BD9285F Datasheet, PDF (17/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
●3.4.2 how to shutdown and set TC54 capacitance
This IC is equipped the discharge function when shutdown is operated.
Datasheet
Figure 23. the shutdown waveform and circuit
○Explanation of shutdown sequence
①When STB=L, DC/DC and TC54 are stop.
②When STB=L, TC54UVLO=H, the DIMOUT logic asserts the PWM logic. The voltage of TC54 (5.4V) will decrease by
the constant current -10uA and is discharged to 2.4V.
③VOUT will be discharged and ILED decresing.
④When the voltage of TC54 pin is under 2.4V(typ.), the IC will shutdown.
○The setting method of TC54 external capacitance
Please use below formula to calculate the shutdown time TOFF.
T_OFF
=
C_REG[F]×3.0 [V ]
10 [uA]
　[Sec]
As shown the above, the PWM signal is required even after STB=L.
The discharge interval of VOUT is the longest in the minimum PWM duty. Please set the Creg value with a enough timing
margin from the end of the VOUT discharge to shutdown.
●3.4.3 The LED current setting
LED current can be adjusted by setting the resistance RISENSE which connects
to ISENSE pin.
○the relationship between RISET and ILED current
With DC dimming
Without DC dimming
R_ISENSE
=
ADIM [V ][_{Ω]　}
I_LED [ A]
R_ISENSE
=
3.5[V ] _[Ω]
I _LED[ A]
[setting example]
If ILED current is 400mA as ADIM is 1.5V, we can calculate RISENSE as below.
R_ISENSE
=
ADIM [V ]
I _LED [ A]
=
1.5[V ]
0.4[ A]
= 3.75[Ω]　　
Vout
DIMOUT
Error AMP
ISENSE
3.5V _ADIM
R_ISENSE
FB
Figure 24. the example of LED
current setting
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TSZ22111・15・001
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TSZ02201-0F1F0C100030-1-2
11.Jul.2012 Rev.001