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Operation
The supply can run in three different modesdepending on the output power:
z
Quasi-Resonant mode (QR) TheQR mode, described above, is used during normal
operation. Thiswill give a high efficiency.
z
Frequency Reductionmode (FR) The FR mode (also called VCO mode) is implementedto
decrease the switching losses at low output loads. In this waythe efficiency at low output
powers is increased, which enables powerconsumption smaller than 3 W during stand-
by. The voltage at thepin 3 (Ctrl) determines where the frequency reduction starts.
Anexternal Ctrl voltage of 1.425 V corresponds with an internal VCOlevel of 75 mV. This
fixed VCO level is called V
VCO,start
. The frequencywill be reduced in relation to the VCO
voltage between 75 mV and 50mV (at levels larger than 75 mV, Ctrl voltage < 1.425V,
theoscillator will run on maximum frequency f
oscH
= 175 kHztypically). At 50 mV (V
VCO,max
)the frequency is reduced to the minimum level of 6 kHz. Valley switchingis still
active in this mode.
z
Minimum Frequencymode (MinF) At VCO levels below 50 mV, the minimum
frequencywill remain on 6 kHz, which is called the MinF mode. Because ofthis low
frequency, it is possible to run at very low loads withouthaving any output regulation
problems.
Figure:
Pa
g
e 19 of 32SPMS
7/8/2004