AL8807AMP-13 Просмотр технического описания (PDF) - Diodes Incorporated.

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AL8807AMP-13

HIGH EFFICIENCY LOW EMI WIDE ANALOG DIMMING RANGE 36V 1A/1.3A BUCK LED DRIVER

Diodes Incorporated. 

AL8807A

Application Information (cont.)

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 39 – typical switching waveform.

Switch ‘On’ time

  tON 

LI

VIN  VLED  IAVG x RS  rL  RSW

Switch ‘Off’ time

  tOFF



VLED 

LI

VD  IAVG x

RS  rL

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)

ΔI is the coil peak-peak ripple current (A)

{Internally set to 0.3 x Iavg}

VIN is the supply voltage (V)

VLED is the total LED forward voltage (V)

RSW is the switch resistance (Ω) {= 0.25Ω nominal (SOT25)}

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

Off

VIN = 12V

TA =25ºC

2 LEDs

20ns/div

SW Pin: 2V/div

On

Figure 39 Typical Switching Waveform

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

1.6

formula:

PD(MAX) = (TJ(MAX) − TA) / JA

where

TJ(MAX) is the maximum operating junction temperature,

TA is the ambient temperature,

JA is the junction to ambient thermal resistance.

The recommended maximum operating junction temperature, TJ, is +125°C

and so maximum ambient temperature is determined by the AL8807A’s

junction to ambient thermal resistance,JA and device power dissipation.

1.4 MSOP-8EP

51mm x 51mm

1.2

1.0

SOT25

51mm x 51mm

0.8

0.6

JA, is layout dependent and package dependent; the AL8807AW5’s JA on a

25 x 25mm single layer PCB with 1oz copper standing in still air is

approximately +250°C/W and around 130°C/W on a 51mm x 51mm dual layer

0.4

SOT25

25mm x 25mm

0.2

board with maximum coverage top and bottom and 3 vias.

The maximum power dissipation at TA = +25°C can be calculated by the

following formulas:

0

-40 -25 -10 5 20 35 50 65 80 95 110 125

AMBIENT TEMPERATURE (°C)

Figure 40 Derating Curve for Different PCB

PD(MAX) = (+125°C − +25°C) / (250°C/W) = 0.4W for single-layer 25mm x25mm PCB

PD(MAX) = (+125°C − +25°C) / (130°C/W) =0.77W for dual layer 51mm x 51mm PCB

Figure 40, shows the power derating of the AL8807AW5 on two different PCBs and the AL8807AMP on one PCB.

SOT25 – 25mm x 25mm: AL8807AW5’s JA on a 25 x 25mm single layer PCB with 1oz copper

SOT25 – 25mm x 25mm: AL8807AW5’s JA on a 51mm x 51mm dual layer board with maximum coverage top and bottom and 3 vias

MSOP-8EP - 51mm x 51mm: AL8807AMP’s JA on a 51mm x 51mm dual layer board with maximum coverage top and bottom and 4 vias

Figure 40 shows that the MSOP-8EP version of the AL8807A can handle more power than its SOT25 version. So the AL8807AMP is the

preferred variant when operating at larger supply voltage rails (>24V) and/or driving larger LED currents. This is especially true in high power

density/space constraint applications such as high power 24VAC MR16 applications.

AL8807A

Document number: DS35990 Rev. 2 - 2

14 of 20

www.diodes.com

March 2013

© Diodes Incorporated

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