AMS2954
APPLICATION HINTS
(Continued)
+V
IN
I
IN
V
IN
*
100k
ERROR
OUPUT
**SHUTDOWN
INPUT
8
+V
IN
5 ERROR*
V
OUT
1
3
AMS2954
SD
GND
4
FB
7
1.23
V
R
2
R
1
V
OUT
1.2
30V
GND
I
G
IN
OUT
+
1µF
AMS2954
5V
I
L
LOAD
I
IN
= I
L
+I
G
*
.01µ
F
+
3.3µF
* See external capacitors
P
Total
= (V
IN
-5)I
L
+(V
IN
)I
G
V
REF
FIGURE 2. Adjustable Regulator
*See Application Hints.
V
out
= V
REF
×
(1 + R
1
/ R
2
)
**Drive with TTL- high to shut down. Ground or leave if
shutdown feature is not used.
Note:
Pins 2 and 6 are left open.
Reducing Output Noise
In reference applications it may be an advantageous to reduce the
AC noise present at the output. One method is to reduce the
regulator bandwidth by increasing the size of the output
capacitor. This is the only way that noise can be reduced on the 3
lead AMS2954 but is relatively inefficient, as increasing the
capacitor from 1
µF
to 220
µF
only decreases the noise from 430
µV
to 160
µV
rms for a 100 kHz bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor across
R
1
, since it reduces the high frequency gain from 4 to unity. Pick
C
BYPASS
≅
1 / 2πR
1
×
200 Hz
or about 0.01
µF.
When doing this, the output capacitor must be
increased to 3.3
µF
to maintain stability. These changes reduce
the output noise from 430
µV
to 100
µV
rms for a 100 kHz
bandwidth at 5V output. With the bypass capacitor added, noise
no longer scales with output voltage so that improvements are
more dramatic at higher output voltages.
Heatsink Requirements
A heatsink might be required when using AMS2954, depending
on the maximum power dissipation and maximum ambient
temperature of the application. The heatsink must be chosen
considering that under all operating condition, the junction
temperature must be within the range specified under Absolute
Maximum Ratings.
To determine if a heatsink is required, the maximum power
dissipated by the regulator must be calculated. It is important to
consider, that if the regulator is powered from a transformer
connected to the AC line, the maximum specified AC input
voltage must be used.
FIGURE 3. Basic 5V Regulator
Figure 3 shows the voltages and currents which are present in a 5V
regulator circuit. The formula for calculating the power dissipated in the
regulator is also shown in Figure 3.
The next parameter which must be calculated is the maximum allowable
temperature rise, T
R(max)
. This is calculated using the formula:
T
R(max)
=T
J(max)
- T
A(max)
Where T
J(max)
is the maximum allowable junction temperature, and
T
A(max)
is the maximum ambient temperature.
Using the calculated values for T
R(max)
and P
(max)
, the required value for
junction to ambient thermal resistance
θ
(J-A)
, can be determined:
θ
(J-A)
= T
R(max)
/P
(max)
If the value obtained is 60°C/W or higher, the regulator may be operated
without an external heatsink. If the calculated value is below 60°C/W, an
external heatsink is required. To calculate the thermal resistance of this
heatsink use the formula:
θ
(H-A)
=
θ
(J-A)
-
θ
(J-C)
-
θ
(C-H)
where:
θ
(J-C)
is the junction-to-case thermal resistance, which is specified as
3°C/W maximum for the AMS2954.
θ
(C-H)
is the case-to-heatsink thermal resistance, which is dependent on
the interfacing material (if used).
θ
(H-A)
is the heatsink-to-ambient thermal resistance. It is this
specification which defines the effectiveness of the heatsink. The
heatsink selected must have a thermal resistance equal or lower than the
value of
θ
(H-A)
calculated from the above listed formula.
Output Isolation
The regulator output can be left connected to an active voltage source
with the regulator input power turned off, as long as the regulator ground
pin is connected to ground. If the ground pin is left floating, damage to
the regulator can occur if the output is pulled up by an external voltage
source.
Advanced Monolithic Systems, Inc.
6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
