BD9470AEFV Datasheet, PDF (13/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
3. Application of BD9470AEFV・BD9470AFM
3.1 BD9470AEFV・BD9470AFM examination for application
●Start-up and SS capacity setting explanation
This section described the start-up sequence of this IC.
①
5V
STB
OSC
PWM
SS
SLOPE
FB
QD
PWM
SS
COMP
N
Css
SS
SLOPE
SS=FB
Circuit
OSC
DRIVER
N_②
VOUT
③④
ILED
LED_OK
LED
FB
LED_OK
0.3～
0.519V
OVP
KEEP
OVP
PWM
LED_OK
⑤
⑦
⑥
LED_DRIVER
Figure 15. Timing chart of start-up
VOUT
○Description of start-up sequence
①STB=PWM=ON
②System is ON.SS starts to charge.
At this time, a circuit in which SS voltage for slow start is equal to FB voltage regardless of whether the PWM pin is set to
Low or High level.
③Since the FB pin and SS pin reach the lower limit of the internal sawtooth wave, the DC/DC converter operates and
VOUT voltage rising.
Until it reachs a certain voltage even PWM=Low by vlotage maintenance function.
（For detailed OVP maintanence function, please refer to”VOUT(OVP) maintanence function section”.）
⑤Vout voltage continues rising to reach a voltage at which LED current starts flowing.
⑥When the LED current reaches the set amount of current, isolate the FB circuit from the SS circuit. With this, the start-up
operation is completed.（Fast start-up is also diasabled by VOUT maintanence function）
⑦After that, conduct normal operation following the feedback operation sequence with the LED pins.
If the SS pin voltage reaches 3.7V or more, the LED protection function will be activated to forcedly end the SS and
FBequalizing circuit.
○ SS capacity setting method
V
REG58
4.9V
VSS>4.9V FB=Source Mode
Iss[A]
SS
4.7V＜Vss
⇒FB Output Current
=Source Only
V
Css[F]
Finished Start Up
SS=FB
Time
Figure 16. SS setting procedure in FB Source mode
Boot system as above described, because of start-up in the state of FB=SS, the start-up time can be imaged of the time to
reach the point from the feedback voltage FB from STB = ON.If you SS> 4.9V, FB output current mode will become Source
mode operation.
If the feedback voltage of FB is the same as VSS and the time can be calculated as below.
T_ss
=
C_ss [F ] ×VFB[V ]
2[µA]
[Sec]
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TSZ22111・15・001
13/35
TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003