RSEN =

VSENSE

Vo - Vi

L x fS

x D

ISWLIMIT +

ISWLIMIT =

VSENSE - (D x(VSL + 'VSL))

RSEN

RSL >

40 PA

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

21

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Typical Applications (continued)

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,

Equation 19 is recommended, using the VSL ratio value given in Electrical Characteristics.

(19)

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. As described in the slope compensation section,

Equation 20 must hold true for a current mode converter to be stable.

Sf - Se < Sn + Se

(20)

To verify that this equation holds true, use Equation 21.

(21)

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

as described in the section below.

8.2.1.2.5

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 Equation 22.

(22)

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 Equation 23, Equation 24, and Equation 25 below.

VCS = VSENSE – (D x (VSL + ΔVSL))

(23)

Where ΔVSL is the additional slope compensation generated as discussed in the slope compensation ramp

section and calculated using Equation 24.

ΔVSL = 40 µA x RSL

(24)

This changes the equation for current limit (or RSEN) as shown in Equation 25.

(25)

The RSEN and RSL values may have to be calculated iteratively in order to achieve both the desired current limit

and stable operation. In some designs RSL can also help to filter noise on the ISEN pin.

If the inductor is selected such that ripple current is the recommended 30% value, and the current limit threshold

is 120% of the maximum peak, a simpler method can be used to determine RSEN. Equation 26 below will provide

optimum stability without RSL, provided that the above 2 conditions are met.

(26)

8.2.1.2.6

Power Diode Selection

Observation of the boost converter circuit shows that the average current through the diode is the average load

current, and the peak current through the diode is the peak current through the inductor. The diode should be

rated to handle more than its peak current. The peak diode current can be calculated using Equation 27.

ID(Peak) = IOUT/ (1−D) + ΔIL

(27)