AN3008

High voltage power supply based on STOD2540

Doc ID 16021 Rev 2

5/14

The output ripple can be approximately given by the following equation.

Equation 6

The magnitude of the ripple will typically be linearly proportional to the output capacitance

present. For the best output voltage filtering, a low ESR output capacitor is recommended.

1.6

Diode selection

The output diode in a boost converter conducts current only when the power switch is off.

The average current is equal to the output current and the maximum current is equal to the

peak inductor current.

To maximize efficiency, we recommend using a Schottky diode characterized by:

1.

a small forward voltage drop.

2.

a rated current larger than the peak inductor current.

3.

a reverse voltage larger than the output voltage.

4.

a small reverse leakage current.

1.7

Single inductor circuit based on STOD2540 derives 35 V/70 V

The circuit shown in Figure 2 is capable of deriving +35 / +70 V from a [3; 5.5] input voltage

range. The STOD2540 DC-DC converter generates the 35 V output voltage. The addition of

an external charge pump consisting of two Schottky diodes (D2 and D3) and two capacitors

(C1 and C2) allows delivering output voltages of over 70 V.

the ON time the main switch is closed and the current flows from the input to ground through

L1 and the internal switch. During this time, the voltage at node SW is 0 V and C1 is charged

up to 35 V. In these conditions, D1 is reverse-biased, D2 is forward-biased, D3 is reverse-

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

+

×

−

×

=

Δ

IN

D

OUT

PK

SW

OUT

OUT

OUT

V

V

V

L

I

F

1

C

I

V

Figure 2.

70 V

COUT

CIN

C1

C2

D3

D2

L1

D1

35 V

SW = 0 V

+

35 V

+

70 V

COUT

CIN

C1

C2

D3

D2

L1

D1

35 V

SW = 0 V

+

35 V

+