MP2452―1A, 36V, 1MHz, STEP-DOWN CONVERTER

MP2452 Rev. 1.0

www.MonolithicPower.com

13

9/27/2010

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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 could cause system

unstable. A good rule of thumb is to set the

crossover frequency to approximately one-tenth

of the switching frequency. If an electrolytic

capacitor is used, the loop bandwidth is no higher

given by:

ESR

ESR

R

2

C

2

1

f

×

×

π

=

The Table 3 lists the typical values of

compensation components for 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 at

given conditions.

Table 3—Compensation Values for Typical

Output Voltage/Capacitor Combinations

(V)

L (µH)

C2

(µF)

R2

(kΩ)

C3

(pF)

1.2

3.3

10

243

6.8

2.5

4.7

10

57.6

6.8

3.3

4.7

10

39.2

10

5

6.8

10

23.7

10

12

10~15

10

8.87

10

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. In below cases, an

external BST diode is recommended from the 5V

to BST pin:

There is a 5V rail available in the system;

This diode is also recommended for high duty

applications.

The bootstrap diode can be a low cost one such

as IN4148 or BAT54.

MP2452

BS

SW

5V

0.1uF

Figure 2—External Bootstrap Diode

At no load or light load, the converter may

operate in pulse skipping mode in order to

maintain the output voltage in regulation. Thus

there is less time to refresh the BS voltage. In

order to have enough gate voltage under such

should be greater than 3V. For example, if the

than 3.3V+3V=6.3V to maintain enough BS

voltage at no load or light load. To meet this

requirement, EN pin can be used to program the

input UVLO voltage to Vout+3V.