ISL9212B

FN6399 Rev 0.00

Page 9 of 11

March 21, 2007

battery terminal, in case the ISL9212B fails. The

recommended value should be between 200k

 to 1M.

With 200k

 resistance, the worst case current flowing from

the VB pin to the charger output is:

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

worst case current, but at the same time increases the error

for the 4.4V battery OVP threshold.

The error of the battery OVP threshold is the original

accuracy at the VB pin, (given in “Electrical Specifications”

pin leakage current. The VB pin leakage current is less than

20nA, as given in “Electrical Specifications” on page 2. With

the 200k

 resistor, the worst-case additional error is 4mV

and with a 1M

 resistor, the worst-case additional error is

20mV.

Interfacing to MCU

The ISL9212B 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 ISL9212B 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 ISL9212B pins under a failure mode. The

recommended resistance ranges from 10k

 to 100k. The

pin voltage is above the disable threshold when the GPIO

output of the MCU is high.

Capacitor Selection

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 PFET of the ISL9212B has a step-down change.

Figure 25 shows an equivalent circuit for the ISL9212B

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

off the power PFET slowly during the OCP, the battery OVP

event, and when the device is disabled via the EN pin.

Because of such design, the input over shoot during those

events is not significant. During an input OVP, however, the

PFET is turned in less than 1µs and can lead to significant

over shoot. Higher capacitance reduces this type of over

shoot.

The over shoot caused by a hot insertion is not very

dependent on the decoupling capacitance value, especially

when ceramic type capacitors are used for decoupling. In

theory, the over shoot can rise up to twice of the DC output

30V 4.2V

–

 200k





130

A

=

(EQ. 2)

FIGURE 23. LITHIUM-SAFE OPERATING REGIONS

5

0

1000

BATTERY VOLTAGE (V)

12

3

4

ISL9212

LIMITS

ISL6292C

LIMITS

6

FIGURE 24. DIGITAL SIGNAL INTERFACE BETWEEN

ISL9212B AND MCU

WRN

R

5

Q

4

Q

5

EN

ISL9212

MCU

R

PU

R

WRN

R

EN

VIO

FIGURE 25. EQUIVALENT CIRCUIT FOR THE ISL9212B INPUT

AC/DC

ISL9212

ADAPTER

CABLE

HANDHELD SYSTEM

C1

L

R

C2