bd6581gu ROHM Co. Ltd., bd6581gu Datasheet - Page 9

• Manufacturer Part Number: bd6581gu
• Description: White Backlight Led Driver
• Manufacturer: ROHM Co. Ltd.

BD6581GU
© 2010 ROHM Co., Ltd. All rights reserved.
www.rohm.com
4) External SBD open detect and over voltage protection
5) Thermal shut down
6) Over Current Limit
BD6581GU 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
・Over voltage protection
BD6581GU 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.
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 BD6581GU 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>
<The estimate of the current value which need for the normal operation >
(Example 1)
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).
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.
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
As over current detector of BD6581GU 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
Ipeak = (VIN / L) x (1 / fsw) x (1-(VIN / VOUT))
Iave=(VOUT x IOUT / VIN) / eff
Ton=(Iave x (1-VIN/VOUT) x (1/fsw) x (L/VIN) x 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) x (1/fsw) < Ton→ peak current = Ipeak /2 + Iave
(1-VIN/VOUT) x (1/fsw) > Ton→ peak current = Ipeak
In case of, VIN=6.5V, L=4.7µH, fsw=1MHz, VOUT=39V, IOUT=80mA, Efficiency=85%
Ipeak = (6.5V / 4.7µH) x (1 / 1MHz) x (1-(6.5V / 39V)) =1.08A
Iave = (39V x 80mA / 6.0V) / 85% = 0.61A
Ton = (0.61A x (1-6.0V / 39V) x (1 / 1MHz) x ( 4.7µH /6.0V) x 2)
(1-VIN/VOUT) x (1/fsw)=0.85µs < Ton
Peak current = 1.08A/2+0.61A = 1.15A
○ Switching frequency = fsw
○ Output voltage = VOUT
○ Average current of coil = Iave ○ Peak current of coil = Ipeak
○ Efficiency = eff
○ ON time of switching transistor = Ton
(Please set up having margin, it refers to data on p.3.)
○ Inductance value of coil = L
MIN=0.8MHz, Typ=1MHz, MAX=1.2MHz
1/2
9/26
○ Total LED current = IOUT
1/2
= 0.90µs
2010.03 - Rev.A
Technical Note