E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
Fop ≈ Duty ≅ 1000kHz ⋅ Duty
TONM
[5]
If the duty cycle is greater than 33%, the off-time (TOFFM) sets the switching frequency. The operating fre-
quency is approximately:
Fop ≈ (1 − Duty ) ≅ 500kHz ⋅ (1 − Duty )
TOFFM
[6]
In both cases the voltage is regulated by the voltage comparator (VCMP). For low duty cycles (<33%) the
internal P-channel MOS transistor is turned on for the minimum on-time (TONM), causing fixed on-time ope-
ration. During the internal P-channel MOS transistor's on-time the output voltage rises. Once the internal P-
channel MOS transistor is turned off, the voltage drops to the regulation threshold set by the external voltage
divider (RFB1 and RFB2), and another cycle is initiated. For high duty cycles (>33%) the internal P-channel
MOS transistor remains off for the minimum off-time (TOFFM) causing fixed off-time operation. In this case
the internal P-channel MOS transistor remains on, until the output voltage rises to the regulation threshold
set by the external voltage divider (RFB1 and RFB2). Then the internal P-channel MOS transistor turns off
for the minimum off-time (TOFFM) and the next cycle starts.
Duty > 33%, TOFF = const = TOFFM
Duty < 33%, TON = const = TONM
VINPUT
Figure 7.8.4-1: Relative Switching Frequency in Continuous Conduction Mode
An unique regulation feature of the PFM controller is its automaticly selection of fixed on-time or fixed off-ime
mode. By switching between fixed on-time and fixed off-time operation the E910.27 can operate at high input
to output ratios, yet still operate up to 100% duty cycle for low dropout. When transitioning from fixed on-time
to fixed off-time mode, the output voltage drops slightly due to the output ripple voltage. In fixed on-time ope-
ration the minimum output voltage is regulated, while in fixed off-time operation the maximum output voltage
is regulated. Thus, as the input voltage drops below approximately three times of the output voltage, a
decrease in line regulation can be expected. The output voltage drop is about:
ELMOS Semiconductor AG
Specification 23 / 50
03SP0357E.00 05.09.2006
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