MP4568

―60V, 100mA, SYNCHRONOUS STEP-DOWN CONVERTER

MP4568 Rev. 1.01

www.MonolithicPower.com

12

8/18/2013

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

© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting Peak Current

The peak current can be programmed from

55mA to 260mA by changing the resistor

between ILIM pin and ground. The peak current

formula:

ILIM

peak

22

R

(k )

I



Where 55mA

≤Ipeak≤260mA.

maximum

value 260mA;

Ipeak is clamped to its minimum value 55mA. To

be convenient, ILIM pin can be just shorted to

GND when Ipeak is set at 260mA and floating

when Ipeak is set at 55mA.

When

setting

Ipeak,

be

noticed

that

the

maximum output current is equal to half of the

Ipeak. Make sure the Ipeak is high enough for

the output current requirement. Also be noticed

that higher Ipeak would result in higher inductor

current ripple, larger input and output voltage

ripple, which means bigger components (inductor,

input and output capacitors).

Selecting the Inductor

As the Ipeak is determined, for given input

voltage and output voltage, the inductor value

can be determined by the following formula:





s

peak

IN

OUT

IN

OUT

f

I

V

V

-

V

V

L









Where fs is the switching frequency at the

maximal output current.

Larger inductor value results in lower switching

frequency, as well as higher efficiency. However,

the larger value inductor will have a larger

physical size, higher series resistance, and/or

lower saturation current as well as the slow load

transient dynamic performance. There is also a

lower limit of the inductor value, which is

determined by the minimum on time. In order to

keep the inductor working under control, the

inductor value should be chosen higher than

Lmin that is derived from below formula:

peak

ON(MIN)

MAX

IN

MIN

I

T

V

L





)

(

on time.

Switching Frquency

Switching frequency can be estimated by below

equation.





L

V

I

V

-

V

V

Io

2

f

IN

2

peak

OUT

IN

OUT

s













Higher Ipeak and inductor can get lower fs. And

fs inceases as Io increasing. When Io increases

to its maximal value Ipeak/2, fs also reaches its

highest value and can be derived by:





L

V

I

V

-

V

V

f

IN

peak

OUT

IN

OUT

s(max)









Below figures show the calculated fs(max) at

different VIN and inductor with typical Vo (3.3V

and 5V) and Ipeak (220mA).

Figure 3

– f