MP4459 – 1.5A, 4MHz, 36V STEP-DOWN CONVERTER

MP4459 Rev. 1.03

www.MonolithicPower.com

10

4/9/2014

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© 2014 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Frequency

The MP4459 has an externally adjustable

frequency. The switching frequency (fS) can be

set using a resistor at FREQ pin (RFREQ). The

recommended RFREQ value for various fS, see

Table1.

Table 1—fS vs. RFREQ

RFREQ (kΩ)

fS (MHz)

18

4

20

3.8

22.1

3.5

24

3.3

26.7

3

30

2.8

33.2

2.5

39

2.2

45.3

2

51

1.8

57.6

1.6

68

1.4

80.6

1.2

100

1

133

0.8

200

0.5

340

0.3

536

0.2

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to FB pin.

The voltage divider divides the output voltage

down to the feedback voltage by the ratio:

2

R

1

R

2

R

V

V

OUT

FB

+

=

Where VFB is the feedback voltage and VOUT is

the output voltage.

Thus the output voltage is:

2

R

)

2

R

1

R

(

V

V

FB

OUT

+

=

A few µA of current from the high-side BS

circuitry can be seen at the output when the

MP4459 is at no load. In order to absorb this

small amount of current, keep R2 under 40kΩ.

A typical value for R2 can be 40.2kΩ. With this

value, R1 can be determined by:

)

k

)(

8

.

0

V

(

25

.

50

1

R

OUT

Ω

−

×

=

For example, for a 3.3V output voltage, R2 is

40.2kΩ, and R1 is 127kΩ.

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will result in less ripple current that will

result in lower output ripple voltage. However,

the larger value inductor will have a larger

physical size, higher series resistance, and/or

lower saturation current. A good rule for

determining the inductance to use is to allow

the peak-to-peak ripple current in the inductor

to be approximately 30% of the maximum

switch current limit. Also, make sure that the

peak inductor current is below the maximum

switch current limit. The inductance value can

be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

IN

OUT

L

S

OUT

V

V

1

ΔI

f

V

1

L

Where VIN is the input voltage, fS is the switching

frequency, and ΔIL is the peak-to-peak inductor

ripple current. Choose an inductor that will not

saturate under the maximum inductor peak

current. The peak inductor current can be

calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

+

=

IN

OUT

S

OUT

LOAD

LP

V

V

1

1

L

f

2

V

I

I

Where ILOAD is the load current. Table 2 lists a

number of suitable inductors from various

manufacturers. The choice of which style

inductor to use mainly depends on the price vs.

size requirements and any EMI requirement.