With the 200k
resistor, the worst-case additional error is 4mV and with a 1M
resistor, the worst-case additional error is 20mV.
Capacitor Selection
The input capacitor (C
1
in the “TYPICAL APPLICATION CIRCUIT”
on page 1) is for decoupling. Higher value reduces the voltage
drop or the over-shoot during transients.
Two scenarios can cause the input voltage over-shoot. The first
one is when the AC adapter is inserted live (hot insertion) and the
second one is when the current in the power NFET of the
ISL9211A has a step-down change. Figure 16 shows an
equivalent circuit for the ISL9211A input. The cable between the
AC/DC converter output and the handheld system input has a
parasitic inductor. The parasitic resistor is the lumped sum of
TABLE 2. R
LIM
VALUE vs OCP THRESHOLD
R
LIM
(k)
OCP
(mA)
R
LIM
(k)
OCP
(mA)
82.5 300 21 1200
61.9 400 19.1 1300
49.9 500 16.5 1400
41.2 600 15.4 1500
35.7 700 14 1600
31.6 800 12.4 1700
28 900 11.3 1800
24.9 1000 10.5 1900
22.6 1100 9.53 2000
24V 4.2V–200k 99A=
(EQ. 1)
FIGURE 15. LITHIUM-SAFE OPERATING REGIONS
50
1000
BATTERY VOLTAGE (V)
CHARGE CURRENT (mA)
134
ISL9211A
LIMITS
ISL6292C
LIMITS
62
