Technical Note

10/28

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2011.06 - Rev.B

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BD6150MUV

・

Over Current Limit

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

PGND, Current sense voltage turns more than detection voltage decided with OCPSET, 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 This IC is detected peak current, current more than over current setting value does not flow.

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

The range of over current setting is from 0.5A to 2.5A.

<Derivation sequence of detection resistor>

R (OCPSET)=34kΩ×Over current setting

TYP value of over current is 2A, MIN = 1.4A and MAX = 2.6A 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 typ value was set at 2A is given as shown below.

Detection resistor =34kΩ×2A=68kΩ

MAX current dispersion of this detection resistor value is

MAX current = 2A×1.3=2.6A

For example, 34kΩ  1A, 68kΩ  2A

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

As over current detector of This IC 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=1.0MHz, Typ=1.25MHz, MAX=1.5MHz

○ Output voltage = VOUT

○ Total LED current = IOUT

○ Average current of coil = Iave

○ Peak current of coil = Ipeak

○ Efficiency = eff

○ 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 = (VIN / L) ×Ton

(Example 1)

In case of, VIN=6.0V, L=4.7µH, fsw=1.25MHz, VOUT=39V, IOUT=80mA, Efficiency=85%

Ipeak = (6.0V / 4.7µH) × (1 / 1.25MHz) × (1-(6.0V / 39V)) =0.86A

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

Ton = (0.61A × (1-6.0V / 39V) × (1 / 1.25MHz) × (4.7µH /6.0V) × 2)

(1-VIN/VOUT) × (1/fsw)=0.68µs < Ton

Peak current = 0.68A/2+0.61A = 1.04A

(Example 2)

In case of, VIN=12.0V, L=4.7µH, fsw=1.25MHz, VOUT=39V, IOUT=80mA, Efficiency=85%

Ipeak = (12.0V / 4.7µH) × (1 / 1.25MHz) × (1-(12V / 39V)) =1.41A

Iave = (39V × 80mA / 12.0V) / 85% = 0.31A

Ton = (0.31A × (1-12 V / 39V) × (1 / 1.25MHz) × (4.7µH /12V) × 2)

(1-VIN/VOUT) × (1/fsw)=0.55µs > Ton

Peak current = 12V/4.7µH × 0.36µs = 0.92A

* When too large current is set, output overshoot is caused, be careful enough because it is led to break down of the IC in

case of the worst.

Current Sence

OCPSET

+

-

detect

R(OCPSET)

Fig. 24 Architecture