E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
7.8.6 Run Away at Start-up and Output Short
Under start-up or output short condition, the PFM will switch the internal P-channel MOS transistor always
on to bring the output voltage in tolerance. When the voltage across RS caused by the high switch current
reaches the threshold level (VCLIM) of the comparator CCMP, it resets the PFM and therefore the internal
P-channel MOS transistor is immediately switched off. After the fixed off-time (TOFFM) the internal P-chan-
nel MOS transistor starts to conduct again. The required duty cycle in this short circuit situation is:
Duty Short
≈
VINPUT
Vd DSW + VRLCK
− Von PMOS − VRS + Vd DSW
[12]
Depending on input voltage VS (especially at high input voltages), inductance values for the choke, internal
P-channel MOS transistor´s on resistance and impedance of the rest of external components it is not always
possible to keep the duty to the required small value. When this happens the current through the circuit is
rising from period to period, until the output voltage reaches a value for the duty cycle that the E910.27 can
handle, or reaches a maximum current, limited by the cumulated resistances in the buck path only. In this
inrush current situation, the external components may be over stressed and / or the internal thermo protec-
tion switches off the whole SMPS. Normally this start-up time period is very short and the effect takes no
matter.
To handle this current run away, the user can select with pin SDLY=true, a longer off-time option (TOFFMS),
that takes place the minimum off-time (TOFFM) in case of triggering the current comparator CCMP only.
Due to the reduced stored energy per switching cycle, ramp up duration of the whole supply takes somewhat
longer. However, be careful there may be combinations of external component values (especially too low or
too high inductance values for the choke), which prevents successful start-up of the supply.
ELMOS Semiconductor AG
Specification 25 / 50
03SP0357E.00 05.09.2006
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