High voltage power supply based on STOD2540

AN3008

4/14

Doc ID 16021 Rev 2

Equation 2

Where

The following equation gives the switching frequency at the nominal load current.

Equation 3

The CF capacitor increases the amplitude of the voltage ripple on the FB pin, causing a

deterioration of the line regulation; therefore, the value of CF should be as small as possible.

1.4

Inductor selection

Since the hysteretic control scheme is inherently stable, the inductor value does not affect

the stability of the regulator. Using the PFM peak current control scheme, the converter

operates in discontinuous conduction mode (DCM).

The inductance value must be calculated so as to ensure that the inductor current reaches

the current limit before the maximum ON time expires. The following equation can be used

to calculate the maximum value of the inductance.

Equation 4

In this case the maximum value of the load current is given by Equation 5.

Equation 5

1.5

The output voltage ripple very much depends on the application conditions. The output

capacitor has a significant effect on the output voltage ripple magnitude because it supplies

the load current through the charge stored during the ON state.

The output voltage ripple consists of two parts: the first is caused by the ESR, the second by

the charging and discharging process of the output capacitor.

U

SW

R

20

F

2

1

CF

×

×

π

×

=

2

PK

IN

OUT

LOAD

LOAD

SW

I

L

)

V

V

(

I

2

)

I

(

F

×

−

×

×

=

MAX

_

ON

PK

MIN

_

IN

T

I

V

L

×

≤

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

×

×

−

+

×

×

=

MIN

IN

PK

IN

OUT

2

PK

MAX

_

LOAD

toff

V

L

I

)

V

Vd

V

(

2

L

I

I