SPMS | Page 19 of 32 |
Operation
The supply can run in three different modesdepending on the output power:
z
zFrequency 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.
zMinimum 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:
7/8/2004
