MP4561 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4561 Rev. 1.0

www.MonolithicPower.com

13

11/5/2012

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© 2012 MPS. All Rights Reserved.

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. To optimize the

compensation components for conditions, the

following procedure can be used.

Table 3—Compensation Values for Typical

Output Voltage/Capacitor Combinations

(V)

L (µH)

C2

(µF)

R3

(kΩ)

C3

(pF)

C7

(pF)

1.8

4.7

47

62

1000

47

2.5

4.7 - 6.8

22

36

680

None

3.3

6.8 -10

22

51

470

None

5

15 - 22

33

82

680

None

12

10

33

40.2

330

2

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

COUT

EA

CS

FB

2 π C2 f

V

R3

GG

V

××

=×

×

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

4

C3

2 π R3 f

>

××

3. Determine if the second compensation

capacitor (C6) 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:

S

ESR

f

1

2π C2 R

2

<

××

If this is the case, then add the second

the location of the ESR zero. Determine the C6

value by the equation:

ESR

C2 R

C6

R3

×

=

High Frequency Operation

The switching frequency of MP4561 can be

programmed up to 2MHz by an external resistor.

The minimum on time of MP4561 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

determines the input/output ripple voltage at

higher switching frequency. As a result of that,

high frequency ceramic capacitor is strongly

recommended as input decoupling capacitor and

output filtering capacitor for such high frequency

operation.

Layout becomes more important when the device

switches at higher frequency. It is essential to

place the input decoupling capacitor, catch diode

and the MP4561 (VIN pin, SW pin and PGND) as

close as possible, with traces that are very short

and fairly wide. This can help to greatly reduce

the voltage spike on SW node, and lower the EMI

noise level as well.