BD9486F View Datasheet(PDF) - ROHM Semiconductor
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BD9486F Datasheet PDF : 37 Pages
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BD9486F
Datasheet
●3.2 External Components Selection
●3.2.1 Start Up Operation and Soft Start External Capacitance Setting
The below explanation is the start up sequence of this IC
1
STB
OSC
PWM
SS
FB
SLOPE
5V
SS
Css
SS=FB
Circuit
OSC
COMP
DRIVER
GATE 2
VOUT
LED_OK
FB
3
ILED
LED_OK
4
6
5
GATE
CS
DIMOUT
ISENSE
PWM
Figure 20. Startup waveform
Figure 21. Circuit behavior at startup
VOUT
○Explanation of start up sequence
1. Reference voltage REF50 starts by STB=H.
2. SS starts to charge at the time of first PWM=H. At this moment, the SS voltage of slow-start starts to equal FB
voltage,and the circuit becomes FB=SS regardless of PWM logic.
3. When FB=SS reaches the lower point of internal sawtooth waveform, GATE terminal outputs pulse and starts to boost
VOUT.
4. It boosts VOUT and VOUT reaches the voltage to be able to flow LED current.
5. If LED current flows over decided level, FB=SS circuit disconnects and startup behavior completes.
6. Then it works normal operation by feedback of ISENSE terminal. If LED current doesn't flow when SS becomes over
3.7V, SS=FF circuit completes forcibly and FBMAX protection starts.
○Method of setting SS external capacitance
According to the sequence described above, start time Tss that startup completes with FB=SS condition is the time that
FB voltage reaches the feedback point.
The capacitance of SS terminal is defined as Css and the feedback voltage of FB terminal is defined as VFB. The
equality on TFB is as follows.
Tss
Css[μF] VFB [V]
3 [μA]
[sec]
If Css is set to a very small value, rush current flows into the inductor at startup.
On the contrary, if Css is enlarged too much, LED will light up gradually.
Since Css differs in the constant set up with the characteristic searched for and differs also by factors, such as a voltage
rise ratio, an output capacitance, DCDC frequency, and LED current, please confirm with the system.
【Setting example】
When Css=0.1μF,Iss=3μA,and startup completes at VFB=3.7V, SS setting time is as follows.
Tss
0.1 106 [F] 3.7[V]
3 1 06 [A]
0.123
[sec]
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TSZ22111・15・001
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TSZ02201-0F1F0C100240-1-2
13.Feb.2014 Rev.004
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