Dear all
Please teach me point of mode variation of this device.
ex. (my guess)
Buck -> Buck-Boost
/ VIN has begun to go down
/ Buck FET is max. duty (uncontrol)
/ lower limit FB voltage(written in the datasheet)
/ start BUCK-BOOST control
Buck-Boost -> Buck
/ VIN has begun to go up
/ Buck FET is max. duty (control)
/ upper limit FB voltage(written in the datasheet)
/ stop BUCK-BOOST control
Regards,
PAN-M
PAN-M,
Buck -> Buck-Boost
- VOUT to VIN ratio is .75 (See datasheet parameter Buck-Boost Characteristics)
- At this point the boost switch begins to have a small duty cycle
- As VIN continues to decrease the boost switch duty cycle becomes larger while the buck duty cycle decreases.
- When both the buck switch and boost switch duty cycles are equal the part has entered buck-boost mode (This should happen before VIN = VOUT)
- As VIN decreases further the buck switch and boost switch duty cycle will be the same and stay in buck-boost mode
Buck-Boost -> Buck
- As VIN increases above VOUT the boost switch duty cycle begins to decrease while the buck switch begins to increase.
- At the VOUT to VIN ratio is .75 there will be no switching on boost switch. The part has entered buck mode.
Here is a picture of the transition mode, figure 9 in the datasheet. The typical transition range happens from .75< VOUT/VIN <.866.
Thanks,
Garrett
