BD9479FV ROHM [Rohm], BD9479FV Datasheet - Page 13

• Manufacturer Part Number: BD9479FV
• Description: White LED Driver for large LCD Panels (DCDC Converter type)
• Manufacturer: ROHM [Rohm]

www.rohm.com
© 2012 ROHM Co., Ltd. All rights reserved.
TSZ22111・15・001
BD9479FV
● ● ● ● Start-up and soft start (SS) capacitance setting
The following describes the start-up sequence of this IC.
○ ○ ○ ○ Description of start-up sequence
○ ○ ○ ○ SS capacitance setting procedure
only in the section in which the PWM pin is maintained at High level. Consequently, setting the PWM duty cycle to the
minimum will extend the start-up time.In addition, start-up time also varies with application of output capacitance selection,
LED current, output voltage, and others.
time of VOUT with a 100% DUTY and take this value as Trise_100.The startup time “Trise_min” for the relevant
application with the minimum duty cycle is given by the following equation.
Please set a time「T
①Set STB from Low to High
②System will be activated while PWM=H.SS start to charge.
③Since the FB pin and SS pin reach the lower limit of the internal sawtooth wave of the IC, the DC/DC converter
④The Vout voltage continues rising to reach a voltage at which LED current starts flowing.
⑤When the LED current reaches the amount of current, isolate the FB circuit from the SS circuit. With
⑥After that, normal operation is controlled by following the feedback voltage of LED pins.
Start-up time at minimum duty cycle can be approximated according to the following method.First, measure the start-up
However, this calculation method is just for approximation, use it only as a reference.
Assuming that the FB pin voltage is VFB, the time is given by the following equation:
As a result, it is recommended to make SS capacitance setting so that “TDCDCSS” will be longer than “Trise_min”
At this time, a circuit in which SS pin voltage for slow start becomes equal to FB pin voltage operates to equalize the
FB pin and SS pin voltages regardless of whether the PWM pin is set to Low or High l eve.
operates to start VOUT voltage rising.
startup operation is completed.
If the SS pin voltage reaches 4V or higher, the LED protection function will be activated to forcedly end the SS and FB
equalizing circuit.
As mentioned above, this IC stops DC/DC converter when the PWM pin is set to Low level.conducts step-upoperation
T
T
rise
DCDCSS
_
min
DCDCSS
C
Min
SS
T
[
F
rise
_
」 during which the SS pin voltage reaches the FB pin voltage longer than this star-tup.
[ 2
]
Duty
_
100
VFB
A
[sec]
]
[
ratio
[
V
]
]
[sec]
[sec]
SLOPE
SS
13/26
SS
SS=FB
Circuit
5V
Q
D
PWM
LED_DRIVER
COMP
TSZ02201-0F1F0C100020-1-2
DRIVER
9.Aug.2012 Rev.001
PWM
BSx
CLx
N
Datasheet
this, the
VOUT