L6911C View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6911C' PDF : 20 Pages View PDF
L6911C
s From the above equations, it results:
R8
=
-∆----V----+----⋅---R-----2--
VPROG
⋅
∆--R--V--L--D--⋅--R-I--M-O----AO---X-P--
;
R9 = R8 ⋅ -∆-R--V--L--D--⋅--R-I--M-O----AO---X-P-- ⋅ 1-----+------∆--R-----V----L--1--D----⋅----R--I----M--O--------AO------X--P---- ;
Where IMAX is the maximum output current.
s The component R3 must be chosen in order to obtain R3<<R8//R9 to permit these and successive
simplifications.
Therefore, with the droop function the output voltage decreases as the load current increases, so the DC output
impedance is equal to a resistance ROUT. It is easy to verify that the output voltage deviation under load tran-
sient is minimum when the output impedance is constant with frequency.
To choose the other components of the compensation network, the transfer function of the voltage loop is con-
sidered. To simplify the analysis is supposed that R3 << Rd, where Rd = (R8//R9).
Figure 8. Compensation network definition
|A v |
|R |
R0
fD
|G lo o p|
G0
2
fLC
fCE
f2
f1
fEC
fCC
f
f3
f
fc
f
ConverterS ingularity
fLC = 1 / 2π ⋅ LC
fCE = 1 / 2π ⋅ ESR ⋅ C OUT
f = 1 / 2π ⋅ ESR ⋅ Cceramic
EC
f = 1 / 2π ⋅ Rceramic ⋅ Cceramic
CC
doublepole
ESRzero
Introduced by
CompensationNetworkS ingularity
CeramicCapacitor
f1 = 1 / 2π ⋅ R 4 ⋅ C 20
f2 = 1 / 2π ⋅ (R 3 + R 4) ⋅ C 20
f3 = 1 / 2π ⋅ R 3 ⋅ C 25
fd = 1 / 2π ⋅ Rd ⋅ C 25
The transfer function may be evaluated neglecting the connection of R8 to PHASE because, as will see later,
this connection is important only at low frequencies. So R4 is considered connected to VOUT. Under this as-
sumption, the voltage loop has the following transfer function:
12/20
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