MAX11504/MAX11505
Four-Channel, Standard-Definition
Video Filters
8 _______________________________________________________________________________________
Applications Information
Output Configuration
The MAX11504/MAX11505 outputs may be either DCor AC-coupled. If AC-coupled, choose a capacitor that
passes the lowest frequency content of the video signal,
and keep the line-time distortion within desired limits. The
capacitor value is a function of the input leakage and
impedance of the circuit being driven. Common industry
practice is to use a 220µF or larger coupling capacitor. If
any or all outputs are driving two parallel loads, see the
Junction Temperature Calculations
section.
The MAX11504/MAX11505 outputs are fully protected
against short circuits either to the ground or the positive
supply of the device. The short-circuit protection circuitry limits the output current to 80mA (typical) per output. Shorting more than one output simultaneously can
exceed the maximum package power dissipation.
Junction Temperature Calculations
Die temperature is a function of quiescent power dissipation and the power dissipation in the output drivers.
Calculate the power dissipated PDusing:
where PDSis the quiescent power dissipated in the die,
and given by:
and where P
DOn
is the power dissipated in the nthdri-
ver stage and given by:
where V
ORMSn
is the RMS output voltage and RLis the
load resistance.
Example - Assuming these conditions:
1) Video standard = 525/60/2:1.
2) Video format = RGB with syncs on all channels.
3) Picture content = 100% white.
4) The input signal is AC-coupled.
5) The output signal is DC-coupled.
6) VCC= 5.0V.
7) ICC= 24mA.
A sync tip exists at 280mV and peak white exists at
2.28V. The RMS voltage will be approximately 1.88V on
each output (80% of the peak-peak voltage, plus the
offset) giving:
and
The junction temperature is given by:
where T
J
= junction temperature, TA= ambient temper-
ature (assume +70°C) and R
θJA
= thermal resistance
junction to ambient.
From the
Absolute Maximum Ratings
section of the
data sheet, the derating factor is 8.8mW/°C above
+70°C. R
θJA
= 1/(derating factor) = 1/(8.8mW/°C) =
113°C/W.
Therefore:
If there is only one video load on each output, the junction temperature lowers to:
The above calculations assume the use of a multilayered board with extensive ground planes for high thermal efficiency. Using such a board is especially
important in applications where there are two video
loads on each channel.
(.).
.