Technical Note

4/19

www.rohm.com

2011.6 - Rev.C

© 2011 ROHM Co., Ltd. All rights reserved.

BD6586MUV

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Description of Functions

1) PWM current mode DC/DC converter

While BD6586MUV is power ON, the lowest voltage of LED1, 2, 3, 4 is detected, PWM duty is decided to be 0.5V

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.

BD6586MUV has many safety functions, and their detection signals stop switching operation at once.

2) Soft start

BD6586MUV 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 EN1, EN2 are changed L H, soft start becomes effective for within 1ms and soft start doesn't become effective

even if EN1, EN2 are changed L H after that.

And, when the H section of PWM is within 1ms, soft start becomes invalid when PWM is input to H more than three times.

The invalid of the soft start can be canceled by making EN1, EN2, PWM  L.

And, a soft start function doesn't work after a protection function release.

3) External SBD open detect and over voltage protection

BD6586MUV 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 internal 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.

4) Thermal shut down

BD6586MUV has thermal shut down function.

The thermal shut down works at 175

C or higher, and while holding the setting of EN1, EN2 control from the outside, turns

into non operation condition from operation condition. And at 175

C or below, the IC gets back to its normal operation.

5) Over Current Limit

Over current flows the current detection resistor between switching transistor source and PGND, then the voltage of that

resistor 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 BD6586MUV is detected peak current, current more than over current setting value does not

flow.

6) Under Voltage Lock Out(UVLO)

When VBAT declines in 2.35V (Typ.) from the condition of the power-on, DC/DC and a current driver are changed from a

state of movement to the condition at the time of the non-movement. And, it is returned in a state of movement when VBAT

is raised beyond 2.55V (Typ.).