R1234D Datasheet, PDF (12/26 Pages) RICOH electronics devices division – PWM/VFM Step-Down DC/DC CONVERTER with Synchronous Rectifier

R1234D
OPERATION of step-down DC/DC converter and Output Current
The step-down DC/DC converter charges energy in the inductor when LX transistor is ON, and discharges the
energy from the inductor when LX transistor is OFF and controls with less energy loss, so that a lower output
voltage than the input voltage is obtained. The operation will be explained with reference to the following
diagrams:
IL
ILmax
VIN
Pch Tr
i1
VOUT
L
ILmin
topen
Nch Tr
i2
CL
GND
ton
toff
T=1/fosc
⋅ Step 1: P-channel Tr. turns on and current IL (=i1) flows, and energy is charged into CL. At this moment, IL
increases from Ilmin (=0) to reach ILmax in proportion to the on-time period (ton) of P-channel Tr.
⋅ Step 2: When P-channel Tr. turns off, Synchronous rectifier N-channel Tr. turns on in order that L maintains IL
at ILmax, and current IL (=i2) flows.
⋅ Step 3: IL (=i2) decreases gradually and reaches IL=ILmin=0 after a time period of topen, and N-channel Tr.
Turns off. Provided that in the continuous mode, next cycle starts before IL becomes to 0 because toff
time is not enough. In this case, IL value increases from this Ilmin (>0).
In the case of PWM control system, the output voltage is maintained by controlling the on-time period (ton),
with the oscillator frequency (fosc) being maintained constant.
• Discontinuous Conduction Mode and Continuous Conduction Mode
The maximum value (ILmax) and the minimum value (ILmin) of the current flowing through the inductor are the
same as those when P-channel Tr. turns on and off.
The difference between ILmax and ILmin, which is represented by ∆I;
∆I=ILmax−ILmin=VOUT×topen/L=(VIN−VOUT)×ton/L ........................................................Equation 1
Where, t=1/fosc=ton+toff
duty (%)=ton/t×100=ton×fosc×100
topen ^<= toff
In Equation 1, VOUT×topen/L and (VIN−VOUT) ×ton/L are respectively shown the change of the current at ON,
and the change of the current at OFF.
When the output current (IOUT) is relatively small, topen < toff as illustrated in the above diagram. In this case,
the energy is charged in the inductor during the time period of ton and is discharged in its entirely during the time
period of toff, therefore ILmin becomes to zero (ILmin=0). When IOUT is gradually increased, eventually, topen
becomes to toff (topen=toff), and when IOUT is further increased, ILmin becomes larger than zero (ILmin>0). The
former mode is referred to as the discontinuous mode and the latter mode is referred to as continuous mode.
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