AN4501 datasheet(25/48 Pages) STMICROELECTRONICS

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AN4501

Selection of the freewheeling diode

Schottky diodes are recommended to maximize the efficiency of the DC-DC converter

thanks to their low forward voltage and their fast recovery time.

Figure 10. Typical waveforms of the freewheeling diode in CCM

Diode current capability

The freewheeling diode conducts only during the converter’s off-phase (see iD on Figure 7

and Figure 10).Therefore, the average diode current over one switching period is equal to

the output current. However, the diode peak current is equal to the inductor’s peak current.

The maximum rating of the Schottky diode must be chosen accordingly.

ID,MAX = IOUT

ID,PEAK,MAX = IL,PEAK,MAX (see Equation 14)

Diode power dissipation and temperature

The power dissipation in the freewheeling diode is PD = VFDIODE x IOUT

The cooling of the diode must guarantee that its max. junction temperature is not exceeded

even at the maximum ambient temperature:

TJ,DIODE = TAMB+ RTHJ-AMB,DIODE x PD

TJ,DIODE,MAX = TAMB,MAX+ RTHJ-AMB,DIODE x PD

Diode breakdown voltage

The diode is reverse biased during the on-phase of M1. The maximum reverse voltage in

normal operation must be at least higher than the max. LED voltage (neglecting the voltage

drop across RSENSE and M2). However, the diode can see a reverse voltage, which is even

higher in case of open load.

Indeed, as the output current goes to zero, the L99LD01 increases the duty cycle and the

output voltage until an over-voltage condition on the output is detected.

The freewheeling diode must also withstand a reverse voltage up to the output over-voltage

threshold (VOUT,OVTH).

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