R1211X Datasheet, PDF (12/33 Pages) RICOH electronics devices division – Step-up DC/DC Controller

continuous. The continuous current through the inductor is described as Iconst, then,
IOUT = fOSC ×VIN²Ã—tON²/(2×L×(VOUT-VIN))+VIN×Iconst/VOUT
Formula 6
In this moment, the peak current, ILxmax flowing through the inductor and the driver Tr. is described as follows:
ILxmax = Iconst +VIN×Ton/L
Formula 7
With the formula 4,6, and ILxmax is,
ILxmax = VOUT/VIN×IOUT+VIN×Ton/(2×L)
Formula 8
Therefore, peak current is more than IOUT. Considering the value of ILxmax, the condition of input and output, and
external components should be selected.
In the formula 7, peak current ILxmax at discontinuous mode can be calculated. Put Iconst=0 in the formula.
The explanation above is based on the ideal calculation, and the loss caused by Lx switch and external components
is not included. The actual maximum output current is between 50% and 80% of the calculation. Especially, when the
ILx is large, or VIN is low, the loss of VIN is generated with the on resistance of the switch. As for VOUT, Vf (as much as
0.3V) of the diode should be considered.
s TIMING CHART
q R1211X002A/R1211X002C
VREF
DTC
SS
VOUT
VFB
AMPOUT
EXT
R1
EXT
R2
q R1211X002B/R1211X002D
VREF
OP AMP
PWM Comparator
DTC
SS
VOUT
VFB
AMPOUT
EXT
R1
EXT
R2
OP AMP
<Soft-start Operation>
Soft-start operation is starting from power-on as follows:
PWM Comparator
(Step1)
The voltage level of SS is rising gradually by constant current circuit of the IC and a capacitor. VREF level which is
input to OP AMP is also gradually rising. VOUT is rising up to input voltage level just after the power-on, therefore, VFB
voltage is rising up to the setting voltage with input voltage and the ration of R1 and R2. AMPOUT is at “L”, and
switching does not start.
(Step2)
Rev. 1.10
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