8

FN6400.0

March 21, 2007

Theory of Operation

The ISL9209B 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 P-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 P-Channel MOSFET when the

temperature exceeds +140°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

ISL9209B protects up to 30V input voltage.

Power-Up

The ISL9209B has a power-on reset (POR) threshold of

2.6V with a built-in hysteresis of 125mV. Before the input

voltage reaches the POR threshold, the internal power PFET

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 PFET to reduce the inrush current as well as the

input voltage drop during the transition. The power-up

behavior 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

protection threshold is set to 5.85V. When the input voltage

exceeds the threshold, the CP1 outputs a logic signal to turn

off the power PFET 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 “Electrical Specifications” on page 2.

When the input overvoltage condition is removed, the

ISL9209B 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 (nominal) 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 PFET is turned off.

The control logic contains a 4-bit binary counter that if the

battery overvoltage event occurs 16 times, the power PFET

is turned off permanently, as shown in Figure 7. Recycling

the input power or toggling the enable (EN) input will reset

the counter and restart the ISL9209B.

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

Ω to 1MΩ be used.

Overcurrent Protection (OCP)

The current in the power PFET 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 FET is

turned on. The reference of the OCP is generated using a

sensing FET Q2, as shown in Figure 1. The current in the

sensing FET is forced to the value programmed by the ILIM

pin. The size of the power FET Q1 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.

The OCP threshold can be calculated using Equation 1:

where the 0.8V is the regulated voltage at the ILIM pin. The

OCP comparator CP2 has a built-in 170µs 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 PFET is

turned off permanently, unless the input power is recycled or

the enable pin is toggled. Figure 8 and Figure 9 illustrate the

waveforms during the power-up when the output is

short-circuited to ground.

Internal Over-Temperature Protection

The ISL9209B monitors its own internal temperature to

prevent thermal failures. When the internal temperature

reaches +140°C, the IC turns off the P-Channel power

MOSFET. The IC does not resume operation until the

internal temperature drops below +90°C.

External Enable Control

The ISL9209B offers an enable (EN) input. When the EN pin

is pulled to logic HIGH, the protection IC is shut down. The

internal control circuit as well as the power PFET are turned

off. Both 4-bit binary counters for the battery OVP and the

OCP are reset to zero when the IC is re-enabled. The EN pin

has an internal 200k

Ω pull-down resistor. Leaving the EN pin

floating or driving it to below 0.4V enables the IC.

I

LIM

0.8V

R

ILIM

--------------- 31250

25000

R

ILIM

----------------

=

⋅

=

(EQ. 1)

ISL9209B