R1240K003A-TR Ver la hoja de datos (PDF) - RICOH Co.,Ltd.
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R1240K003A-TR Datasheet PDF : 19 Pages
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R1240x
NO.EA-190-170609
OPERATING DESCRIPTIONS
OPERATION OF STEP-DOWN DC/DC CONVERTER AND OUTPUT CURRENT
The step-down DC/DC converter charges energy in the inductor (L) when the LX transistor turns on, and
discharges the energy from the inductor when LX transistor turns off and controls with less energy loss, so that
a lower output voltage (VOUT) than the input voltage (VIN) can be obtained. The operation of the step-down
DC/DC converter is explained in the following figures.
VIN
Nch Tr.
Diode
i1
VOUT
L
i2
COUT
GND
Basic Circuit
IL
ILmin
ILmax
topen
ton
toff
T=1/fosc
Inductor Current flowing through Inductor
Step1. The Nch transistor turns on and the inductor current (i1) flows, L is charged with energy. At this
moment, i1 increases from the minimum inductor current (ILmin), which is 0 A, and reaches the
maximum inductor current (ILmax) in proportion to the on-time period (ton) of the Nch transistor.
Step2. When the Nch transistor turns off, L tries to maintain IL at ILmax, so L turns the diode on and the inductor
current (i2) flows into L.
Step3. i2 decreases gradually and reaches ILmin after the open-time period (topen) of the Nch transistor,
and then the diode turns off. This is called discontinuous current mode.
As the output current (IOUT) increases, the off-time period (toff) of the Nch transistor runs out before
IL reaches ILmin. The next cycle starts, and the Nch transistor turns on and the diode turns off, which
means IL starts increasing from ILmin. This is called continuous current mode.
In the case of PWM mode, VOUT is maintained by controlling ton. During PWM mode, the oscillator frequency
(fosc) is being maintained constant.
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