MP4569―75V, 0.3A, SYNCHRONOUS STEP-DOWN CONVERTER

MP4569 Rev. 1.0

www.MonolithicPower.com

12

9/12/2014

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

© 2014 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Inductor

As the Ipeak is fixed, 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

×

=

)

(

Switching Frequency

Switching frequency can be estimated by below

equation.

()

L

V

I

V

-

V

V

Io

2

f

IN

2

peak

OUT

IN

OUT

s

×

×

×

×

×

=

Larger inductor can get lower fs. And fs

increases 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)

×

×

×

=

Setting the Output Voltage

The output voltage is set using a resistive voltage

divider from the output voltage to FB pin. As

shown in figure 3.

To get the desired output voltage, divider resistor

can be chosen through below formula:

1

-

V

V

R5

R4

REF

OUT

=

The current flows into divider resistor would

increase the supply current, especially at no load

and light load condition. The Vin supply current

caused by the feedback resistors can be

calculated from:

η

1

V

V

R5

R4

V

I

IN

OUT

OUT

IN_FB

×

×

+

=

Where

To reduce this current, resistors in the megohm

range are recommended. The recommended

value of the feedback resistors are shown in

Table 1.

Table 1—Resistor Selection for Common

Output Voltages

R4 (kΩ)

R5 (kΩ)

3.3

1200

523

5

1200

300

Under Voltage Lock Out Point Setting

MP4569 has internal fixed under voltage lock out

(UVLO) threshold: rising threshold is about 4.2V

while trailing threshold is about 3.75V. External

resistor divider between EN and VIN as shown in

Figure 4 can be used to get higher equivalent

UVLO threshold.

Figure 4 – Adjustable UVLO using EN pin