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SLVS677C – JULY 2008 – REVISED OCTOBER 2015

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Typical Application (continued)

Table 6. Design Parameters

DESIGN PARAMETERS

EXAMPLE VALUES

No. of LED strings

16

No. of LEDs per string

3

LED current (mA)

20

Forward voltage of each LED (V)

3.5

Junction-to-ambient thermal resistance (°C/W)

40

Ambient temperature of application (°C)

115

VDD (V)

5

IDD (mA)

17

Max operating junction temperature (°C)

150

10.2.2 Detailed Design Procedure

TJ = TA + θJA × PD_TOT

where

•

TJ is the junction temperature

•

TA is the ambient temperature

•

θJA is the junction-to-ambient thermal resistance

•

PD_TOT is the total power dissipation in the IC

(12)

space

PD_TOT = PD_CS + IDD × VDD

where

•

PD_CS is the power dissipation in the LED current sinks

•

IDD is the IC supply current

•

VDD is the IC supply voltage

(13)

space

PD_CS = IO × VO × nCH

where

•

IO is the LED current

•

VO is the voltage at the output pin

•

nCH is the number of LED strings

(14)

space

VO = VLED – (nLED × VF)

where

•

VLED is the voltage applied to the LED string

•

nLED is the number of LEDs in the string

•

VF is the forward voltage of each LED

(15)

space

VO should not be too high as this will cause excess power dissipation inside the current sink. However, VO

should also not be loo low as this will not allow the full LED current (refer to the output voltage vs. output current

graph). With VLED = 12 V:

VO = 12 V – (3 × 3.5 V) = 1.5 V

(16)

PD_CS = 20 mA × 1.5 V × 16 = 0.48 W

(17)

Using PD_CS, calculate:

PD_TOT = PD_CS + IDD × VDD = 0.48 W + 0.017 A × 5 V = 0.565 W

(18)

Using PD_TOT, calculate:

22

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