ISL9211A

8

FN6702.1

August 25, 2010

Theory of Operation

The ISL9211A 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 ISL9211A protects

up to 26V input voltage.

Power-Up

The ISL9211A 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 1.

Input Overvoltage Protection (OVP)

The input voltage is monitored by the comparator CP1 in

the “Block Diagram” on page 2. CP1 has an accurate

reference of 1.2V from the bandgap reference. The OVP

the CP1 outputs a logic signal to turn off the power NFET

within 1µs (see Figure 2) 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 4.

When the input overvoltage condition is removed, the

ISL9211A re-enables the output by running through the

soft-start, as shown in Figure 4. 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 5.

Battery Overvoltage Protection

The battery voltage OVP is realized with the VB pin. The

comparator CP3, as shown in the “Block Diagram” on

page 2, 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 16 times, the power

NFET is turned off permanently, as shown in Figure 6.

Recycling the input power will reset the counter and

restart the ISL9211A.

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

200kΩ to 1MΩ 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 the it is turned on. The reference of the

as shown in the “Block Diagram” on page 2. The current

FIGURE 13. ON-RESISTANCE vs TEMPERATURE AT DIFFERENT INPUT VOLTAGES

0

50

100

150

200

250

300

350

-50 -30 -10

10

30

50

70

90 110 130

TEMPERATURE (°C)

4.3V

5.0V

3.0V