BD93942F Datasheet, PDF (10/30 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD93942F
Datasheet
○ISET (12pin)
The ISET pin is an output current setting resistor. Output current ILED varies in inverse proportion to resistance.
The relation between output current ILED and the resistance of ISET pin connection resistor RISET is given by the following
equation:
I _LED [mA] 
R
3000
ISET ^[kΩ]

V
ADIM
[V
]
(ADIM=0.2V to2.7V)
I _LED [mA]

7500
R_ISET [kΩ]
(ADIM>4V to 35V)
Output current setting should be made in the range of 30mA to 150mA.
It prepares automatically to suitable LED feedback voltage that can output LED current set by ISET pin.
In short LED feedback voltage is dropped when the LED current is small and the IC heating is held automatically.
In case of a large current is needed, raise the LED pin feedback voltage. And it adjusts automatically to LED pin voltage
that can be flow large LED current.
The calculation is as below.
VLED  3 I_LED [ A] [V ]　　(ILED  117mA)
VLED 0.35[V ](　typ)　 (ILED  117mA)
The LED feedback voltage (VLED) is clamped to 0.35V (typ.) when the LED current (ILED) is less than 117mA.
ADIM input range is from 0V to 35V. And the range which the LED currents change with linearity is from 0.2V to 2.7V.
When it reaches under VISET×0.90V(typ.), the LED current is off to prevent from passing a large current to the LED pin
when the RISET is shorted and the ISET pin is shorted to the GND. And as the ISET pin returns to a normal state, the LED
current returns.
○SSFB (13pin)
The SSFB pin is used to make setting of soft start time and duty for soft start, and DC/DC current mode control error
amplifier. It performs constant current charge of 10uA to the external capacitor connected to SSFB terminal, which enables
soft-start of DC/DC converter.
The SSFB pin detects the voltages of LED pins (1 to 4) and controls inductor current so that the pin voltage of the LED
located in the row with the highest Vf will come to 0.45V(ADIM=2.5V,ISET=50kΩ,ILED=150mA). As a result, the pin
voltages of other LEDs become higher by Vf variation. After completion of soft start, the SSFB pin is put into the
high-impedance state with the PWM signal being in the low state, thus maintaining the SSFB voltage.
Since the LED protection function (OPEN/SHORT detection) works when it turns to the LED feedback mode.
○RT (14pin)
The RT pin is used to connect a DC/DC frequency setting resistor. DC/DC drive frequency is determined by connecting the
RT resistor.
Drive frequency vs. RT resistance (Ideal)
R _RT

15000
f _SW [kHz ]
[k]
When RT is 100kΩ, Fsw is 150kHz(typ.). However, drive frequency setting should be made in the range of 100 kHz to 800
kHz.
○ADIM (15pin)
ADIM pin is for analog dimming. Output current is proportionality with input voltage. Basically, ADIM pin assumes the
voltage inputted externally using high accuracy of resistive divider and etc., IC internally is in OPEN (High impedance)
condition. Cannot use in an OPEN condition.
If you don't use analog dimming, please connect pull-up resistor to over 5V (for example REG58).
○VCC (16pin)
The VCC pin is used to supply power for the IC in the range of 9 to 35V.
If the VCC pin voltage reaches 7.5V (Typ.) or more, the IC will initiate operation. If it reaches 7.2V (Typ.) or less, the IC will
be shut down.
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