MP173– NON-ISOLATED OFFLINE REGULATOR

MP173 Rev. 1.0

www.MonolithicPower.com

14

8/19/2015

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oo

oo

FB

12

o

1

2

o

VC

VC

1

C

2R

R

I

R

R

I









(11)

Dummy Load

A dummy load is required to maintain the load

regulation. This ensures sufficient inductor

energy to charge the sample and hold capacitor

to detect the output voltage. Normally a 3 mA

dummy load is needed and can be adjusted

according to the regulated voltage. There is a

compromise

between

small,

no-load

consumption and good, no-load regulation,

especially for applications that require 30 mW no-

load consumption. Use a zener to reduce no-load

consumption if no-load regulation is not a

concern.

Auxiliary VCC Supply

MP172

VCC

FB

SOURCE

SOURCE

C2

R1

R2

D2

C3

L1

VOUT

R3 D3

achieves the 30 mW no-load power requirement

by adopting an external VCC supply to reduce

overall power consumption (see Figure 7).

This auxiliary VCC supply is derived from the

resistor connected between C2 and C3. C3

should

be

set

larger

than

the

value

recommended above. D3 is used in case VCC

interferes with FB. R3 is determined Using

Equation (12).

oFW

S

VV

5.8V

R3

I





                                                 (12) 

applications, R3 should be adjusted to meet the

application’s

specific

In

a

particular

Surge Performance

An appropriate input capacitor value should be

chosen to obtain good surge performance. Figure

8 shows the half-wave rectifier. Table 2 shows

the capacitance required under normal conditions

for different surge voltages. FR1 is a 20 Ω/2 W

fused resistor, and L1 is 1 mH for this

recommendation.

L1

C1

C2

N

L

FR1

Figure 8—Half-wave rectifier

Table 2—Recommended capacitance

Surge

Voltage

500 V

1000 V

2000 V

C1

1 μF

2.2 μF

3.3 μF

C2

1 μF

2.2 μF

3.3 μF

PCB Layout Guidelines

Efficient PCB layout is critical for reliable

operation, good EMI, and thermal performance.

For best results, follow the guidelines below:

1) Minimize the loop area formed by the input

capacitor, IC, freewheeling diode, inductor,

and output capacitor.

2) Place the power inductor far away from the

input filter while keeping the loop area to the

inductor at a minimum (see example below).

3) Place a capacitor valued at several hundred

pF between FB and SOURCE as close to the

IC as possible.

4) Connect the exposed pads or large copper

area

with

DRAIN

to

improve

thermal

performance.