R1212D Datasheet, PDF (14/41 Pages) RICOH electronics devices division – STEP-UP DC/DC CONTOLLER

R1212D
TIMING CHART
<Soft-start Operation>
The timing chart below describes the state of each pin from the power-on until the IC entering the stable
operation.
By raising the voltage of the DTC pin slowly, the switching duty cycle is limited, and prevent the drastic voltage
rising (over-shoot) and inrush current.
When the VIN voltage becomes equal or more than the UVLO released voltage (VUVLO+VHYS), VREFOUT operation
starts. Following with the increase of the voltage level of VREFOUT, the internal oscillator begins to operate, then
the DTC voltage is also rising, then, soft-start operation starts. When the DTC voltage crosses the chopping
wave level inside the IC, EXT pin starts switching, then, step-up operation begins. During this term, the output
voltage does not reach the set output voltage. Therefore the output of the amplifier is "H". Besides, the protection
circuit may work and the IC charges the DELAY pin. Because of this, the soft-start time should be set shorter
than the latch protection delay time.
After the initial stage, when the output voltage reaches the set output voltage, the level of AMPOUT becomes
the normal state. In other words, the level is determined with the input voltage, the output voltage, and the output
current. When the level of AMPOUT becomes falling, charging the DELAY pin stops and discharges to the GND.
The soft-start time (the time for the DTC pin voltage becoming to VDTC level) can be estimated with the next
formula.
T≅1/α×ln(VDTC×α/β+1), herein, α=−1/C7×(1/R5+1/R6), and β=VREFOUT/(C7×R5).
(VUVLO+VHYS)
VIN
VREFOUT
OSC
Soft-start Time
DTC
AMPOUT
DELAY
EXT
VREFOUT
DTC
R5
R6
C7
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