Datasheet

12/16
Technical Note
L
C
+
V
CC
VO
L
R
ESR
C
VCC
VO
(1) LC resonant circuit (2) With ESR provided
fr =
1
[Hz]
At this resonance point, a -180˚
A -90˚ phase-delay occurs.
phase-delay occurs.
(3) Insert feedback resistance in the C. (4) Insert the R3 in integrator
.
C1
V
O
R1
R2
C2
FB
INV
-
+
A
V
O
R1
R2
R3
C2
FB
INV
-
+
A
3. For output capacitors having low ESR, such as low impedance electrolyte capacitor or OS-CON
(1) Phase compensation with secondary phase lead
VO
R1 C1
R2
R3
C2
FB
INV
-
+
A
For output capacitors that have high ESR (i.e., several Ω), the phase compensation setting procedure becomes
comparatively simple. Since the DC/DC converter application has a LC resonant circuit attached to the output, a -180˚
phase-delay occurs in that area. If ESR component is present, howeve r, a +90˚ phase-lead occurs to shift the phase
delay to -90˚. Since the phase delay should be set within 150˚, it is a very effective method but tends to increase
the ripple component of the output voltage.
To cancel the LC resonance, the frequency to insert the phase lead should be set close to the LC resonant frequency.
The settings above have are estimated. Consequently, the settings may be adjusted on the actual system. Furthermore,
since these characteristics vary with the layout of PCB loading conditions, precise calculations should be made on the
actual system.
In order to use capacitors with low ESR (i.e., several tens of mΩ), two phase-leads should be inserted so that a -180˚
phase-delay, due to LC resonance, will be compensated. The following section shows a typical phase compensation
procedure.
To set phase lead frequency, insert both of the phase leads close to the LC resonant frequency. According to empirical rule,
setting the phase lead frequency f
Z2 with R3 and C2 lower than the LC resonant frequency fr, and the phase lead frequency
f
Z1 with the R1 and C1 higher than the LC resonant frequency fr, will provide stable application conditions.
According to changes in phase characteristics, due to the ESR, only one phase lead should be inserted.
For this phase lead, select either of the methods shows below:
2. For output capacitors having high ESR, such as electrolyte capacitor
( )
2010.02 - Rev. B
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