LTC4100_09 datasheet(26/30 Pages) LINER

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LTC4100

4100fb

APPLICATIONS INFORMATION

CONNECTOR

TO BATTERY

TO SYSTEM

4100 F10

VDD

Figure 10. Recommended SMBus Transient Protection

4100 F11

VBAT

VIN

HIGH

FREQUENCY

CIRCULATING

PATH

BAT

SWITCH NODE

VIAS TO CSP AND BAT

4100 F12

DIRECTION OF CHARGING CURRENT

RSENSE

Figure 11. High Speed Switching Path

Figure 12. Kelvin Sensing of Charging Current

Protecting SMBus Inputs

The SMBus inputs, SCL and SDA, are exposed to uncon-

trolled transient signals whenever a battery is connected

to the system. If the battery contains a static charge, the

SMBus inputs are subjected to transients which can cause

damage after repeated exposure. Also, if the battery’s posi-

tive terminal makes contact to the connector before the

negative terminal, the SMBus inputs can be forced below

ground with the full battery potential, causing a potential

for latch-up in any of the devices connected to the SMBus

inputs. Therefore it is good design practice to protect the

SMBus inputs as shown in Figure 10.

SafetySignal (Thermistor) Value

The SafetySignal (typical application on back page), is a

multifunction signal the communicates three pieces of

information in order of importance:

1) Presence of the Smart Battery

2) The maximum time duration of the wake-up charge

allowed.

3) An optional and redundant temperature measurement

system.

The value of the resistance to ground communicates all

this information. The resistance ranges and what it does

is covered by the SBS Smart Battery Charger standard in

Section 6. Basically if you have a battery chemistry, such

as Li-ion, that cannot safely withstand an inﬁnite dura-

tion wake-up charge, the SafetySignal resistance value

needs to be less than 425Ω. The popular value to use is

a ﬁxed 300Ω resistor. Otherwise the resistance value is

10k which is normally expected to be done using a 10k

NTC resistor.

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