LTC3786 datasheet(20/44 Pages) LINER

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LT8710

8710f

For more information www.linear.com/LT8710

ThecurrentthroughRSENSE2issensingthecurrentthrough

MP which is turning on and off every clock cycle. Since

the current through RSENSE2 is chopped, a filter capacitor

connected from the IMON pin to ground is needed to filter

the voltage at the IMON pin before heading to EA3. Below is

the equation to calculate the required IMON pin capacitor:

CIMON≥

100µA •DCMAX

5mV • f

whereDCMAXisthemaximumdutycycleoftheconverter’s

application (VIN at the lowest of its input range) and f is

the switching frequency.

To prevent start-up issues, the IMON capacitor should

charge up faster than the SS capacitor. It is recommended

to size the SS capacitor at least 5x greater than the IMON

capacitor.

Output Current Monitoring

The voltage at the IMON pin is a gained up version of the

voltage seen across the ISP and ISN pins. Below are the

equations relating the RSENSE2 current to the IMON pin

voltage. Assume the current through RSENSE2 is steady

state and that its time average current is approximately

equal to the converter’s load current:

VIMON =11.9• IRSENSE2(AVE)•RSENSE2 +51.8mV

(

)

IOUT ≈IRSENSE2(AVE)=

VIMON

11.9

– 51.8mV

RSENSE2

Output Current Limiting

As shown in Figure 9, IMON voltages exceeding 1.213V

(typical) causes the VC voltage to reduce, thus limiting

the inductor current. This voltage on IMON corresponds

to an average voltage of 50mV across RSENSE2. Below is

the equation for selecting the RSENSE2 resistor for limiting

the output current at steady state:

RSENSE2 =

50mV

IOUT(LIMIT)

If it is not desirable to limit the output current, size

RSENSE2 by setting IOUT(LIMIT) at least 60% higher than

themaximumoutputcurrentoftheconverter.Thiscurrent

sense resistor is needed if using the synchronous PFET

in the converter. If the PFET is replaced with a Schottky,

then RSENSE2 is not needed if output current limiting or

monitoring isn’t required.

Note that if the INTVEE LDO is in UVLO and SS > 1.8V (typi-

cal), then the reference voltage at EA3 reduces to 916mV,

and the output current is limited to about half its set point.

Output Overcurrent

As shown in Figure 9, a comparator monitors the voltage

at the IMON pin and triggers a reset condition if the IMON

pin voltage exceeds 1.38V (typical). This corresponds to

an average voltage of 63.6mV (typical) across the ISP

and ISN pins:

IOUT(OVERCURRENT)=

63.6mV

RSENSE2

IOUT(OVERCURRENT)=1.27•IOUT(LIMIT)

applicaTions inForMaTion

Figure 9. Output Current Monitor and Control

8710 F09

ISN

TO SYSTEM

VOUT

RSENSE2

51.8mV

–

1.213V

11.9K

1.38V

OVER

CURRENT

EA3

GND

IMON

CIMON

ISP

+–

CHRG

666.5mV

1mA/V

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