isl9212 Intersil Corporation, isl9212 Datasheet - Page 9

• Manufacturer Part Number: isl9212
• Description: Charging System Safety Circuit
• Manufacturer: Intersil Corporation

R
The R
battery terminal, in case the ISL9212 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,
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
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 the Electrical Specification
plus the voltage built across the R
current. The VB pin leakage current is less than 20nA, as
given in the Electrical Specification. With the 200kΩ resistor,
the worst-case additional error is 4mV and with a 1MΩ
resistor, the worst-case additional error is 20mV.
Interfacing to MCU
The ISL9212 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 ISL9212 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
ISL9212 pins under a failure mode. The recommended
resistance ranges from 10kΩ to 100kΩ. The selection of the
R
should be selected so that the ISL9212 EN pin voltage is
above the disable threshold when the GPIO output of the
MCU is high.
1000
EN
VB
is dependent on the IO voltage (VIO) of the MCU. R
Selection
0
VB
FIGURE 23. LITHIUM-SAFE OPERATING REGIONS
prevents a large current from the VB pin to the
1
ISL9212
LIMITS
ISL6292C
LIMITS
2
BATTERY VOLTAGE (V)
9
3
VB
by the VB pin leakage
VB
4
value reduces the
5
EN
6
ISL9212
Capacitor Selection
The input capacitor (C1 in the Typical Application Circuit) 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 ISL9212 has a step-down change. Figure
25 shows an equivalent circuit for the ISL9212 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
decoupling capacitor C2. The ISL9212 is designed to turn 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.
FIGURE 25. EQUIVALENT CIRCUIT FOR THE ISL9212 INPUT
ISL9212
AC/DC
FIGURE 24. DIGITAL SIGNAL INTERFACE BETWEEN ISL9212
Q
ADAPTER
5
C1
Q
4
AND MCU
R
5
L
WRN
EN
CABLE
R
R
PU
R
R
WRN
EN
VIO
C2
HANDHELD SYSTEM
ISL9212
May 8, 2006
MCU
FN9280.1