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BD6583MUV-A

White Backlight LED Driver for Medium to Large LCD Panels (Switching Regulator Type)

ROHM Semiconductor 

BD6583MUV-A

Technical Note

4) External SBD open detect and over voltage protection

BD6583MUV-A has over boost protection by external SBD open and over voltage protection. It detects VDET voltage and

is stopped output Tr in abnormal condition. Details are as shown below.

▪ External SBD open detect

In the case of external SBD is not connected to IC, the coil or external Tr may be destructed. Therefore, at such an error as

VOUT becoming 0.1V or below, the Under Detector shown in the figure works, and turns off the output Tr, and prevents the

coil and the IC from being destructed.

And the IC changes from activation into non-activation, and current does not flow to the coil (0mA).

▪ Over voltage protection

At such an error of output open as the output DC/DC and the LED is not connected to IC, the DC/DC will boost too much

and the VDET terminal exceed the absolute maximum ratings, and may destruct the IC. Therefore, when VDET becomes

sensing voltage or higher, the over voltage limit works, and turns off the output Tr, and the pressure up made stop.

At this moment, the IC changes from activation into non-activation, and the output voltage goes down slowly. And, when

the output voltage becomes the hysteresis of the over voltage limit or below, the output voltage pressure up to sensing

voltage once again and unless the application error is recovered, this operation is repeated.

5) Thermal shut down

BD6583MUV-A has thermal shut down function.

The thermal shut down works at 175C or higher, and the IC changes from activation into non-activation. Because

non-activation is different from RSTB=L, it doesn’t’ be reset inside IC. Moreover, even if thermal shut down function works,

soft start, FAILSEL, selection the number of LED lines of the current driver and starting current setting at PWMDRV=L

related RSTB are hold.

6) Over Current Limit

Over current flows the current detection resistor that is connected to switching transistor source and between GND,

SENSP pin voltage turns more than detection voltage, over current protection is operating and it is prevented from flowing

more than detection current by reducing ON duty of switching Tr without stopping boost.

As over current detector of BD6583MUV-A is detected peak current, current more than over current setting value does not flow.

And, over current value can decide freely by changing over current detection voltage.

<Derivation sequence of detection resistor>

Detection resistor =Over current detection voltage / Over current setting value

TYP value of over current detection voltage is 100mV, MIN = 70mV and MAX = 130mV and after the current value which

was necessary for the normal operation was decided, detection resistor is derived by using MIN value of over current

detection value.

For example, detection resistor when necessary current value was set at 1A is given as shown below.

Detection resistor =70mV / 1A = 70mΩ

MAX current dispersion of this detection resistor value is

MAX current = 130mV / 70mΩ = 1.86A

<The estimate of the current value which need for the normal operation>

As over current detector of BD6583MUV-A is detected the peak current, it have to estimate peak current to flow to the coil

by operating condition.

In case of, ○ Supply voltage of coil = VIN

○ Inductance value of coil = L

○ Switching frequency = fsw MIN=0.8MHz, Typ=1MHz, MAX=1.2MHz

○ Output voltage = VOUT

○ Total LED current = IOUT

○ Average current of coil = Iave

○ Peak current of coil = Ipeak

○ Efficiency = eff (Please set up having margin, it refers to data on p.3.)

○ ON time of switching transistor = Ton

Ipeak = (VIN / L) × (1 / fsw) × (1-(VIN / VOUT))

Iave=(VOUT × IOUT / VIN) / eff

Ton=(Iave × (1-VIN/VOUT) × (1/fsw) × (L/VIN) × 2)1/2

Each current is calculated.

As peak current varies according to whether there is the direct current superposed, the next is decided.

(1-VIN/VOUT) × (1/fsw) < Ton  peak current = Ipeak /2 + Iave

(1-VIN/VOUT) × (1/fsw) > Ton  peak current = Ipeak

(Example 1)

In case of, VIN=6.5V, L=4.7µH, fsw=1MHz, VOUT=39V, IOUT=80mA, Efficiency=85%

Ipeak = (6.5V / 4.7µH) × (1 / 1MHz) × (1-(6.5V / 39V)) =1.08A

Iave = (39V × 80mA / 6.0V) / 85% = 0.61A

Ton = (0.61A × (1-6.0V / 39V) × (1 / 1MHz) × ( 4.7µH /6.0V) × 2)1/2 = 0.90µs

(1-VIN/VOUT) × (1/fsw)=0.85µs < Ton

Peak current = 1.08A/2+0.61A = 1.15A

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2011.06 - Rev.C

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