MPQ4456 –INDUSTRIAL GRADE, 1A, 4MHz, 36V STEP-DOWN CONVERTER

MPQ4456 Rev. 1.0

www.MonolithicPower.com

9

2/8/2013

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

© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Frequency

The MPQ4456 has an externally adjustable

see Table1.

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

+

=

the output voltage.

Thus the output voltage is:

2

R

)

2

R

1

R

(

V

V

FB

OUT

+

=

A few

A of current from the high-side BS

circuitry can be seen at the output when the

MPQ4456 is at no load. In order to absorb this

small amount of current, keep R2 under 40k .

A typical value for R2 can be 40.2k . 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

40.2k , and R1 is 127k .

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

∆I

f

V

1

L

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

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.