MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER

MP4560 Rev. 1.0

www.MonolithicPower.com

13

11/5/2012

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© 2012 MPS. All Rights Reserved.

The

system

may

have

another

zero

of

importance, if the output capacitor has a large

capacitance and/or a high ESR value. The zero,

due to the ESR and capacitance of the output

capacitor, is located at:

ESR

ESR

R

C2

2π

1

f

×

×

=

In this case, a third pole set by the compensation

capacitor (C5) and the compensation resistor (R3)

is used to compensate the effect of the ESR zero

on the loop gain. This pole is located at:

3

R

5

C

π

2

1

P

×

×

=

The goal of compensation design is to shape the

converter transfer function to get a desired loop

gain. The system crossover frequency where the

feedback loop has the unity gain is important.

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.

Table 3—Compensation Values for Typical

Output Voltage/Capacitor Combinations

(V)

L (µH)

C2

(µF)

R3

(kΩ)

C3

(pF)

C6

(pF)

1.8

4.7

33

32.4

680

None

2.5

4.7 - 6.8

22

26.1

680

None

3.3

6.8 -10

22

68.1

220

None

5

15 - 22

33

47.5

330

None

12

10

22

16

470

2

To optimize the compensation components for

conditions not listed in Table 3, the following

procedure can be used.

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

π

2

3

R

×

×

×

×

=

2. Choose the compensation capacitor (C3) to

achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

crossover frequency provides sufficient phase

margin. Determine the C3 value by the following

equation:

C

f

3

R

π

2

4

3

C

×

×

>

3. Determine if the second compensation

capacitor (C5) is required. It is required if the

ESR zero of the output capacitor is located at

less than half of the switching frequency, or the

following relationship is valid:

2

f

R

C2

2π

1

S

ESR

<

×

×

If this is the case, then add the second

the location of the ESR zero. Determine the C5

value by the equation:

3

R

R

2

C

5

C

ESR

×

=

High Frequency Operation

The switching frequency of MP4560 can be

programmed up to 2MHz by an external resistor.

The minimum on time of MP4560 is about 100ns

(typ). Pulse skipping operation can be seen more

easily at higher switching frequency due to the

minimum on time.

Since the internal bootstrap circuitry has higher

impedance, which may not be adequate to

charge the bootstrap capacitor during each

(1-D)×Ts charging period, an external bootstrap

charging diode is strongly recommended if the

switching frequency is about 2MHz (see External

Bootstrap

Diode

section

for

detailed

implementation information).

With higher switching frequencies, the inductive

reactance (XL) of capacitor comes to dominate,

so that the ESL of input/output capacitor