LM5177 datasheet(39/112 Pages) TI

39 / 112 page

8.3.11.2 Output Current Limitation

In this configuration the current sense has an internal feedback to the peak current limit of the device. The ILIM

circuit regulates the ILIMCOMP voltage to V(ISET), and overrides the voltage loop regulation.

the peak current limit down as long as the differential voltage between ISNSP and ISNSN exceeds the internal

offset voltage of the ILIM circuit. The ILIM threshold can be set via the register programming or via the an ISET

resistor in the ILIMCOMP pin.

If the current limit threshold is selected by a resistor the regulation overwrites the voltage loop once V(ISET) is

increasing to the threshold voltage (typ. 1V). The threshold voltage for ISET can be calculated with:

VISET = VISNSP −VISNSN ∙gmILIMCOMP ∙RILIMCOMP

(15)

Hence the resistor value to select the current limit threshold voltage calculates with:

RISET =

VISET

VISNSP − VISNSN ∙gmILIMCOMP

(16)

For high frequency noise suppression a capacitor based on the following equation should be placed in parallel to

R(ISET)

CISET = 1

π∙ fSW ∙RISET

(17)

If the threshold for the current limit is programmed by the internal DAC the bandwidth of the current limit control

loop can be optimized for different loads with a resistor and capacitor network on the ILIMCOMP pin. A simple

integrator compensation for resistive loads can be selected according the following equations:

CO2= 5

2∙π∙fbw∙RLOAD

(18)

Where CO2 is the capacitance after the average current sense resistor R(SNS)

fbw is the bandwidth of the voltage loop compensation (see Voltage Regulation Loop)

CO1 =CO − CO2

(19)

Where CO is the total output capacitance determined by the voltage loop calculation and the applications voltage

ripple requirement.

Where CO1 is the capacitance before the average current sense resistor R(SNS)

fp= 1

2∙π∙RSNS ∙CO2

(20)

fbwilim=fp∙10−0.25

(21)

CILIMCOMP =gmILIMCOMP

2π∙fbwilim

(22)

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