TLC5926-Q1_16 Datasheet, PDF (23/36 Pages) Texas Instruments – TLC592x-Q1 16-Channel Constant-Current LED Sink Drivers

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TLC5926-Q1, TLC5927-Q1
SLVS973A – SEPTEMBER 2009 – REVISED JULY 2015
10.2.1.2 Detailed Design Procedure
T_J = T_A + θ_JA × P_{D_TOT}
where
• T_J is the junction temperature
• T_A is the ambient temperature
• θ_JA is the junction-to-ambient thermal resistance
• P_{D_TOT} is the total power dissipation in the IC
(12)
space
P_{D_TOT} = P_{D_CS} + I_DD × V_DD
where
• P_{D_CS} is the power dissipation in the LED current sinks
• I_DD is the IC supply current
• V_DD is the IC supply voltage
(13)
space
P_{D_CS} = I_O × V_O × n_CH
where
• I_O is the LED current
• V_O is the voltage at the output pin
• n_CH is the number of LED strings
(14)
space
V_O = V_LED – (n_LED × V_F)
where
• V_LED is the voltage applied to the LED string
• n_LED is the number of LEDs in the string
• V_F is the forward voltage of each LED
(15)
space
V_O must not be too high as this will cause excess power dissipation inside the current sink. However, V_O must
also not be too low as this will not allow the full LED current (refer to the output voltage vs. output current graph).
With V_LED = 12 V:
V_O = 12 V – (3 × 3.5 V) = 1.5 V
(16)
P_{D_CS} = 20 mA × 1.5 V × 16 = 0.48 W
(17)
Using P_{D_CS}, calculate:
P_{D_TOT} = P_{D_CS} + I_DD × V_DD = 0.48 W + 0.017 A × 5 V = 0.565 W
(18)
Using P_{D_TOT}, calculate:
T_J = T_A + θ_JA × P_{D_TOT} = 115°C + 39.7°C/W × 0.565 W = 137.6°C
(19)
This design example has demonstrated how to calculate power dissipation in the IC and ensure that the junction
temperature is kept below 150°C.
NOTE
This design example assumes that all channels have the same electrical parameters
(n_LED, I_O, V_F, V_LED). If the parameters are unique for each channel, then the power
dissipation must be calculated for each current sink separately. Then, each result must be
added together to calculate the total power dissipation in the current sinks.
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Product Folder Links: TLC5926-Q1 TLC5927-Q1