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BW7381 Datasheet, PDF (9/11 Pages) Bruckewell Technology LTD – Universal High Brightness LED Driver
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BW7381
Universal High Brightness LED Driver
3-5. OVP Threshold Voltage
The BW7381 perform output over voltage
protection though VDD pin. Once VDD pin
exceed in 28V, BW7381 turns off and latch out the
MOSFET switcher. The OVP threshold voltage
can be set by the Zener diode, ZD1.
ZD1 OVP  28V
3-6. VDD, Supply Current Circuit
At the beginning in start-up, the output
voltage is not high enough to charge VDD
capacitor. Therefore VDD capacitor have to save
enough energy for IC operating bias during
start-up. To determine the capacitance, we need
to calculate the IC operating current, IDD first.
IDD # Ciss u 20u freq 1.5mA
In general, assume the VDD capacitor need
to keep enough energy for 50ms. Then the
capacitance can be calculated.
C2
t
IDD u 50ms
18V  8.5V
C3 100nF
Place the VDD capacitor C2 and bypass
capacitor C3 as close to IC as possible. Morevoer,
a current spike filter resistor R11 is suggest
(10 : ~22 : ) to place between VDD and C3.
3-7. Start-up Resistor
At the initial stage, the VDD capacitor get
charge through start-up resistor towards the
start-up threshold voltage. The smaller start-up
resistor the larger power dissipation. The larger
start-up resistor the longer charge time.
Rev.1.4 Jan. 2013Ä¡
R1
t
t(charg e) uVac
C2  C3 u18
2
3-8. Off-Time setting
The duty cycle in buck-boost converter is a
function of Vin and Vo.
Duty( pk)
VO
Vac 2  VO
The off-time can be set manually by adjusting
the size of an external resistor (RT). The BW7381
automaticall adjusts the off-time with the line
voltage (Vac). To calculat the operating off-time
and RT, the maximum switching frequency at
peak of line cycle must be determined first.
toff H
1  Duty( pk)
freq
toff L
toff H
1.4
R6
toff H
21Ps
u
50K:
where :
toffH is the off-time at high-line (180Vac to
300Vac),
toffL is the off-time at low-line (90Vac to
135Vac),
3-9. Power Inductor
Assume a target efficiency to estimate input
power and input peak current.
Pin
VO u IO
K
Iin( pk )
Pin u 2
Vac
 ▷