BD6590MUV
Technical Note
●Description of Functions
1)PWM current mode DC/DC converter
While BD6590MUV is power ON, the lowest voltage of LED terms is detected, PWM duty is decided to be 0.7V and output
voltage is kept invariably. As for the inputs of the PWM comparator as the feature of the PWM current mode, one is
overlapped with error components from the error amplifier, and the other is overlapped with a current sense signal that
controls the inductor current into Slope waveform to prevent sub harmonic oscillation. This output controls internal Nch Tr
via the RS latch. In the period where internal Nch Tr gate is ON, energy is accumulated in the external inductor, and in the
period where internal Nch Tr gate is OFF, energy is transferred to the output capacitor via external SBD.
BD6590MUV has many safety functions, and their detection signals stop switching operation at once.
2)Soft start
BD6590MUV has soft start function.
The soft start function prevents large coil current.
Rush current at turning on is prevented by the soft start function.
After PWMPOW, RSTB is changed L→ H, soft start becomes effective for within 4ms and soft start doesn't become
effective even if PWMPOW is changed L→H after that.
And, when the H section of PWMPOW is within 4ms, soft start becomes invalid when PWMPOW is input to H more than
three times. The invalid of the soft start can be canceled by making PWMPOW, PWMDRV →L.
PWMDRV
PWMPOW
VREG
PWMPOW
Max 1ms Max 3ms
OFF ON
OFF
ON
OFF
OFF
ON
Soft start
VREG
Soft start
reset
OFF
OFF
OFF
Soft start
OFF
ON
OFF
Reset
Reset
Fig.3 Soft start
Fig.4 Soft start reset and set
3)FAILFLAG
When the error condition occurs, boost operating is stopped by the protection function, and the error condition is outputted
from FAILFLAG. After power ON, when the protection function is operating under about 1ms have passed.
Object of protect function is as shown below.
・Over-voltage protection
・External SBD open detect/ Output Short protection
・LED terminal open/short protection
・Over current limit
PWMPOW
about 1ms
FAILFLAG
Protection
function
Boost
operating
un-detection
normal
detect
un-detection
off
off
boost stop
normal
normal
Fig.5 FAILFLAG operating description
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© 2011 ROHM Co., Ltd. All rights reserved.
2011.07 - Rev.B
5/26
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