AN3008

High voltage power supply based on STOD2540

Doc ID 16021 Rev 2

3/14

1

High voltage power supply based on STOD2540

1.1

STOD2540 function description

The STOD2540 uses a PFM control scheme to reach high efficiency in low load conditions.

The DC-DC has a current mode control scheme that uses a minimum OFF time and a

maximum ON time.

The converter monitors the output voltage through the resistor dividers R1 and R2 by

comparing the feedback voltage with the internal reference voltage of 1.24 V.

The integrated main power switch is turned on as soon as the feedback voltage falls below

the internal reference. The switch stays on until the inductor current reaches the peak

current limit or for a maximum ON time equal to 5.5 µsec. The peak current limit value is

adjustable through an external resistor connected between the RSET pin and GND. The

main switch stays off for at least a minimum OFF time (300 ns typical) and remains in the off

state for as long as the feedback voltage remains above the internal reference voltage.

During the ON time, the load current is only supplied by the charge stored in the output

capacitor until the feedback voltage drops below the reference voltage again.

PFM regulation is particularly useful when output currents are low and the part is prevalently

in the OFF state.

1.2

Load disconnect

When the device is in shutdown mode, a DC current path exists between the power source

and the load. A high-side switch LDS isolates the load from the source when the device is

disabled.

1.3

Output adjust

Choose the R4 value in the range of 10 to 200 k

Ω. The value of R3 can be calculated from

the following equation.

Equation 1

Where

The STOD2540 shows a pulses burst behavior that causes a high output voltage ripple. To

decrease the output ripple it is possible to insert a capacitor across the upper feedback

resistor. The following formula can be used to obtain a first estimation of the value of the

capacitor.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

=

1

V

V

R

R

FB

OUT

L

U