MPQ2451―INDUSTRIAL-GRADE 36V, 2MHz, 0.6A, STEP-DOWN CONVERTER, AEC-Q100 QUALIFIED

MPQ2451 Rev 1.02

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13

5/3/2013

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Compensation Components

The goal of compensation design is to shape the

converter transfer function to get a desired loop

gain. Lower crossover frequencies result in

slower line and load transient responses, while

higher crossover frequencies can cause system

instability. Generally, set the crossover frequency

to equal approximately one-tenth of the switching

frequency. If using an electrolytic capacitor,

select a loop bandwidth is no higher than 1/4 of

given by:

ESR

ESR

R

2

C

2

1

f

×

×

π

=

The Table 3 lists the typical values of

compensation components of some standard

output voltages with various output capacitors

(ceramic) and inductors. The values of the

compensation components have been optimized

for fast transient responses and good stability

under the given conditions.

Table 3: Compensation Values for Typical

L (µH)

C2 (µF)

R2 (kΩ)

C3 (pF)

1.2

2.2

10

249

22

2.5

2.2

10

57.6

22

3.3

2.2

10

40.2

33

5

3.3

10

23.7

33

12

6.2

10

8.87

47

Note:

With the compensation, the control loop has the bandwidth at about

1/10 switching frequency and the phase margin higher than 45

degree.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator. Connect an external

BST diode from 5V to the BST pin if:

There is a 5V rail available to the system;

This diode is also recommended for high-duty–

A low-cost bootstrap diode, such as IN4148 or

BAT54, is suitable for such applications.

MPQ2451

BS

SW

5V

Figure 2: External Bootstrap Diode

At no load or light load, the converter may

operate in pulse-skipping mode in order to

regulate the output voltage and leave less time to

refresh the BST voltage. To ensure sufficient

than 3.3V+3V=6.3V at no load or light load. To

meet this requirement, the EN pin can be used to