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RSL >

40 2A

RSEN x (Vo - 2VIN)

2 x fS x L

- VSL

RSEN <

2 x VSL x fS x L

Vo - (2 x VIN)

RSEN =

VSENSE - (D x VSENSE x VSLratio)

ISWLIMIT

ISWLIMIT =

IOUT

(1-D)

(D x VIN)

(2 x fS x L)

RSEN =

VSENSE - (D x VSL)

ISWLIMIT

LM3488, LM3488-Q1

www.ti.com

SNVS089N – JULY 2000 – REVISED DECEMBER 2014

8.2.1.2.3

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 20). 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:

(18)

RSEN is part of the current mode control loop and has some influence on control loop stability. Therefore, once

the current limit threshold is set, loop stability must be verified. To verify stability, use the following equation:

(19)

If the selected RSEN is greater than this value, additional slope compensation must be added to ensure stability,

as described in Current Limit with External Slope Compensation.

8.2.1.2.4

Current Limit with External Slope Compensation

RSL is used to add additional slope compensation when required. It is not necessary in most designs and RSL

should be no larger than necessary. Select RSL according to the following equation:

where

•

RSEN is the selected value based on current limit. With RSL installed, the control signal includes additional

external slope to stabilize the loop, which will also have an effect on the current limit threshold. Therefore, the

current limit threshold must be re-verified, as illustrated in the equations below :

(20)

VCS = VSENSE – (D x (VSL + ΔVSL))

where

•

ΔVSL is the additional slope compensation generated and calculated as:

(21)

ΔVSL = 40 µA x RSL

(22)

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