SGM9146 Datasheet, PDF (9/13 Pages) SG Micro Corp – 4-Channel, Video Filter Driver

SGM9146
APPLICATION INFORMATION
Application Circuits
The SGM9146 video filter provides 6dB gain from input to
output. In addition, the input is slightly offset to optimize the
output driver performance. The offset is held to the minimum
required value to decrease the standing DC current into the
load.
The SGM9146 provides an internal clamp circuit for CVBS
channel and a bias circuit for YPbPr three channels to support
AC-coupled input signals. For CVBS channel, if the input signal
does not go below ground, the input clamp does not operate.
This allows DAC outputs to directly drive the SGM9146 without
an AC coupling capacitor. When the input is AC-coupled, the
clamp sets the sync tip (or lowest voltage) just below ground.
The worst-case sync tip compression due to the clamp cannot
exceed 7mV. The input level set by the clamp, combined with
the internal DC offset, keeps the output within its acceptable
range. For YPbPr channels, AC input is recommended to be
used.
For a DC-coupled DAC drive with DC-coupled outputs, use the
configuration in Figure 1.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
75Ω
Clamp Inactive
Figure 1. DC-Coupled Inputs and Outputs
Alternatively, if the DAC’s average DC output level causes the
signal to exceed the range from 0V to 1.4V, it can be
AC-coupled as shown in Figure 2.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
LCVF
75Ω
0.1µF
Clamp Active
Figure 2. AC-Coupled Inputs, DC-Coupled Outputs
4-Channel, Video Filter Driver
for SD/HD (1080i)/HD (1080p)
The same circuits can be used with AC-coupled outputs if
desired.
DVD or
STB
SoC
DAC
Output
0V - 1.4V
0.1μF
LCVF
Clamp Active
75Ω ^220μF
Figure 3. AC-Coupled Inputs and Outputs
NOTE: The video tilt or line time distortion is dominated by the
AC coupling capacitor. The value may need to be increased
beyond 220μF to obtain satisfactory operation in some
applications.
Power Dissipation
The SGM9146 output drive configuration must be considered
when calculating overall power dissipation. Care must be taken
not to exceed the maximum die junction temperature. The
following equations can be used to calculate the power
dissipation and internal temperature rise.
TJ = TA + PD • θJA
(1)
where:
PD = PCH1 + PCH2 + PCH3
(2)
P_CHX = V_CC • I_CH - (V_O2/R_L)
(3)
where:
V_O = 2V_IN + 0.55V
(4)
ICH = (ICC/3) + (VO/RL)
(5)
V_IN = RMS value of input signal
ICC = 58mA (1080i)
V_CC = 5.0V
RL = channel load resistance
Board layout can also affect thermal characteristics. Refer to
the Layout Considerations section for details.
The SGM9146 is specified to operate with output currents
typically less than 50mA, more than sufficient for a dual (75Ω)
video load. Internal amplifiers are current limited to a maximum
of 80mA and should withstand brief duration of short-circuit
conditions. This capability is not guaranteed.
SG Micro Corp
9
www.sg-micro.com