APW1173 Ver la hoja de datos (PDF) - Anpec Electronics
Número de pieza
componentes Descripción
Fabricante
APW1173 Datasheet PDF : 21 Pages
| |||
APW1173
Application Description (Cont.)
Frequency Compensation (Cont.)
The problem is the phase nearly –180 degrees at the
natural frequency especially in the high Q situation. If
the Q factor is high, the phase decrease vary sharp at
the location of the double poles. This problem leads
the regulator oscillating when use ceramic one as the
output capacitor without compensation. The purpose
of the compensation is saving the phase. The manner
is added additional zeros to achieve the goal. A zero
have the ability that contribute the maximum phase of
90 degrees. According this characteristic, needs two
zeros to compensate the phase loss. The PID
compensator is good for this.It shows as Figure-4.
C3
R2
C1
C2
R3
FB
R1
EA
Vref
COMP
Figure-4
The transfer function H(s) is
H (s)
=
(SC2R3 +1)[SC1(R1 + R2 )+1]
S(SC1R2 +1)[SC2C3R3 + (C2 + C3 )]
zero2
=
2π
1
⋅ C2 R3
zero3
=
2π
1
⋅ C1(R1
+
R2 )
pole3
=
2π
1
⋅ C1R2
pole4
=
C2 + C3
2π ⋅ C2C3R3
The frequency response of the PID compensator pre-
sented as Figure-5:
slope=-20db/
0db
decade
270d
180d
phase
90d
45d
0d
f
Zero2 Zero3 Pole3 Pole4
Figure-5
The assumption is 10(zero2)<zero3,10(zero3)<pole3,
10(pole3)<pole4.In order to compensate the phase,
place the two zeros closely and located before the
natural frequency. In general
zero2 ≅ zero3 = k ⋅ pole1,2
(11)
Where k is a constant, the value of k is almost 0.7 to
0.8.
The useful rules are:
(1) Determine the value of C2,the value must smaller
than 5nF to get fast response time.
(2) Find R3 by the equation
R3 = (2π ⋅ C2 ⋅ k ⋅ pole1,2 )−1
(3) Determine the value of C1 from 470pF to 1uF. This
range of C1 is for reference.
(4) The range of pole3 is from 150KHz to 300KHz.
Use this range to find the value of R2.
(5) Find R1 by the equation
R1 = (2π ⋅ C1 ⋅ k ⋅ pole1,2 )−1 − R2
(6) The location of pole4 is 5 times pole3. Use this
result to find the value of R3.
Copyright © ANPEC Electronics Corp.
16
Rev. A.5 - Aug., 2005
www.anpec.com.tw
Share Link: 