BD93942F Datasheet, PDF (11/30 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD93942F
Startup operation and soft start (SSFB) capacitance setting
The following section describes the sequence for the startup of this IC.
①
STB
OSC
SSFB
Soft-Start(ISS=10uA)
IFB(Sink, Source)=±100uA)
5V
Q D _PWM
COMP
OSC
DRIVER
Datasheet
VOUT
N
CS
PWM
N
VOUT
ILED
LED_OK
②
③
④
⑥
⑤
Figure 11. Startup waveform
SSFB
RSSFB
CSSFB
LED_OK
VLED
LED_DRIVER
LED
PWM
Figure 12. Circuit behavior at startup
Description of startup sequence
(1) Set the STB and PWM pin to “ON”.
(2) Set all systems to “ON”, SSFB charge will be initiated.
(3) Since the SSFB 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, the startup operation is completed.
(6) After that, conduct normal operation following the feedback operation sequence with the LED pins.
If the SSFB pin sink/source current is ±100uA, the LED protection function will be activated.
SSFB 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.
Assuming that the SSFB pin voltage is VSSFB, the time is given by the following equation:
T_SSFB

C_SSFB[F ] VSSFB[V
10[A]
]
[Sec]
As a result, it is recommended to make SSFB capacitance setting so that “T_SSFB” will be greater than “Trise_min”
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12.Aug.2015 Rev.002