ncp5389 ON Semiconductor, ncp5389 Datasheet - Page 24

• Manufacturer Part Number: ncp5389
• Description: 2/3 Phase Buck Controller For Vr11 Pentium Iv Processor Applications
• Manufacturer: ON Semiconductor

which includes a more detailed board parasitic for this
demo board.
Compensation and Output Filter Design
start for any similar output filter solution. The dynamic
performance can then be adjusted by swapping out various
individual components.
significantly different, it's best to use the available PSPICE
models to design the compensation and output filter from
scratch.
2.0 MHz. The phase switching frequency is currently set to
300 kHz. It can easily be seen that the board impedance of
0.75 mW between the load and the bulk capacitance has a
large effect on the output filter. In this case the ten 560 mF
and ceramic capacitor type output filter.
A complex switching model is available by request
The values shown on the demo board are a good place to
If the required output filter and switching frequency are
The design target for this demo board was 1.0 mW out to
By matching the following equations a good set of starting compensation values can be found for a typical mixed bulk
-100
-20
-40
-60
-80
80
60
40
20
0
100
RF/RFB
2p · CF · RF
2p · CFBI · (RFBI ) RFB)
1000
1
1
+
1/(2*PI*(RBRD+ESRBulk)*CBulk)
1/(2*PI*CFB1*(RFB1+RFB))
2p · (RBRD ) ESRBulk) · CBulk
1mOhm
http://onsemi.com
10000
1/(2*PI*CF*RF)
Figure 18.
NCP5389
+
Frequency
1/(2*PI*CCer*(RBRD+ESRBulk))
2p · CCer * (RBRD ) ESRBulk)
24
1
bulk capacitors have an ESR of 7.0 mW. Thus the bulk ESR
plus the board impedance is 0.7 mW + 0.75 mW or
1.45 mW. The actual output filter impedance does not drop
to 1.0 mW until the ceramic breaks in at over 375 kHz. The
controller must provide some loop gain slightly less than
one out to a frequency in excess 300 kHz. At frequencies
below where the bulk capacitance ESR breaks with the
bulk capacitance, the DC-DC converter must have
sufficiently high gain to control the output impedance
completely. Standard Type-3 compensation works well
with the NCP5389. RFB1 should be kept above 50 W for
amplifier stability reasons.
resulting gain generates constant output impedance from
DC up to the frequency where the ceramic takes over
holding the impedance below 1.0 mW. See the example of
the locations of the poles and zeros that were set to optimize
the model above.
The goal is to compensate the system such that the
100000
1/(2*PI*SQRT(ESL_Cer*CCer))
Zout Open Loop
Zout Closed Loop
Open Loop Gain with Current loop Closed
Voltage Loop Compensation Gain
1
RF/RFB1
1000000
1/(2*PI*RF*CH)
Error Amp
Open Loop
Gain
10000000
(eq. 9)