BD6590MUV
Technical Note
・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 BD6590MUV 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.
Current Sence
+
detect
-
<Derivation sequence of detection resistor>
OCPSET
R (OCPSET)=34kΩ×Over current setting
R(OCPSET)
Fig. 8 Architecture
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 BD6590MUV 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
○ Switching frequency = fsw
○ Inductance value of coil = L
MIN=1.0MHz, Typ=1.25MHz, MAX=1.5MHz
○ Total LED current = IOUT
○ Output voltage = VOUT
○ 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
Ton
=
=
(VOUT × IOUT / VIN) / eff
(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→
(1-VIN/VOUT) × (1/fsw) > Ton→
peak current = Ipeak /2 + Iave
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
Ton
= (39V × 80mA / 6.0V) / 85% = 0.61A
= (0.61A × (1-6.0V / 39V) × (1 / 1.25MHz) × (4.7µH /6.0V) × 2)1/2 = 0.81µs
(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)1/2 = 0.36µs
(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.
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2011.07 - Rev.B
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