TLC5916-Q1_16 Datasheet, PDF (26/34 Pages) Texas Instruments – 8-Bit Constant-Current LED Sink Drivers

TLC5916-Q1, TLC5917-Q1
SLVS814A – JANUARY 2008 – REVISED MAY 2015
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10.2.1.2 Detailed Design Procedure
Use the following equations to determine the design parameters.
T_J = T_A + R_θJA × P_{D_TOT}
where
• T_J is the junction temperature
• T_A is the ambient temperature
• R_θJA is the junction-to-ambient thermal resistance
• P_{D_TOT} is the total power dissipation in the IC.
(6)
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.
(7)
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.
(8)
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.
(9)
V_O must not be too high as this causes excess power dissipation inside the current sink. However, V_O also must
not be too low as this does not allow the full LED current Figure 21.
With V_LED = 12 V:
V_O = 12 V – (3 × 3.5 V) = 1.5 V
(10)
P_{D_CS} = 20 mA × 1.5 V × 8 = 0.24 W
(11)
Using P_{D_CS}, calculate:
P_{D_TOT} = P_{D_CS} + I_DD × V_DD = 0.24 W + 0.01 A × 5 V = 0.29 W
(12)
Using P_{D_TOT}, calculate:
T_J = T_A + R_θJA × P_{D_TOT} = 115°C + 86.9°C/W × 0.29 W = 140°C
(13)
This design example demonstrates 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, add each result
together to calculate the total power dissipation in the current sinks.
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