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Design Design
increases as the inductance (L) is reduced. As ω CASE STUDY – THE MPQ4430
L
increases, the phase delay caused by this pole is Let’s explore these principles in a real part. The
also pushed further out from the BW frequency. MPQ4330 from MPS is a 36V, 3.5A, synchro-
The reduction in the negative phase induced by nous step-down converter with integrated FETs
this pole results in an increase in the system’s and integrated compensation network. Figure 3
overall phase, and consequently the system’s PM. shows the typical application schematic of the
MPQ4430 and the internal compensation net-
Therefore, if COMP-Z is set based on the mini- work. The switching frequency is set using a
mum configurable switching frequency, the Phase resistor at the FREQ pin. Based on the resistor
Margin (PM) will increase as the switching fre- value, the switching frequency of this part can
quency increases. range between 350kHz and 2.5MHz.
Figure 3: The MPQ4430 Typical Application and Internal Compensation Network.
Unlike COMP-Z, the COMP-P frequency is set Since the switching frequency is configurable in this
PARTS WITH CONFIGURABLE ▶ COMP-P is set based on the maximum con- based on the maximum configurable switching part, the approach discussed in the previous section
SWITCHING FREQUENCY figurable switching frequency. frequency. As discussed earlier, the magnitude/ must be followed. COMP-Z must be set based on
Similar approaches can be applied to parts with angle due to a pole starts descending at 10% of the minimum switching frequency, which is 350kHz.
configurable switching frequencies, as long as COMP-Z is set based on the minimum configur- the frequency of that pole. Let’s assume that
two critical points are considered: able switching frequency because as the switch- f COMP-P was set based on the minimum switch- Assuming a 350kHz switching frequency, the
ing frequency increases, the inductor size is re- ing frequency. Now, if the part was configured to target BW would be 10% of that, which is 35kHz.
▶ COMP-Z is set based on the minimum con- duced proportionally. When looking at the second operate at the maximum switching frequency, Now, let’s calculate the COMP-Z frequency based
figurable switching frequency. pole in a PCM buck regulator power stage (repre- the phase reduction due to f COMP-P (which starts on the compensation network passive compo-
sented by Equation (2)), the pole frequency (ω ) taking effect from 0.1 x f COMP-P ) would occur inside nents. In this part, R COMP and C COMP are 460kΩ and
L
its BW. This is not recommended, as it places an- 52pF, respectively. Using Equation (6), this results
other pole inside the BW. Since there is only one in f COMP-Z being 6.6kHz. This value is within the ac-
zero available in a Type II compensation network, ceptable range of 4kHz to 8kHz based on Equa-
it cannot compensate for this pole. Therefore, tion (9), which means the first requirement is met.
the COMP-P frequency must be set based on the
maximum switching frequency of the part. Next, the COMP-P frequency is checked against
Figure 4: Bode Diagram of MPQ4430 at Three Switching Frequencies.
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