R1224N Datasheet, PDF (8/42 Pages) RICOH electronics devices division – PWM/VFM step-down DC/DC Converter

R1224N
When you use these ICs, consider the following issues;
⋅As shown in the block diagram, a parasitic diode is formed in each terminal, each of these diodes is not formed
for load current, therefore do not use it in such a way. When you control the CE pin by another power supply, do
not make its “H” level more than the voltage level of VIN pin.
⋅Set external components as close as possible to the IC and minimize the connection between the components
and the IC. In particular, a capacitor should be connected to VOUT pin with the minimum connection. Make
sufficient ground and reinforce supplying. A large switching current could flow through the connection of power
supply, an inductor and the connection of VOUT. If the impedance of the connection of power supply is high, the
voltage level of power supply of the IC fluctuates with the switching current. This may cause unstable operation
of the IC.
⋅Protection circuit may work if the maximum duty cycle continue for the time defined in the electrical
characteristics. Once after stopping the output voltage, output will restart with soft-start operation. If the
difference between input voltage and output voltage is small, the protection circuit may work.
⋅Use capacitors with a capacity of 22µF or more for VOUT pin, and with good high frequency characteristics such
as tantalum capacitors. We recommend you to use output capacitors with an allowable voltage at least twice as
much as setting output voltage. This is because there may be a case where a spike-shaped high voltage is
generated by an inductor when an external transistor is on and off.
⋅Choose an inductor that has sufficiently small D.C. resistance and large allowable current and is hard to reach
magnetic saturation. And if the value of inductance of an inductor is extremely small, the ILX may exceed the
absolute maximum rating at the maximum loading.
Use an inductor with appropriate inductance.
⋅Use a diode of a Schottky type with high switching speed, and also pay attention to its current capacity.
⋅Do not use this IC under the condition with VIN voltage at equal or less than minimum operating voltage.
⋅When the threshold level of an external power MOSFET is rather low and the drive-ability of voltage supplier is
small, if the output pin is short circuit, input voltage may be equal or less than UVLO detector threshold. In this
case, the devise is reset with UVLO function that is different from the reset-protection function caused by
maximum duty cycle.
⋅With the PWM/VFM alternative circuit, when the on duty cycle of switching is 35% or less, the R1224N alters
from PWM mode to VFM mode (Pulse skip mode). The purpose of this circuit is raising the efficiency with a light
load by skipping the frequency and suppressing the consumption current. However, the ratio of output voltage
against input voltage is 35% or less, (ex. VIN>8.6V and VOUT=3.0V) even if the large current may be loaded, the
IC keeps its VFM mode. As a result, frequency might be decreased, and oscillation waveform might be unstable.
These phenomena are the typical characteristics of the IC with PWM/VFM alternative circuit.
ÌThe 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.
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