LP3913 datasheet(61/70 Pages) TI1

61 / 70 page

LP3913

www.ti.com

SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013

Resistors

Battery Thermistor

The LP3913 battery thermistor bias provided by the TS pin is tailored to thermistors with the following

specification:

•

Negative Temperature Coefficient

•

100 k

Ω resistance

A suitable solution is available from AVX thermistors:

AVXNB21250104 http://www.avxcorp.com/docs/Catalogs/nb21-23.pdf

I2C Pullup Resistors

I2C_SDA, I2C_SCL terminals need to have pullup resistors connected to the VDDIO pin. VDDIO must be connected

to a power supply that is less than or equal to VDD, such as Buck2. The values of the pull-up resistors (typ. ≈1.8

Ω) are determined by the capacitance of the bus. Too large of a resistor combined with a given bus capacitance

will result in a rise time that would violate the max rise time specification. Too small of a resistor will result in a

contention with the pull-down transistor on either slave(s) or master.

RIREF Resistor

The current through this resistor is used as a reference current that biases many analog circuits inside the

LP3913 and needs to have a resistance of 121 k

Ω ±1%

RISENSE Resistor

The current through this resistor is used as a reference current for the charge current. The accuracy of the ADC

is dependent on the tolerance of this resistor. RISENSE needs to have a resistance of 4.64 kΩ ±1% tolerance.

Operation without I2C Interface

Operation of the LP3913 without the I2C interface is possible if the system can operate with default values for the

DC/DC converters and the charger. (Read below: Factory programmable options). The I2C-less system must use

the POWERACK pin to power cycle the LP3913.

I2C Master Power Concern

The processor that contains the I2C master should be powered by BUCK1 or LDO2 as these converters require

no I2C access to enable/disable them. If the I2C master were to be powered by a DC/DC converter that is

enable/disabled through a control register, then a corrupted application software execution could by accident

disable the power to the I2C master, which in this case has no means to recover. It is possible that the regulator

connected to VDDIO could accidentally disable, in which case the processor should be able to recognize that

communication has been broken and then power down the system to allow for a clean restart.

System Operation When the Load Current Exceeds the USB or Adapter Current Limit

In the event that the system requires current that exceeds the current limit of either the USB or the adapter

source, then the battery can provide the extra power provided that it has been charged. It is clear that a long

sustained overload will eventually discharge the battery such that its extra power will no longer be sufficient to

properly operate the system. This will be the case when the system is for instance operated from a USB host

with a 100 mA current limit.

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