AL8807B datasheet(13/18 Pages) DIODES

13 / 18 page

AL88070B

Document number: DS36191 Rev. 1 - 2

13 of 18

www.diodes.com

March 2014

AL8807B

Applications Information (cont.)

Inductor Selection (cont.)

The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean

output current.

Suitable coils for use with the AL8807B are listed in the table below:

Part No.

(µH)

DCR

(V)

ISAT

(A)

Manufacturer

MSS1038-333

0.093

2.3

CoilCraft www.coilcraft.com

MSS1038-683

0.213

1.5

NPIS64D330MTRF

0.124

1.1

NIC www.niccomp.com

The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times over the supply voltage and load current range.

The following equations can be used as a guide, with reference to Figure 38 – typical switching waveforms.

Switch ‘On’ time



AVG

LED









Switch ‘Off’ time



AVG

LED

OFF







Where:

L is the coil inductance (H)

rL is the coil resistance (Ω)

RS is the current sense resistance (Ω)

Iavg is the required LED current (A)

∆I is the coil peak-peak ripple current (A)

{Internally set to 0.3 x Iavg}

VIN is the supply voltage (V)

Figure 38 Typical Switching Waveform

VLED is the total LED forward voltage (V)

RSW is the switch resistance (Ω) {=0.5Ω nominal}

VD is the diode forward voltage at the required load current (V)

Thermal Protection

The AL8807B includes Over-Temperature Protection (OTP) circuitry that will turn off the device if its junction temperature gets too high. This is to

protect the device from excessive heat damage. The OTP circuitry includes thermal hysteresis that will cause the device to restart normal

operation once its junction temperature has cooled down by approximately 55°C.

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:

PD(MAX) = (TJ(MAX) − TA) / JA

where

TJ(MAX) is the maximum operating junction temperature  Maximum recommended = 125°C

TA is the ambient temperature, and

JA is the junction to ambient thermal resistance.

JA, is layout dependent and package dependent; the AL8807BMP’s JA on an FR4 51x51mm PCB with 2oz copper standing in still air is

approximately 69°C/W.

So the maximum power dissipation at TA = +25°C is:

PD(MAX) = (125°C − 25°C) / (69°C/W) = 1.41W for the above dimensioned PCB

VIN = 12V

TA =25ºC

2 LEDs

20ns/div

SW Pin: 2V/div

Off

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