L6911E datasheet(19/34 Pages) STMICROELECTRONICS

19 / 34 page

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

Where VPROG is the output voltage of the digital to analog converter (i.e. the set point) and

RL is the inductance resistance. The second term of the equation allows a positive offset at

zero load (

∆V+); the third term introduces the droop effect (∆V

DROOP). Note that the droop

effect is equal the ESR drop if:

Equation 14

Figure 9.

Compensation network

OUT

PROG

R8 // R9

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

R8 // R9

⋅

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

OUT

⋅

–

⋅

R8 // R9

⋅

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

ESR

C1 8

C2 0

R 4

ES R

C6 - 1 5

PW M

C2 5

V IN

V PRO G

R L

Z F

Z I

V OU T

V P H ASE

V CO M P

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