UCC2913 datasheet(4/9 Pages) TI

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UCC1913

UCC2913

UCC3913

OVERCURRENT

COMPARATOR

SENSE

VSS

INPUT VOLTAGE

OUTPUT

LOAD

µA

H=CLOSE

µA

1mA

SENSE

IMAX

H=CLOSE

0.5V

2.5V

VSS

TO OUTPUT

DRIVE

H=OFF

OVERLOAD COMPARATOR

FAULT TIMING CIRCUITRY

0.2V

5.0V

50mV

APPLICATION INFORMATION

Figure 1. Fault timing circuitry for the UCC1913, including power limit overload.

Figure 1 shows the detailed circuitry for the fault timing

function of the UCC1913. For the time being, we will dis-

cuss a typical fault mode, therefore, the overload com-

parator, and current source I3 does not work into the

operation. Once the voltage across the current sense re-

sistor, RS, exceeds 50mV, a fault has occurred. This

causes the timing capacitor to charge with a combination

of 36

µA plus the current from the power limiting amplifier.

The PL amplifier is designed to only source current into

the CT pin and to begin sourcing current once the volt-

age across the output FET exceeds 5V. The current IPL

is related to the voltage across the FET with the following

expression:

FET

−5

Where VFET is the voltage across the NMOS pass de-

vice.

Later it will be shown how this feature will limit average

power dissipation in the pass device. Note that under a

condition where the output current is more than the fault

level, but less than the max level, VOUT ~ VSS (input

voltage), IPL = 0, the CT charging current is 36µA.

During a fault, CT will charge at a rate determined by the

internal charging current and the external timing capaci-

tor. Once CT charges to 2.5V, the fault comparator

switches and sets the fault latch. Setting of the fault latch

causes both the output to switch off and the charging

switch to open. CT must now discharge with the 1

µA cur-

rent source, I2, until 0.5V is reached. Once the voltage at

CT reaches 0.5V, the fault latch resets, which re-enables

the output and allows the fault circuitry to regain control

of the charging switch. If a fault is still present, the fault

comparator will close the charging switch causing the cy-

cle to begin. Under a constant fault, the duty cycle is

given by:

Duty Cycle

Average power dissipation in the pass element is given

by:

IMAX

FET avg

FET

() =•

•

where VFET >> 5V IPL can be approximated as :

FET

and where IPL>>36µA, the duty cycle can be approxi-

mated as :

UDG-99004

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