isl9211a Intersil Corporation, isl9211a Datasheet - Page 8

• Manufacturer Part Number: isl9211a
• Description: Charging System Safety Circuit
• Manufacturer: Intersil Corporation

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
Protection section.
R
The R
battery terminal, in case the ISL9211A fails. The
recommended value should be between 200kΩ to 1MΩ.
With 200kΩ resistance, the worst case current flowing from
the VB pin to the charger output is shown in Equation 2,
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
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 2 plus the voltage built across the R
VB pin leakage current. The VB pin leakage current is less
than 20nA, as given in the “Electrical Specifications” table.
With the 200kΩ resistor, the worst-case additional error is
4mV and with a 1MΩ resistor, the worst-case additional error
is 20mV.
(
24V 4.2V
VB
–
Selection
VB
prevents a large current from the VB pin to the
)
⁄
(
200kΩ
VB
value reduces the worst case current,
)
ILIM
=
99μA
is given in the Overcurrent
8
VB
by the
(EQ. 2)
ISL9211A
Capacitor Selection
The input capacitor (C
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 various components, such as
the cable, the adapter output capacitor ESR, the connector
contact resistance, and so on.
During the load current step-down transient, the energy
stored in the parasitic inductor is used to charge the input
decoupling capacitor C
off the power NFET slowly during the OCP and the battery
OVP event. Because of such design, the input over-shoot
during those events is not significant. During an input OVP,
however, the NFET is turned in less than 1µs and can lead to
significant over-shoot. Higher capacitance reduces the
over-shoot.
FIGURE 16. EQUIVALENT CIRCUIT FOR THE ISL9211A INPUT
1000
AC/DC
ADAPTER
0
FIGURE 15. LITHIUM-SAFE OPERATING REGIONS
C1
1
ISL9211A
LIMITS
ISL6292C
LIMITS
L
1
2
CABLE
2
in the “Typical Application Circuit” on
. The ISL9211A is designed to turn
BATTERY VOLTAGE (V)
R
3
4
C2
HANDHELD SYSTEM
5
ISL9211A
April 2, 2009
FN6702.0
6