BD9489F Datasheet, PDF (16/39 Pages) Rohm – DCDC converter with current mode

BD9489F
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
5V
SS
Css
SS=FB
Circuit
OSC
COMP
DRIVER
GATE
CS
LED_OK
FB
DIMOUT
ISENSE
VOUT
PWM
Figure 24. Startup waveform
Figure 25. Circuit behavior at startup
○Explanation of start up sequence
1. Reference voltage REF58 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. The retaining function of the output voltage enables the quick startup by boosting the VOUT to a certain level
nevertheless PWM=L. Please refer the section “2.8 The Retaining Function of The Output Voltage”.
5. It boosts VOUT and VOUT reaches the voltage to be able to flow LED current.
6. If LED current flows over decided level, FB=SS circuit disconnects and startup behavior completes.
7. 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 T_FB is as follows.
T_ss

C_ss[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=0.1uF,Iss=3μA,and startup completes at VFB=3.7V, SS setting time is as follows.
T_ss

0.1　 10^6 [F]  3.7[V]
3  1　0^6 [A]
　
　0.123
[sec]
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TSZ22111・15・001
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TSZ02201-0F1F0C100250-1-2
29.Oct 2014 Rev.002