ꢀ
ꢀ
If the mode is switched over into PWM mode from VFM mode during the operation, change the mode at light load
current. If the load current us large, output voltage may decline.
Reinforce the VIN, PGND, and VOUT lines sufficiently. Large switching current may flow in these lines. If the
impedance of VIN and PGND lines is too large, the internal voltage level in this IC may shift caused by the
switching current, and the operation might be unstable.
ꢀ The performance of power source circuits using these ICs extremely depends upon the peripheral circuits.
Pay attention in the selection of the peripheral circuits. In particular, design the peripheral circuits in a way that the
values such as voltage, current, and power of each component, PCB patterns and the IC do not exceed their
respected rated values.
ꢀ 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:
<Basic Circuits>
<Current through L>
i1
ILmax
IOUT
ILmin
ton
topen
L
Pch Tr
Nch Tr
VIN
VOUT
i2
CL
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
Wherein 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 respectively show 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.
In the continuous mode, when Equation 1 is solved for ton and assumed that the solution is tonc,
Rev. 1.12
- 9 -
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
