DocID18265 Rev 8

5/25

AN3319

Boost switching application

2

Boost switching application

A step-up (or boost) converter is a switching DC-DC converter able to generate an output

voltage higher than (or at least equal to) the input voltage.

Referring to

Figure 1, the switching element (S

w

) is typically driven by a fixed frequency

square waveform generated by a PWM controller.

When S

w

is closed (t

on

) the inductor stores energy and its current increases with a slope

depending on the voltage across the inductor and its inductance value. During this time the

output voltage is sustained by C

OUT

and the diode does not allow any charge transfer from

the output to input stage.

When S

w

is open (t

off

), the current in the inductor is forced, flowing toward the output until

voltage at the input is higher than the output voltage. During this phase the current in the

inductor decreases while the output voltage increases.

Figure 3 shows the behavior of inductor current.

Figure 3. Inductor current in continuous mode

The energy stored in the inductor during t

on

is ideally equal to the energy released during

t

off

, therefore the relation between t

on

and t

off

can be written as follows:

where “D” is the duty cycle of the square waveform driving the switching element.

Boost applications can work in two different modes depending on the minimum inductor

current within the switching period, that is if it is not null or null respectively:

•

Continuous mode (CM)

•

Discontinuous mode (DCM)

D

t

on

t

on

t

of f

+

()

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

=