MP4558

– 1A, 2MHz, 55V STEP-DOWN CONVERTER

MP4558 Rev. 1.01

www.MonolithicPower.com

15

10/28/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

The system may have another important zero if

the output capacitor has a large capacitance or a

high ESR value. The zero, due to the ESR and

the output capacitor value, is located at:





ESR

ESR

1

f

2

π C2 R

In this case, a third pole set by the compensation

capacitor (C5) and R3 compensates for the effect

of the ESR zero on the loop gain. This pole is

located at:





P3

1

f

2

π C5 R3

The compensation network shapes the converter

transfer function for a desired loop gain. The

feedback-loop unity gain at the system crossover

frequency

is

important:

Lower

crossover

frequencies result in slower

line and load

transient

responses,

while

higher

crossover

frequencies

can

destabilize

the

system.

Generally,

set

the

crossover

frequency

to

approximately 1/10 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, use the following

procedure.

1. Choose R3 to set the desired crossover

frequency. Determine the R3 value from the

following equation:







C

OUT

EA

CS

FB

2

π C2 f

V

R3

G

G

V

2. Choose C3 to achieve the desired phase

margin. For applications with typical inductor

values, set the compensation zero

—f

the crossover frequency to provide sufficient

phase margin. Determine C3 from the following

equation:





C

4

C3

2

π R3 f

3. Determine if C5 is required

—if the ESR zero of

or if the following relationship is valid:





S

ESR

f

1

2

π C2 R

2

at the location of the ESR zero. Determine the

C5 value by the equation:





ESR

C 2 R

C5

R3

High-Frequency Operation

The

MP4558

switching

frequency

can

be

programmed up to 2MHz by an external resistor.

The minimum MP4558 ON-time is typically about

100ns. Pulse-skipping operation can be seen

more easily at higher switching frequencies due

to the minimum ON-time.

Since the internal bootstrap circuitry has higher

impedance that may not be adequate to charge

the

bootstrap

capacitor

during

each

charging

period,

add

an

external

bootstrap

charging

diode

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 the capacitor dominates so that

the ESL of the input/output capacitor determines

the input/output ripple voltage at higher switching

frequencies. Because of this ripple, use high-

frequency

ceramic

capacitors

for

the

input

decoupling capacitor and output the filtering

capacitor for high-frequency operation.

Layout becomes more important when the device

switches at higher frequencies. For best results,