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L6911D

Figure 6. Output transient response without (a) and with (b) the droop function

As shown in figure 6, the ESR drop is present in any case, but using the droop function the total deviation of the

output voltage is minimized. In practice the droop function introduces a static error (Vdroop in figure 6) propor-

tional to the output current. Since a sense resistor is not present, the output DC current is measured by using

the intrinsic resistance of the inductance (a few m

Ω). So the low-pass filtered inductor voltage (that is the induc-

tor current) is added to the feedback signal, implementing the droop function in a simple way. Referring to the

schematic in figure 7, the static characteristic of the closed loop system is:

resistance. The second term of the equation allows a positive offset at zero load (

the droop effect (

Figure 7. Compensation network

Considering the previous relationships R2, R3, R8 and R9 may be determined in order to obtain the desired

droop effect as follow:

s

Choose a value for R2 in the range of hundreds of K

Ω to obtain realistic values for the other

components.

(a)

(b)

ESR DROP

ESR DROP

V

OUT

V

PRO G

V

PRO G

R3

R 8 // R9

+

R2

-------------------------------------

R

L

R8 // R9

⋅

R8

----------------------------------- I

OUT

⋅

–

⋅

+

=

R

L

R8 // R9

⋅

R8

-----------------------------------

ESR

=

C1 8

C2 0

R 4

R3

R9

L2

ES R

C6 - 1 5

R8

R2

PW M

C2 5