6
FN6658.4
January 13, 2009
Theory of Operation
The ISL9211 is an integrated circuit (IC) optimized to provide
a redundant safety protection to a Li-ion battery from
charging system failures. The IC monitors the input voltage,
the battery voltage, and the charge current. When any of the
above three parameters exceeds its limit, the IC turns off an
internal N-channel MOSFET to remove the power from the
charging system. In addition to the above protected
parameters, the IC also monitors its own internal
temperature and turns off the N-channel MOSFET when the
temperature exceeds +150°C. Together with the battery
charger IC and the protection module in a battery pack, the
charging system has triple-level protection from
overcharging the Li-ion battery and is two-fault tolerant. The
ISL9211 protects up to 26V input voltage.
Power-Up
The ISL9211 has a power-on reset (POR) threshold of
2.47V (max). Before the input voltage reaches the POR
threshold, the internal power NFET is off. Approximately
10ms after the input voltage exceeds the POR threshold, the
IC resets itself and begins the soft-start. The 10ms delay
allows any transients at the input during a hot insertion of the
power supply to settle down before the IC starts to operate.
The soft-start slowly turns on the power NFET to reduce the
inrush current as well as the input voltage drop during the
transition. The power-up sequence is illustrated in Figure 2.
Input Overvoltage Protection (OVP)
The input voltage is monitored by the comparator CP1 in the
Block Diagram (Figure 1). CP1 has an accurate reference of
1.2V from the bandgap reference. The OVP threshold is set
by the resistive divider consisting of R
1
and R
2
. When the
input voltage exceeds the threshold, the CP1 outputs a logic
signal to turn off the power NFET within 1µs (see Figure 3) to
prevent the high input voltage from damaging the electronics
in the handheld system. The hysteresis for the input OVP
threshold is given in the “Electrical Specifications” table on
page 2. When the input overvoltage condition is removed,
the ISL9211 re-enables the output by running through the
soft-start, as shown in Figure 5. Because of the 10ms
second delay before the soft-start, the output is never
enabled if the input rises above the OVP threshold quickly,
as shown in Figure 6.
Battery Overvoltage Protection
The battery voltage OVP is realized with the VB pin. The
comparator CP3, as shown in Figure 1, monitors the VB pin
and issues an overvoltage signal when the battery voltage
exceeds the 4.34V battery OVP threshold. The threshold
has 30mV built-in hysteresis. The comparator CP3 has a
built-in 180µs blanking time to prevent any transient voltage
from triggering the OVP. If the OVP situation still exists after
the blanking time, the power NFET is turned off. The control
logic contains a 4-bit binary counter that if the battery
overvoltage event occurs 16x, the power NFET is turned off
permanently, as shown in Figure 7. Recycling the input
power will reset the counter and restart the ISL9211.
The resistor between the VB pin and the battery, R
VB
, as
shown in the “Typical Application Circuit” on page 1, is an
important component. This resistor provides a current limit in
case the VB pin is shorted to the input voltage under a failure
mode. The VB pin leakage current under normal operation is
negligible to allow a resistance of 200k to 1M be used.
Overcurrent Protection (OCP)
The current in the power NFET is limited to prevent charging
the battery with an excessive current. The current is sensed
using the voltage drop across the power FET after it is
turned on. The reference of the OCP is generated using a
sensing FET Q
2
(Mirror to Q
1
), as shown in Figure 1. The
current in the sensing FET is forced to match the value
programmed by the ILIM pin. The size of the power FET Q
1
is 31,250x the size of the sensing FET. Therefore, when the
current in the power FET is 31,250x the current in the
sensing FET, the drain voltage of the power FET falls below
FIGURE 14. ON-RESISTANCE vs TEMPERATURE AT DIFFERENT INPUT VOLTAGES
Typical Operating Performance The test conditions for the Typical Operating Performance are: V
IN
= 5V, T
A
= +25°C,
R
ILIM
= 24.9k, R
VB
= 200k, Unless Otherwise Noted. (Continued)
0
50
100
150
200
250
300
350
-50-30-101030507090110130
TEMPERATURE (°C)
r
DS(ON)
(m)
4.3V
5.0V
3.0V
ISL9211
