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CS5127 View Datasheet(PDF) - Cherry semiconductor

Part Name
Description
MFG CO.
'CS5127' PDF : 24 Pages View PDF
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 VCONTROL transfer function, the gain of the feedback
resistor divider and the negative inverse of the compensa-
tion loop transfer function. That is,
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.
TLOOP = - (TVC-VO ´ TDIVIDER ´ TCOMPENSATION)
or
TLOOP =
[ ] R ´ VIN ´ (sCRC + 1)
s2LC (R + RC) + s[L + RLC(R + RC) + RCRC] + R + RC
[ ] [ ] [ ] ´ 1 ´
VR
RB
RA + RB
´ sC1 (R1 + R2)+ 1
sC2 R1 (sC1 R2 + 1)
Bode plots for this transfer function are shown below.
100
C1
R1
C2
R3 C3
From VOUT
R2
VFB
Figure 11: Two poleÐtwo zero compensation network.
COMP
The two zeros are placed close to the resonant frequency of
the LC output circuit. That is,
1
1
1
Å
Å
2¹ LC 2¹ C1 R2 2¹ R3 C3
60
The two poles are placed near half the switching frequen-
cy, or
20
fSW Å
1
Å
1
-20
2 2¹ C1 R1 2¹ R3 C2
-60.0
-100.0
1
10
102
103
104
105
106 107
Frequency (Hz)
Figure 9: Bode plot of gain response for compensated voltage mode
system.
90
0
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.
-90
-180
-270.0
1
10
102
103
104
105
106 107
Frequency (Hz)
Figure 10: Bode plot of phase response for compensated voltage mode
system.
Thermal Management for Semiconductor Components
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
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