MP4568

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

MP4568 Rev. 1.01

www.MonolithicPower.com

14

8/18/2013

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© 2013 MPS. All Rights Reserved.

Soft Start Capacitor

There is an internal 1ms soft start when float SS

pin and this is the shortest soft start time. To get

longer soft start, it needs to add capacitor

between SS pin and GND. The soft start time is

the duration when SS is charged from 0 to FB

reference voltage 1V by an internal 3

μA current

source. So the capacitor at SS pin can be chosen

according to below formula:

SS

SS

C3

Τ ( F)

 



To get the soft start time longer than 1ms, a

capacitor bigger than 3nF is needed. When using

a capacitor smaller than 3nF, the soft start time is

the internal 1ms soft start.

Feedforward Capacitor

As described above that the PWM control

scheme of MP4568 is very special and the HS

turn on when FB drops lower than reference

voltage.

This

brings

good

load

transient

performance. However, this also makes the HS

turn on moment is very sensitive to the FB

voltage. Once there is noise on FB, the moment

HS turns on is easy to be affected, and then Fsw

jitter would occur. The Fsw jitter is easy to

happen especially when Vo ripple is very small.

To improve the jitter performance, a small

feedforward capacitor between Vo and FB can

be used and typical 39pF is recommended.

PCB Layout

1) Keep the path of switching current short and

minimize the loop area formed by input

capacitor, high-side, low-side MOSFET and

output capacitor.

2) Bypass ceramic capacitors should be as

close as possible to the VIN pin.

3) Make sure that all feedback connections are

short and direct. Place the feedback resistors

as close to the chip as possible.

4) Keep SW away from sensitive analog areas

such as FB.

5) For better thermal performance and long-term

reliability consideration, VIN, SW and GND

should be connected to a large copper area

respectively to cool the chip.

C2

R3

GND

OUT

IN

R1

R2

C3

C1

R1

Figure 7

– PCB Layout Example

Design Example

Below

is

a

design

example

following

the

application guidelines for the specifications:

Table 2

— Design Example

4.5V

– 45V

3.3V

Io

100mA

The detailed application schematic is shown in

Figure 8. The typical performance and circuit

waveforms have been shown in the Typical

Performance Characteristics section. For more

device applications, please refer to the related

Evaluation Board Datasheets.

VIN

EN

C1

SW

2

EN

9

ENHY

8

ILIM

6

IN

10

FB

5

BIAS

4

MP4568

SS

7

U1

R3

C2

L1

C3

VOUT

3.3V/100mA

2M

10uF

22uF

100uH

0.1uF

R1

523k

R2

1.2M

R4

100k

C4

10nF

C5

39pF

Figure 8

— Detailed Application Schematic