L6911C Ver la hoja de datos (PDF) - STMicroelectronics

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componentes Descripción

Fabricante

L6911C

5 BIT PROGRAMMABLE STEP DOWN CONTROLLER WITH SYNCHRONOUS RECTIFICATION

STMicroelectronics 

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

ESR DROP

ESR DROP

VNOM

VMAX

VMIN

(a)

(b)

L6911C

VDROOP

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 mΩ). 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:

VOUT

=

VPROG

+

VPROG

⋅

R-----3-----+-----RR-----82------/-/----R----9--

–

R-----L----⋅---R-----8------/--/---R----9--

R8

⋅

IOUT

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 (∆VDROOP). Note that the droop effect is equal the ESR drop if:

-R----L----⋅---R--R---8--8----/--/---R----9-- = ESR

Figure 7. Compensation network

V IN

V COM P

C18

ZF

V PROG

PW M

C 20

R4

R3

R2

V PHASE

L2

RL

R8

R9

C 25

ZI

V O UT

ESR

C 6-15

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 KΩ to obtain realistic values for the other

components.

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