AN1621 APPLICATION NOTE

10/14

Sp

is determined by the load (Ro) and the output capacitance

Sz

is determined by the ESR and the value of the output capacitance

n

is the current transformer turn number

Rs

is the sense resistance

A load regulation of 1.5% has been imposed that corresponding to a maximum output voltage variation of

360mV. The power block DC gain is:

the

The E/A DC gain and the voltage divisor must be selected to fit this condition

The output voltage divider usually sinks a current of about 2mA and the resistance values are:

R11 = 12k

Ω

P1+ R12 = 1k

Ω

The R7 value is:

It is necessary to introduce a pole at about one third of the zero frequency and C9 is:

2.12 Overvoltage protection

The voltage divider, R9 and R10, is providing for fixing the threshold for overvoltage protection intervention. With

the selected values, the threshold is fixed at 30V.

C18 is fixing the min. overvoltage time intervention to avoid OVP triggering from spikes.

stitute the R11 resistor with a voltage divider, and to connect the common point to pin14.

2.13 Evaluation results

Fig 7 show the efficiency values obtained in different working conditions of input voltages and output currents.

The efficiency is from ac mains to output dc voltage, at a measured switching frequency of 200kHz.

G

pw

V

o

∆

V

c

∆

----------

nR

o

⋅

3R

s

⋅

---------------

==

V

c

∆

V

o

3R

s

⋅⋅

n

--------------------------

1

R

omin

----------------

1

R

omax

-----------------

–





⋅

2.7V

==

V

o

∆

V

c

∆

----------

7.5

=

R7

V

o

∆

V

c

∆

----------

P1

R12

+

()

⋅

80k

Ω

==

C9

1

2

π F

z

⋅⋅

3R7

⋅

---------------------

----------------------

1.8nF

==