R1234D Datasheet, PDF (15/26 Pages) RICOH electronics devices division – PWM/VFM Step-Down DC/DC CONVERTER with Synchronous Rectifier

R1234D
External Components
1.Inductor
Select an inductor that peak current does not exceed ILmax. If larger current than allowable current flows,
magnetic saturation occurs and makes transform efficiency be worse.
Supposed that the load current is at the same, the smaller value of L is used, the larger the ripple current is.
Provided that the allowable current is large in that case and DC current is small, therefore, for large output
current, efficiency is better than using an inductor with a large value of L and vice versa.
2.Capacitor
As for CIN, use a capacitor with low ESR (Equivalent Series Resistance) Ceramic type of a capacity at least
10µF for stable operation.
COUT can reduce ripple of Output Voltage, therefore as much as 10µF ceramic type is recommended.
3.Diode
If VFM mode is chosen at light load, use a Schottky diode with small VF. A diode with small VF makes the
efficiency of the circuit improved. Small reverse direction current, IR is an important factor, however, VF has more
important priority than IR.
TIMING CHART
CE pin Voltage
Intemal Opertional
Amplifier Output
Intemal
Soft-start
Set Voltage
Output
Short
Output Short
Intemal Oscillator Waveform
Lx Pin Output
Soft-start Time
Stable
Delay Time of Protection
The timing chart as shown above describes the waveforms starting from the IC is enabled with CE and latched
with protection. During the soft-start time, until the level is rising up to the internal soft-start set voltage, the duty
cycle of LX is gradually wider and wider to prevent the over-shoot of the voltage. During the term, the output of
amplifier is "H". After the output voltage reaches the set output voltage, they are balanced well. Herein, if the
output pin would be short circuit, the output of amplifier would become "H" again, and the condition would
continue for 2.0ms (Typ.), latch circuit would work and the output of LX would be latched with "OFF". (Output
="High-Z")
If the output short is released before the latch circuit works (within 2ms after output shorted), the output of
amplifier is balanced in the stable state again.
Once the IC is latched, to release the protection, input "L" with CE pin, or make the supply voltage at UVLO
level or less.
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