AD1991ASV Analog Devices Inc, AD1991ASV Datasheet - Page 8

• Manufacturer Part Number: AD1991ASV
• Description: IC AMP AUDIO PWR 40W STER 52TQFP
• Manufacturer: Analog Devices Inc
• Type: Class Dr

Specifications of AD1991ASV

• Rohs Status: RoHS non-compliant
• Output Type: 2-Channel (Stereo) or 4-Channel (Quad)
• Max Output Power X Channels @ Load: 40W x 1 @ 4 Ohm; 20W x 2 @ 8 Ohm
• Voltage - Supply: 6.5 V ~ 22.5 V
• Features: Depop, Mute, Short-Circuit and Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 52-TQFP, 52-VQFP

AD1991
APPLICATION CONSIDERATIONS
Good board layout and decoupling are vital for correct operation
of the AD1991. Due to the fact that the part switches high currents,
there is the potential for large PV
tor transitions. This can cause unpredictable operation of the part.
To avoid this potential problem, close chip decoupling is essen-
tial. It is also recommended that the decoupling capacitors be
placed on the same side of the board as the AD1991 and connected
directly to the PV
capacitors on the other side of the board and decoupling through
vias, the effectiveness of the decoupling is reduced. This is
because vias have inductive properties and, therefore, prevent
very fast discharge of the decoupling capacitors. Best operation
is achieved with at least one decoupling capacitor on each side of
the AD1991 or optionally two capacitors per side can be used to
further reduce the series resistance of the capacitor. If these
decoupling recommendations cannot be followed and decoupling
through vias is the only option, the vias should be made as large
as possible to increase surface area, thereby reducing inductance
and resistance.
Figures 5 and 6 show two possible layouts to provide close chip
decoupling. In both cases, the PV
close as possible to the pins of the AD1991. One solution uses
surface-mount capacitors that offer low inductance; however, each
output (OUTA, OUTB, OUTC, and OUTD) must be brought
through vias to another layer of the board to be brought to the
LC filter. The other solution uses through-hole capacitors that
have higher inductance but allow the outputs to connect directly
to the LC filter. In this solution, the inductor for OUTA and
OUTC would span the PV
decoupling capacitors from PV
not be necessary if capacitors with low series resistance are
used. Another close chip capacitor is used for AV
decoupling, with the actual power connections to the capacitors
being done through vias. This is quite acceptable since AV
a low current stable supply. Finally, a close chip capacitor is used
to decouple DV
is a digital supply whose current will change dynamically and,
therefore, requires good decoupling. For both PV
additional reservoir capacitors should be used to augment the
close chip decoupling, especially for PV
large transients.
THERMAL CONSIDERATIONS
Careful consideration must be given to heat sinking the AD1991,
particularly in applications where the ambient temperature can
be much higher than normal room temperature. The three
thermal resistances of
order to correctly heat sink the part. These values specify the
temperature difference between two points, per unit power
dissipation.
JC
DD
specifies the temperature difference between the
DD
to DGND. This is quite important since DV
and PGND pins. By placing the decoupling
JC
,
DD
CA
trace. These diagrams show four
DD
, and
DD
DD
to PGND; however, this may
bounce each time a transis-
to PGND decoupling is as
JA
DD
should be known in
, which usually has very
DD
DD
and DV
to AGND
DD
DD
DD
is
,
–8–
junction (die) and the case (package) for each watt of power
dissipated in the die. The AD1991 is specified with a
1°C/W, which means that for each watt of power dissipated in
the part, the junction (or die) temperature will be 1ºC higher
than the case (or package) temperature.
The value of
temperatures, is entirely dependent on the size of heat sink
attached to the case, the material used, the method of attach-
ment, and the airflow over the heat sink. The value of
specified as 26°C/W for no heat sink and no airflow over the device.
Finally,
between 1°C/W and 27°C/W depending on the heat sink used.
This is the temperature difference between the junction (die) and
ambient temperature around the case (package) for each watt
dissipated in the part.
The AD1991 is specified to have a thermal shutdown of typically
150°C die temperature. Good design procedures allow for a
margin, so the system should be designed such that the AD1991
die never goes above 140°C. Knowing the maximum desirable
die temperature, the efficiency of the AD1991, the maximum
ambient temperature, and the maximum power that will be
delivered to the load, the necessary
8 Ω load, the AD1991 has a typical efficiency of 87%, which
can be reduced slightly to be conservative. For this example,
assume an 85% efficiency. If the power delivered to the loads is
to be 2
the AD1991 can be calculated as follows:
If the ambient temperature can reach 85°C maximum, the allowable
difference between the die temperature and ambient temperature
is (140 – 85) = 55°C. This gives a
7.9°C/W. This requires a heat sink that gives a
The size and type of heat sink required can now be calculated.
If adequate heat sinking is not applied to the AD1991, the system
will suffer from the AD1991 going into thermal shutdown. It is
advisable to also use the thermal warning output on the AD1991
to attenuate the power being delivered to help prevent thermal
shutdown.
POWER-UP CONSIDERATIONS
Careful power-up is necessary when using the AD1991 to
ensure correct operation and to avoid possible latch-up issues.
The AD1991 should be held in RESET with MUTEB asserted
until all three power supplies have stabilized. Once the supplies
have stabilized, the part can be brought out of RESET, and
following this, MUTEB can be negated.
Total Power Supplied to the AD1991 = (40/85
JA
20 W rms continuous power, the power dissipated in
Power Dissipated in the AD1991 = 7 W rms
is the sum of the
CA
Power Supplied to Loads = 40 W rms
, the difference between the case and ambient
JC
and
JA
CA
CA
requirement of (55/7) =
can be calculated. For an
values, and will be
100) = 47 W rms
CA
of 6.9°C/W.
JC
CA
REV. 0
of
is