BD9394FP Datasheet, PDF (12/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9394FP, BD9394EFV
●Startup operation and soft start (SS) capacitance setting
The following section describes the sequence for the startup of this IC.
Datasheet
SS
SLOPE
SS
Css
FB
5V
QD
PWM
COMP
OSC
SS=FB
Circuit
DRIVER
LED_OK
0.8V
LED_DRIVER
N
LED
PWM
VOUT
cDescription of startup sequence
(1) Set the STB and PWM pin to “ON”.
(2) Set sll systems to “ON”, SS charge will be initiated.
At this time, a circuit in which SS pin voltage for soft start becomes equal to FB pin voltage operates to equalize the
FB pin and SS pin voltages regardless of whether the PWM pin is et to Low or High level.
(3) Since the FB pin and SS pin reach the lower limit of the internal sawtooth wave of the IC, the DC/DC converter
operates to start VOUT voltage rising.
(4) The Vout voltage continues rising to reach a voltage at which LED current starts flowing.
(5) When the LED current reaches the set amount of current, isolate the FB circuit from the SS circuit. With this, the
startup operation is completed.
(6) After 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
FB equalizing circuit.
cSS capacitance setting procedure
As aforementioned, this IC stops DC/DC converter when the PWM pin is set to Low level and conducts step-up operation
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 startup time. The startup time also varies with application settings of output capacitance, LED
current, output voltage, and others.
Startup time at minimum duty cycle can be approximated according to the following method:
Make maeasurement of VOUT startup time with a 100% duty cycle, first. Take this value as “Trise100”.
The startup time “Trise_min” for the relevant application with the minimum duty cycle is given by the following equation.
T_{rise_ min}
=
T_{rise_100}[Sec]
Min_ Duty[ratio]
[Sec]
However, since this calculation method is just for approximation, use it only as a guide.
Make setting of time during which the SS pin voltage reaches the FB pin voltage longer than this startup time.
Assuming that the FB pin voltage is VFB, the time is given by the following equation:
T_ss
=
C_ss[F]×VFB[V ]
2[µA]
[Sec]
As a result, it is recommended to make SS capacitance setting so that “Tss” will be greater than “Trise_min”
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TSZ22111・15・001
12/28
TSZ02201-0F1F0C100180-1-2
09.Nov.2012 Rev.002