To account for delay to output, equation (B1) should be rewritten as follows:

(B2),

where the current overflow

∆I is:

∆I =

If the offset voltage is selected so that:

(B4),

the term

and the effect of the delay to output is eliminated.

garded as the difference between the actual voltage

applied on the current sense pin of the L5991 and

The compensation can be easily realized with the

circuit shown in fig. B2. R2 is often used along with

the capacitor C to smooth the leading edge spikes

occurring when the switch turns on. In such a case

only R1 will be added.

that, therefore, R1 >> R2 (R2 is typically 1k

Ω, R1 will

be in the M

Ω), perfect delay compensation will be

achieved when the ratio of the two resistors is:

R2

R1

(B5).

The resistor R3, connected to the 5V reference volt-

age externally available on pin 4, is used for addi-

tional offsetting the voltage on the current sense

pin.

3

3

3

a)

b)

Figure B1. Effect of the delay to output (a) and its compensation by means of Vo (b)

Vin

Rs

L5991

R2

R1

C

Vref

R3

4

10

13

12

Figure B2. Compensation of the delay to out-

put.

AN1049 APPLICATION NOTE

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