LM3488MM datasheet(19/33 Pages) TI

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ISWLIMIT =

IOUT

(1-D)

(D x VIN)

(2 x fS x L)

RSEN =

VSENSE - (D x VSL)

ISWLIMIT

VOUT

ISEN

COUT

Rfb2

VIN

LM3488

RSEN

Rfb1

LM3488

LM3488-Q1

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SNVS089M – JULY 2000 – REVISED MARCH 2013

Figure 34. Adjusting the Output Voltage

SETTING THE CURRENT LIMIT

The maximum amount of current that can be delivered to the load is set by the sense resistor, RSEN. Current limit

occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage,

VSENSE. When this threshold is reached, the switch will be turned off until the next cycle. Limits for VSENSE are

specified in Electrical Characteristics. VSENSE represents the maximum value of the internal control signal VCS.

This control signal, however, is not a constant value and changes over the course of a period as a result of the

internal compensation ramp (see Figure 22). Therefore the current limit threshold will also change. The actual

current limit threshold is a function of the sense voltage (VSENSE) and the internal compensation ramp:

RSEN x ISWLIMIT = VCSMAX = VSENSE - (D x VSL)

where

•

ISWLIMIT is the peak switch current limit, defined by the equation below. As duty cycle increases, the control

voltage is reduced as VSL ramps up. Since current limit threshold varies with duty cycle, the following equation

should be used to select RSEN and set the desired current limit threshold:

(15)

(16)

The numerator of the above equation is VCS, and ISWLIMIT is calculated as:

(17)

To avoid false triggering, the current limit value should have some margin above the maximum operating value,

typically 120%. Values for both VSENSE and VSL are specified in Electrical Characteristics. However, calculating

with the limits of these two specs could result in an unrealistically wide current limit or RSEN range. Therefore, the

following equation is recommended, using the VSL ratio value given in Electrical Characteristics:

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