LTC4066EPFTRPBF View Datasheet(PDF) - Linear Technology

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Description

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LTC4066EPFTRPBF

USB Power Controller and Li-Ion Linear Charger with Low Loss Ideal Diode

Linear Technology 

LTC4066/LTC4066-1

APPLICATIONS INFORMATION

Any time a battery is connected to the BAT pin and the

SHDN pin is low, the BAT pin current can be monitored

with the following equation:

IBAT

=

VISTAT

RISTAT

• 1000

where |IBAT| is the absolute value of the BAT pin current,

VISTAT is the voltage on the ISTAT pin and RISTAT is the total

resistance from the ISTAT pin to ground.

The POL pin has two states: high impedance and strong

pull-down. High impedance indicates that current is flow-

ing from BAT to OUT (ideal diode function) and strong

pull-down indicates that current is flowing from OUT to

BAT (charging). If an external ADC is used to convert the

ISTAT voltage, then the POL pin can be thought of as a

sign bit.

When the ideal diode function is operating, the ISTAT pin

cannot monitor ideal diode load currents less than about

1mA. For any ideal diode load current less than 1mA, the

ISTAT pin will source a constant current of approximately

1μA. However, when the battery charger function is operat-

ing, the ISTAT pin will continue to source one-thousandth

of the battery charge current even if the charge current

drops to less than 1mA.

When choosing the value of RISTAT, two details must be

considered. For the battery charger function, the value of

RISTAT programs the charge current below which the CHRG

pin transitions to its high impedance state (see CHRG

Status Output Pin). Furthermore, the available common

mode range on the ISTAT pin needed to maintain an accurate

ratio between IBAT and IISTAT is limited. When charging, the

ISTAT pin voltage should not exceed approximately VOUT

– 0.5V. When the ideal diode is functioning, the ISTAT pin

voltage should not exceed approximately VBAT – 0.5V (for

the typical minimum operating voltage for the ideal diode

this value would be 2.8V – 0.5V = 2.3V). Typically, it is this

second case that is the limiting situation since VBAT is typi-

cally lower than VOUT (while charging) and transient ideal

diode loads tend to be greater than typical charge currents

(causing a higher voltage on the ISTAT pin). Therefore,

choosing a value of RISTAT based on the CHRG detection

current may limit the maximum ideal diode load current

that can be sensed accurately. Consider an example:

a) Desired charge current = 850mA

b) Desired CHRG detection current = 100mA

c) Maximum transient ideal diode current = 1.5A

Calculate:

a) RPROG = (1V/850mA) • 50,000 = 59k

b) RISTAT = 100V/100mA = 1k

c) VISTAT(MAX) = 1.5A/1000 • 1k = 1.5V

In this example, there is no common mode problem because

the maximum ISTAT voltage (1.5V) is well below the 2.3V

minimum. However, if, instead of 100mA, the desired CHRG

detection current was lowered to 40mA, then the desired

RISTAT resistor would increase to 2.5k (100V/40mA) and

the maximum ISTAT voltage would increase to 3.75V (as-

suming no change in the 1.5A maximum ideal diode cur-

rent). Therefore, ideal diode currents greater than 920mA

(2.3V/2.5k • 1000) might not be reported accurately. To

calculate the maximum ideal diode current that will be

reported accurately:

IDMON(MAX)

=

VBAT – 0.5V

RISTAT

4066fc

22

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