MP4458 – 1A, 4MHz, 36V STEP-DOWN CONVERTER

MP4458 Rev. 1.02

www.MonolithicPower.com

13

3/4/2013

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Table 4—Compensation Values for Typical

Output Voltage/Capacitor Combinations

L

R3

C3

C6

1.8V

4.7µH

47µF

ceramic

105k

100pF

None

2.5V

4.7µH-

6.8µH

22µF

ceramic

54.9k

220pF

None

3.3V

6.8µH-

10µH

22µF

ceramic

68.1k

220pF

None

5V

15µH-

22µH

22µF

ceramic

100k

150pF

None

12V

22µH-

33µH

22µF

ceramic

147k

150pF

None

Note: The selection of L is based on

refer to “Inductor section” on page7 to select proper

inductor if

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

2

3

R

×

×

×

×

π

=

(which typically has a value no higher than

2. Choose the compensation capacitor (C3) to

achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C

f

3

R

2

4

3

C

×

×

π

>

Where R3 is the compensation resistor value.

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:

2

f

R

2

C

2

1

S

ESR

<

×

×

π

If this is the case, then add the second

at the location of the ESR zero. Determine the

C6 value by the equation:

3

R

R

2

C

6

C

ESR

×

=

High Frequency Operation

The switching frequency of MP4458 can be

programmed up to 4MHz by an external resistor.

Please pay attention to the following if the

switching frequency is above 2MHz.

The minimum on time of MP4458 is about 80ns

(typ). Pulse skipping operation can be seen

more easily at higher switching frequency due

to the minimum on time. Recommended

operating voltage at 4MHz is 12V or below, and

24V or below at 2MHz.

30

25

20

15

10

5

1.5

2.0

2.5

3.0

3.5

4.0

Input Max vs

Switching Frequency