R1211X Datasheet, PDF (13/33 Pages) RICOH electronics devices division – Step-up DC/DC Controller

When the voltage level of SS becomes the setting voltage with the ration of R1 and R2 or more, switching operation
starts. VREF level gradually increases together with SS level. VOUT is also rising with balancing VREF and VFB. Duty
cycle depends on the lowest level among AMPOUT, SS, and DTC of the 4 input terminals in the PWM comparator.
(Step3)
When SS reaches 1V, soft-start operation finishes. VREF becomes constant voltage (=1V). Then the switching
operation becomes normal mode.
SS,VREF _VFB
SS
V_FB,VREF
DTC
AMPOUT
Step1
AMPOUT
Step2
VOUT
V _IN
<Latch Protection Operation>
The operation of Latch protection circuit is as follows: When AMPOUT becomes “H” and the IC detects maximum
duty cycle, charge to an external capacitor, C2 of DELAY pin starts. And maximum duty cycle continues and the
voltage of DELAY pin reaches delay voltage detector threshold, VDLY, outputs “L” to EXT pin and turns off the
external power MOSFET.
To release the latch protection operation, make the IC be standby mode with CE pin and make it active in terms of
R1211X002B/D version. Otherwise, make supply voltage down to UVLO detector threshold or lower, and make it rise
up to the normal input voltage.
During the soft-start time, if the duty cycle may be the maximum, protection circuit does not work and DELAY pin is
fixed at GND level.
The delay time of latch protection can be calculated with C2, VDLY, and Delay Pin Charge Current, IDLY1, as in the
next formula.
t=C2×VDLY/IDLY1
Once after the maximum duty is detected and released before delay time, charge to the capacitor is halt and delay
pin outputs “L”.
Output Short
AMPOUT
VDLY
AMPOUT
DTC
Normal
EXT
Rev. 1.10
DELAY
maxdutyOperation
Latched
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