R1282D002A Datasheet, PDF (5/18 Pages) RICOH electronics devices division – 2CH PWM DC/DC CONTROLLER

R1282D002A
Operation of Step-up DC/DC Converter and Output Current
Step-up DC/DC Converter makes higher output voltage than input voltage by releasing the energy
accumulated during on time of LX Transistor on input voltage.
VIN
GND
<Basic Circuit>
i2
Inductor
Diode
i1
Lx Tr ^CL
IOUT
VOUT
<Current through L>
Discontinuous Mode
IL
IL
ILxmax
Continuous Mode
ILxmax
ILxmin
Tf
t
Ton
Toff
T=1/fosc
ILxmin
Iconst
Ton
T=1/fosc
t
Toff
Step 1. LX Tr. is on, then the current IL=i1 flows, and the energy is charged in L. In proportion to the on time of LX
Tr. (Ton), IL=i1 increases from IL=ILXmin=0 and reaches ILXmax.
Step 2. When the LX Tr. is off, L turns on Schottky Diode (SD), and IL=i2 flows to maintain IL=ILXmax.
Step 3. IL=i2 gradually decreases, and after Tf passes, IL=ILXmin=0 is true, then SD turns off. Note that in the
case of the continuous mode, before IL=ILXmin=0 is true, Toff passes, and the next cycle starts, then LX
Tr. turns on again.
In this case, ILXmin>0, therefore IL=ILXmin>0 is another starting point and ILX max increases.
With the PWM controller, switching times during the time unit are fixed. By controlling Ton, output voltage is
maintained.
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