CS5127 datasheet(13/24 Pages) CHERRY

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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,

TLOOP = - (TVC-VO ´ TDIVIDER ´ TCOMPENSATION)

TLOOP =

[]

[] ´[] ´[]

Bode plots for this transfer function are shown below.

Figure 9: Bode plot of gain response for compensated voltage mode

system.

Figure 10: Bode plot of phase response for compensated voltage mode

system.

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.

Figure 11: Two poleÐtwo zero compensation network.

The two zeros are placed close to the resonant frequency of

the LC output circuit. That is,

ÅÅ

The two poles are placed near half the switching frequen-

cy, or

ÅÅ

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.

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

Thermal Management for Semiconductor Components

Channel 2 ENABLE Feature

2¹ R3 C2

2¹ C1 R1

fSW

2¹ R3 C3

2¹ C1 R2

2¹ LC

From VOUT

VFB

COMP

-270.0

Frequency (Hz)

-90

-180

102

103

104

106

105

107

-60.0

Frequency (Hz)

-20

100

102

103

104

106

105

107

-100.0

sC1 (R1 + R2)+ 1

sC2 R1 (sC1 R2 + 1)

RA + RB

´ VIN ´ (sCRC + 1)

s2LC (R + RC) + s[L + RLC(R + RC) + RCRC] + R + RC

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