L6591 datasheet(20/32 Pages) STMICROELECTRONICS

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Application information

L6591

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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

low-side MOSFET first and, after a dead time Td, turn on the high-side MOSFET. Normally,

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

time Td) in response to the control loop; in case of overload it will be the overcurrent

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

oscillator. Precisely, with reference to the waveforms in Figure 15, where Tsw= 2/fosc, 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.

The dead-time Td equals the duration of the negative-going ramp of the oscillator sawtooth

plus an internal delay of 125 ns; hence it depends on the timing capacitor CT and the

resistor RT and is given by the approximate relationship:

Equation 5

There is an internal 325 ns limit to the minimum Td value, to make sure that no hazardous

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

capacitor values lower than 220 pF.

))

1150

(

osc

≈

()

osc

max

−

125

−

⋅

−

⋅

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