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NXP Semiconductors
PCU9955
16-channel UFm I 2 C-bus 57 mA constant current LED driver
string. Reducing this to a minimum (e.g., 0.8 V) helps to keep the power dissipation down.
Therefore LEDs binning is recommended to minimize LED voltage forward variation and
reduce power dissipation in the device.
In order to ensure that the device will not go into thermal shutdown when operating under
certain application conditions, its junction temperature (T j ) should be calculated to ensure
that is below the overtemperature threshold limit (125 ? C). The T j of the device depends
on the ambient temperature (T amb ), device’s total power dissipation (P tot ), and thermal
resistance.
The device junction temperature can be calculated by using the following equation:
T j = T amb + R th ? j - a ? ? P tot
(6)
where:
T j = junction temperature
T amb = ambient temperature
R th(j-a) = junction to ambient thermal resistance
P tot = (device) total power dissipation
An example of this calculation is show below:
Conditions:
T amb = 50 ? C
R th(j-a) = 31 ? C/W (per JEDEC 51 standard for multilayer PCB)
I LED = 50 mA / channel
I DD(max) = 12 mA
V DD = 5 V
LEDs per channel = 10 LEDs / channel
LED V F(typ) = 3 V per LED (30 V total for 10 LEDs in series)
LED V F mismatch = 0.2 V per LED (2 V total for 10 LEDs in series)
V reg(drv) = 0.8 V (This will be present only in the LED string with the highest LED forward
voltage.)
V sup = LED V F(typ) + LED V F mismatch + V reg(drv) = 30 V + 2 V + 0.8 V = 32.8 V
P tot calculation:
P tot = IC_power + LED drivers_power;
IC_power = (I DD ? V DD )
IC_power = (0.012 A ? 5 V) = 0.06 W
LED drivers_power = [(16 ? 1) ? (I LED ) ? (LED V F mismatch + V reg(drv) )] +
(I LED ? V reg(drv) )
LED drivers_power = [15 ? 0.05 A ? (2 V + 0.8 V)] + (0.05 A ? 0.8 V) = 2.14 W
P tot = 0.06 W + 2.14 W = 2.2 W
PCU9955
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 2 October 2012
? NXP B.V. 2012. All rights reserved.
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