BD6066GU_11 Datasheet, PDF (10/16 Pages) Rohm – White Backlight LED Drivers for Medium to Large LCD Panels (Switching Regulator Type)

BD6066GU,BU6066EKN
Technical Note
●Setting of LED current and current range
LED current can set up resistance value (RISET) connecting to ISET,
LED current setting range is 4mA~30mA.
ISET setting example
RISET
LED current
Setting of LED current is given as shown below.
LED current = 20mA × ( 24kΩ / RISET)
24.0 kΩ
25.5 kΩ
27.0 kΩ
(E24)
(E96)
(E12)
20.0mA
18.8mA
17.8mA
The current in the standard application is as shown below.
RISET=24kΩ, LED current=20mA
28.0 kΩ (E96)
30.0 kΩ (E24)
17.1mA
16.0mA
●Brightness control
33.0 kΩ (E6)
14.5mA
PWM brightness adjustment is made by inputting PWM pulse to PWM pin.
The electric current select setting with ISET in the Hi section, and the Lo section turns off the electric current. Lo.
The average current increases in proportion with the duty cycle of PWM signal. By this method, IC can power off at the OFF
time, the IC and LED both consume no currents, thus providing a high-efficiency operation.
The recommended PWM frequency is 100Hz ~ 300Hz.
<ON time limit>
On time is necessary more than 750μs (at the time of 200Hz, 15%) to require the start up-time of 750μs.
< OFF time limit >
OFF time is approximated by the following parameter.
External output capacitor
: Cvout
LED current setting
: ILED (worst = +5%)
LED sequence number
: NPLED
LED series number
: NSLED
VF tolerance of LED terminal
: ∆VF
Reaction time of internal circuit : 10μs (worst = 15μs)
Necessary time for OFF (typ) = Cvout × ((∆VF × NSLED) + 0.5V)/(ILED × NPLED)+10μs
(example)
Necessary time for OFF (typ) = 2μF × ((0.2V × 10 lights) + 0.5V)/(20mA × 4)+10μs = 72.5μs
<PWM=200Hz, duty1.5% equivalent>
Please use worst value of each parameter when you calculate the worst time.
(example)
Necessary time for OFF (worst) = (2μF × 1.3) × ((0.26V × 10 lights) + 0.7V)/((20mA × 1.05) × 4)+15μs =117μs
<PWM=200Hz, duty2.3% equivalent >
And please don’t use ON time under 750μs or under OFF time calculated in the above of current setting for the brightness
control, because power-on/off has very influential.
●Noise of ceramic capacitor by PWM brightness control
In order to use a ceramic capacitor as the output capacitor, noise of capacitor occurs by the kind of the circuit board and
mounted method, the mounting place.
As for the noise cause, the resonant of ceramic capacitor and the board give cause by output voltage fluctuation.
Measures of this trouble are shown.
(1) Ceramic capacitor is changed to tantalum capacitor.
By changing to tantalum capacitor, noise is stopped because of changing ceramic capacitor itself bringing about the noise.
But, as the output ripple increases by ESR that is characteristic of the tantalum, it needs to select tantalum with low ESR.
Also, it recommends connecting parallel capacitor for the noise measure of 0.1µF
Recommended capacitor is shown next page.
(2) Ceramic capacitor is floated from the top of the circuit board.
Reducing the contact surface of the ceramic capacitor and the circuit board can reduce the noise.
Capacitor that can float a capacitor from the circuit board are as shown below. This capacitor is connected the metal cap both
electrodes of the capacitor, so It can have the effect which be floated because the metal cap is connected to the circuit board.
Recommended capacitor is shown next page.
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The metal cap
CKG45NX7R2A225N (TDK)
The circuit board
Fig.31 The metal cap capacitor
10/15
2011.06 - Rev.B