L5953_07 STMICROELECTRONICS [STMicroelectronics], L5953_07 Datasheet - Page 27

• Manufacturer Part Number: L5953_07
• Description: Multiple switching voltage regulator
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

L5953
6.3.3
6.4
6.5
Output capacitor selection
The criteria for the selection of the capacitor C7 is based on the output voltage ripple
requirements. The ripple on the output voltage is due to a capacitive contribute, often
negligible, equal to
and a resistive contribute given by the ESR of the capacitor and which is equal to
ΔVC fixes the value for C7 while ΔVESR limits the ESR of the capacitor.Usually the capacitor
is chosen so that the total ripple on the output regulated voltage Vo is equal to 1% of the
value of Vo. If V
More often the minimum value of C7 is imposed by other considerations such as to get a
good dynamic behavior of the output voltage in case of large load variations.
Free-wheeling diode
The diode must withstand an average current Id equal to Id = I
current of intervention of the short circuit protection and D
D
Compensation network
In continuous mode, the voltage controlled buck converter shows two poles due to the
output LC filter and one zero due to the ESR of the output capacitor. The suggested
compensation network introduces two zeros and two poles:
the second pole is nominally located in the origin which means an infinite gain at frequency
null. In the reality the DC value of the closed loop gain can not be greater than the DC value
of the EA open loop gain and the pole is located at very low frequency.
The values for the components of the compensation network can be fixed when the inductor
L1 and the output capacitor C7 are chosen. The necessary steps are:
min
–
–
is very low, the current Id can be assumed equal to I
the zeros compensate the double poles of the LC filter
one pole compensates the zero due to ESR of the output capacitor
ripple
is the maximum allowed voltage ripple on Vo then it should result:
L
min
=
V
ΔV
ripple
-------------------- -
ΔI
ΔV
ESR
L max
V
c
,
O
=
≥
=
--------------------------------------- -
8 C7 f
⋅
ΔV
⋅
ESR ΔI
1
ΔI
c
–
2
L max
⋅
----------------- -
V
,
+
⋅
i max
sw min
V
,
ΔV
O
L max
,
,
ESR
⋅
lim
min
2
------------------- -
f
sw min
.
is the minimum duty cycle. As
1
,
lim
(1- D
min
) where I
Application note
lim
is the
27/31