MP103 Rev. 1.01

www.MonolithicPower.com

15

1/23/2014

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EMI

To meet the relevant conducted emissions

standard, slowly rise and fall the driver current is

adopted to turn on and turn off the external BJT.

Using this control method, a smaller X cap

connected between the input ports will pass EMI

with enough margins.

Besides, a capacitor connected between DR and

VB will further slow down the switching process

of external BJT for better EMI performance. The

larger value this capacitor used, the slower

switching process and better EMI performance

will be got, however, the more power losses will

be introduced by this capacitor and the longer

startup process will be, so it is a compromise to

select the value of the capacitor connected.

Generally, with a 100nF X cap connected to pass

EMI, a 2.2uF ceramic capacitor for 12V output

and 4.7uF ceramic capacitor for 5V output is a

good candidate to get good compromise.

Surge

From its working principle, MP103 is working just

when VIN falls into its charging window, so when

surges happen at this moment, then a lot of

energy will be absorbed by BJT and MP103 due

to the slow turn off process. To protect them from

damage, a fast turn off threshold (typically it is

71V) of VIN is set specially to shut down driver

current quickly.

Since there is no bulk capacitor to absorb AC line

transients, MOV should be used to protect the IC

to survive the surge test. Besides the value of

fuse resistor will also affect the surge result, the

larger value used, the better to facilitate to pass

the surge test, but the more power consumption

will be caused, in the meanwhile, the larger value

of fuse resistor used, the easier to trigger its fast

turn off threshold, so 10~20 Ω fuse resistor is

recommended in real application.

To pass 1kV surge test, 750V external BJT is

recommended considering some margin, MP103

can pass 1kV surge test with an appropriate

MOV such as TVR10431 connected between the

line input ports.

Besides, the thermal pad must be connected to

the GND for better surge performance.

PCB Layout Guide

PCB layout is very important to achieve good

regulation, ripple rejection, transient response

and

thermal

performance.

It

is

highly

recommended to duplicate EVB layout for

optimum performance. If change is necessary,

please follow these guidelines and take figure 6

for reference.

1) Keep the trace from positive output rectifier to

VIN as short and wide as possible.

2) Minimize the loop area formed by positive

output of rectifier, emitter of external BJT and

GND.

3) Minimize the loop area formed by VIN,

collector of BJT, emitter of BJT.

4) Ensure all feedback connections are short

and direct. Place the feedback resistors and

compensation components as close to the

chip as possible.

5) Place the NTC resistor as close to external

BJT as possible for temperature detection

and effective protection.

6) Output capacitor should be put close to the

output terminal.

7) Connect the exposed pad with GND to a

large copper area to improve thermal

performance and long-term reliability.

U1

RV

BD1

R4

C4

RF

L

N

VOUT

GND

Q1

R3

RT1

C5

CX

C2

R2

C3

R5

C1

R1

VB

5

NC

7

FB 3

DR

6

RT 1

VIN

8

GND 2

VOUT 4

MP103

GND

GND