CS5127
Applications Information: continued
Now that we have the compensation components chosen,
we can put together a transfer function for the entire con-
trol loop. The transfer function is the product of the VOUT
to V
CONTROL
transfer function, the gain of the feedback
resistor divider and the negative inverse of the compensa-
tion loop transfer function. That is,
T
LOOP
= - (T
VC-VO
´
T
DIVIDER
´
T
COMPENSATION
)
or
T
LOOP
=
C1
R3
R1
R2
V
FB
COMP
Entering the loop transfer function in a mathematics pro-
gram or a spreadsheet and evaluating the performance
from resulting Bode plots may help to further optimize the
compensation network component values.
Compensation may be further optimized by using a two
poleÐtwo zero compensation network as shown below.
C2
C3
[
´
R
´
V
IN
´
(sCR
C
+ 1)
s
2
LC
1
(R + R
C
) + s[L + R
L
C(R + R
C
) + RCR
C
] + R + R
C
´
RB
RA + RB
]
From V
OUT
[ ][
V
R
100
60
Gain, (dB)
][
´
sC1 (R1 + R2)+ 1
sC2 R1 (sC1 R2 + 1)
]
Figure 11: Two poleÐtwo zero compensation network.
The two zeros are placed close to the resonant frequency of
the LC output circuit. That is,
1
2¹
LC
Å
1
2¹ C1 R2
Å
1
2¹ R3 C3
Bode plots for this transfer function are shown below.
The two poles are placed near half the switching frequen-
cy, or
f
SW
2
Å
1
2¹ C1 R1
Å
1
2¹ R3 C2
20
-20
-60.0
-100.0
1
10
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz)
Channel 2 ENABLE Feature
The ENABLE lead controls operation of channel 2. Channel
2 operates normally if the ENABLE lead voltage is greater
than 3.5V. Setting the ENABLE lead voltage below 1.5V
will guarantee that channel 2 is disabled. In this case, the
GATE2 lead will be held low and no switching will occur.
This feature can be used to selectively power up or power
down circuitry that may not always need to be on. For
example, in a laptop computer, channel 1 could power the
microprocessor while channel 2 controlled the disk drive.
Channel 2 could be turned off if the drive was not in use.
Figure 9: Bode plot of gain response for compensated voltage mode
system.
90
Phase, (degree)
0
-90
Thermal Management for Semiconductor Components
-180
-270.0
1
10
10
2
10
3
10
4
10
5
10
6
10
7
Frequency (Hz)
Figure 10: Bode plot of phase response for compensated voltage mode
system.
Semiconductor components will deteriorate in high tem-
perature environments. It is necessary to limit the junction
temperature of control ICs, power MOSFETs and diodes in
order to maintain high levels of reliability. Most semicon-
ductor devices have a maximum junction temperature of
125¡C, and manufacturers recommend operating their
products at lower temperatures if at all possible.
Power dissipation in a semiconductor device results in the
generation of heat in the pin junctions at the surface of the
13
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
