Application information

L6591

20/32

Equation 3

During the negative-going ramp of the sawtooth a clock pulse is released. A T flip-flop, along

with a logic circuit, separates the odd and the even clock pulses. The even ones turn off the

the high-side MOSFET is turned off (and the low-side MOSFET turned on after the dead

comparator to do the job or, in case of open control loop, the odd clock pulses will limit the

maximum ON-time within one oscillator cycle. In this way, the maximum duty cycle will be

limited right below 50 % and the operating frequency of the converter will be half that of the

maximum achievable duty cycle is:

Equation 4

At start-up the first clock pulse will turn on the low-side MOSFET for 10 oscillator cycles to

charge the bootstrap capacitor and then the high-side MOSFET will switch on. When the IC

resumes switching during burst-mode operation the first clock pulse will turn-on the low-side

MOSFET first to charge the bootstrap capacitor, and just after the second clock pulse the

high-side MOSFET will switch-on. In this way the bootstrap capacitor will always be charged

and ready to supply the high-side floating driver. The oscillator waveforms are illustrated in

Figure 15

as well.

Equation 5

condition of shoot-through can be generated, however it is recommended not to use

capacitor values lower than 220 pF.

))

1150

R

(

C

(

39

.

1

f

T

T

osc

+

≈

()

osc

d

SW

d

SW

d

SW

max

f

T

1

5

.

0

T

T

5

.

0

T

T

2

T

D

−

=

−

=

−

=

9

T

3

T

d

10

125

R

05

.

3

10

54

.

2

1

.

2

C

T

−

−

⋅

+

−

⋅

=