ISL59920_15 Datasheet, PDF (16/18 Pages) Intersil Corporation – Triple Analog Video Delay Lines

ISL59920, ISL59921, ISL59922, ISL59923
CENABLE at Power-On
To guarantee proper operation, the CENABLE pin should be held
low for at least 30ms after the power supply has settled to within
5% of its final value.
If CENABLE cannot be guaranteed to be held low during this time,
an RC delay can be inserted between the CENABLE source and
the CENABLE input to meet this requirement as shown in
Figure 37. R₁ and C₁ generate an ~33ms delay, and D₁
discharges any charge on C₁ when the power to the ISL5992x is
removed.
+5V
V_SP
D₁
R₁
CENABLE
33k
C₁
1F
ISL5992x
FIGURE 37. CENABLE RC DELAY
Power Dissipation
As the delay setting increases, additional filter blocks turn on and
insert into the signal path. When the delay per channel
increments, V_SP current increases by 0.9mA while V_SM does not
change significantly. Under the extreme settings, the positive
supply current reaches 141mA and the negative supply current
can be 41mA. Operating at ±5V power supply, the worst-case
ISL5992x power dissipation is shown by Equation 1:
PD = 5  141mA + 5  41mA = 910mW
(EQ. 1)
The minimum _JA required for long term reliable operation of the
ISL5992x is calculated using Equation 2:
_JA = T_J – T_A   PD = 55C  W
(EQ. 2)
Where:
T_J is the maximum junction temperature (+135°C)
T_A is the maximum ambient temperature (+85°C)
For a 20 Ld package on a well laid-out PCB with good
connectivity between the QFN’s pad and the PCB copper area,
31°C/W _JA thermal resistance can be achieved. This yields a
much higher power dissipation of 3.54W using Equation 2 (see
Figure 33). To disperse the heat, the bottom heat spreader must
be soldered to the PCB. Heat flows through the heat spreader to
the circuit board copper then spreads and convects to air. Thus,
the PCB copper plane becomes the heatsink (see TB389). This
has proven to be a very effective technique. A separate
application note, which details the 20 Ld QFN PCB design
considerations, is available.
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FN6826.4
August 27, 2015