L7986TA STMicroelectronics, L7986TA Datasheet - Page 20

• Manufacturer Part Number: L7986TA
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986TA

• Low Dropout Operation: 100% duty cycle

Application informations
6.4.1
20/43
The transfer function on the LC filter is given by:
Equation 19
where:
Equation 20
Equation 21
As seen in
following paragraph the guidelines to select the Type II and Type III compensation network
are illustrated.
Type III compensation network
The methodology to stabilize the loop consists of placing two zeros to compensate the effect
of the LC double pole, so increasing phase margin; then to place one pole in the origin to
minimize the dc error on regulated output voltage; finally to place other poles far away the
zero dB frequency.
If the equivalent series resistance (ESR) of the output capacitor introduces a zero with a
frequency higher than the desired bandwidth (that is: 2π∗ESR∗COUT<1/BW), the type III
compensation network is needed. Multi Layer Ceramic capacitors (MLCC) have very low
ESR (<1mΩ), with very high frequency zero, so type III network is adopted to compensate
the loop.
In
(f
Equation 22
Z1
Figure 9
, f
Z2
) and three poles (f
Chapter 5.3
the type III compensation network is shown. This network introduces two zeros
f
Q
LC
=
=
----------------------------------------------------------------------------------------- -
----------------------------------------------------------------------- -
2π
two different kind of network can compensate the loop. In the two
R
f
Z1
OUT
⋅
P0
L
=
L C
G
, f
+
----------------------------------------------- -
2π C
⋅
⋅
Doc ID 022098 Rev 1
LC
L C
C
P1
OUT
⋅
s ( )
OUT
⋅
, f
1
OUT
P2
3
=
⋅
⋅
⋅
). They expression are:
R
1
(
R
------------------------------------------------------------------------ -
1
OUT
1
⋅
1
+
(
+
R
+
----------------------------
2π Q f ⋅
-------------- -
R
OUT
ESR
R
⋅
OUT
E SR
3
1
⋅
)
s
,
+
+
,
------------------------- -
2π f
ESR
LC
f
⋅
Z2
f
s
+
zESR
zESR
)
⎛
⎝
,
=
------------------ -
2π f
----------------------------- -
2π R
s
=
⋅
⋅
LC
------------------------------------------- -
2π ESR C
1
R
4
⎞
⎠
OUT
⋅
2
⋅
C
4
1
=
⋅
V
--------------
I
OUT
OUT
OUT
L7986TA