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AL8807
Application Information (cont.)
The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.
Switch ‘On’ time
Switch ‘Off’ time
t ON ?
L ? I
V IN ? V LED ? I AVG x ? R S ? r L ? R SW ?
t OFF ?
L ? I
V LED ? V D ? I AVG x ? R S ? r L ?
Where:
L is the coil inductance (H)
r L is the coil resistance ( ? )R S is the current sense resistance ( ? )
I avg is the required LED current (A)
Δ I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
V IN is the supply voltage (V)
V LED is the total LED forward voltage (V)
R SW is the switch resistance ( ? ) {=0.5 ? nominal}
V D is the diode forward voltage at the required load current (V)
Thermal Considerations
For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation
depends on several factors: the thermal resistance of the IC package ? JA , PCB layout, airflow surrounding the IC, and difference between junction
and ambient temperature.
The maximum power dissipation can be calculated using the following formula:
P D(MAX) = (T J(MAX) ? T A ) / ? JA
where
T J(MAX) is the maximum operating junction temperature,
T A is the ambient temperature, and
? JA is the junction to ambient thermal resistance.
The recommended maximum operating junction temperature, T J , is 125°C and so maximum ambient temperature is determined by the AL8807’s
junction to ambient thermal resistance, ? JA and device power dissipation.
? JA , is layout dependent and package dependent; the AL8807W5’s ? JA on a 25x25mm single layer PCB with 1oz copper standing in still air is
approximately 250°C/W (160°C/W on a four-layer PCB).
The maximum power dissipation at T A = 25°C can be calculated by the following formulas:
P D(MAX) = (125°C ? 25°C) / (250°C/W) = 0.4W for single-layer PCB
P D(MAX) = (125°C ? 25°C) / (160°C/W) = 0.625W for standard four-layer PCB
Figure 35, shows the power derating of the AL8807W5 on two (one single-layer and four-layer) different 25x25mm PCB with 1oz copper standing in
still air and the AL8807MP on an FR4 51x51mm PCB with 2oz copper standing in still air.
Figure 35 Derating Curve for Different PCB
AL8807
Document number: DS35281 Rev. 5 - 2
13 of 20
www.diodes.com
March 2013
? Diodes Incorporated
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