L6911E

Device description

19/34

As shown in Figure 8 on page 18, 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 8 on page 18) proportional 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 inductor current) is added to the feedback signal, implementing the droop

function in a simple way. Referring to the schematic in Figure 9, the static characteristic of

the closed loop system is:

Equation 13

zero load (

effect is equal the ESR drop if:

Equation 14

Figure 9.

Compensation network

V

OUT

V

PROG

V

PROG

R3

R8 // 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