ISL9209B Datasheet, PDF (9/11 Pages) Intersil Corporation – Charging System Safety Circuit

ISL9209B
Warning Indication Output
The WRN pin is an open-drain output that indicates a LOW
signal when any of the three protection events happens. This
allows the microprocessor to give an indication to the user to
further enhance the safety of the charging system.
Applications Information
The ISL9209B is designed to meet the “Lithium-Safe” criteria
when operating together with the ISL6292 family Li-ion
battery chargers. The “Lithium-Safe” criteria requires the
charger output to fall within the green region shown in
Figure 23 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 any ISL6292 family chargers, 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 ISL9209B 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 ISL9209B and the ISL6292C chip set can
easily pass the “Lithium-Safe” criteria test procedures.
The ISL9209B 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 the “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 ISL9209B 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:
(30V – 4.2V) ⁄ (200kΩ) = 130μA
(EQ. 2)
assuming the VB pin voltage is 30V 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 “Electrical Specifications” on
page 2) 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 “Electrical Specifications” on page 2. With
the 200kΩ resistor, the worst-case additional error is 4mV
and with a 1MΩ resistor, the worst-case additional error is
20mV.
1000
ISL9209
LIMITS
ISL6292C
LIMITS
0
1
2
3
4
5
6
BATTERY VOLTAGE (V)
FIGURE 23. LITHIUM-SAFE OPERATING REGIONS
Interfacing to MCU
The ISL9209B has the enable (EN) and the warning (WRN)
digital signals that can be interfaced to a microcontroller unit
(MCU). Both signals can be left floating if not used. When
interfacing to an MCU, it is highly recommended to insert a
resistor between the ISL9209B signal pin and the MCU
GPIO pin, as shown in Figure 24. The resistor creates an
isolation to limit the current, in case a high voltage shows up
at the ISL9209B pins under a failure mode. The
recommended resistance ranges from 10kΩ to 100kΩ. The
selection of the R_EN is dependent on the IO voltage (VIO) of
the MCU. R_EN should be selected so that the ISL9209B EN
pin voltage is above the disable threshold when the GPIO
output of the MCU is high.
IISSLL99220099
Q₄
W_WR_RN_N
VIO
RR_PUPU
RR_WWRRNN
MCU
E_EN_N
Q^Q₅5
R^R₅₅
R_REENN
FIGURE 24. DIGITAL SIGNAL INTERFACE BETWEEN
ISL9209B AND MCU
Capacitor Selection
The input capacitor (C₁ in the “Typical Application Circuit” on
page 1) is for decoupling. Higher value reduces the voltage
drop or the over shoot during transients.
9
FN6400.0
March 21, 2007