AMS2954
APPLICATION HINTS (Continued)
IIN
+VIN
5V
IL
VIN
IN
OUT
AMS2954
+
LOAD
*
m
F
1
GND
8
100k
VOUT
+VIN
IG
1.2
30V
5
3
1
ERROR
OUPUT
ERROR*
VOUT
IIN = IL +IG
AMS2954
*
+
**SHUTDOWN
INPUT
* See external capacitors
PTotal = (VIN -5)IL +(VIN)IG
3.3
F
m
SD
GND
R1
.01m
F
FB
1.23
V
4
7
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.
R2
VREF
The next parameter which must be calculated is the maximum allowable
temperature rise, TR(max). This is calculated using the formula:
FIGURE 2. Adjustable Regulator
*See Application Hints.
TR(max) =TJ(max) - TA(max)
Vout = VREF ´ (1 + R1/ R2)
Where TJ(max) is the maximum allowable junction temperature, and
TA(max) is the maximum ambient temperature.
Using the calculated values for TR(max) and P(max), the required value for
**Drive with TTL- high to shut down. Ground or leave if
shutdown feature is not used.
Note: Pins 2 and 6 are left open.
junction to ambient thermal resistance q(J-A), can be determined:
Reducing Output Noise
q(J-A) = TR(max) /P(max)
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 mF to 220 mF only decreases the noise from 430
mV to 160 mV rms for a 100 kHz bandwidth at 5V output.
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:
q(H-A) = q(J-A) - q(J-C) - q(C-H)
Noise could also be reduced fourfold by a bypass capacitor across
R1, since it reduces the high frequency gain from 4 to unity. Pick
where:
q(J-C) is the junction-to-case thermal resistance, which is specified as
3°C/W maximum for the AMS2954.
q(C-H) is the case-to-heatsink thermal resistance, which is dependent on
the interfacing material (if used).
q(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 q(H-A) calculated from the above listed formula.
CBYPASS @ 1 / 2pR1 ´ 200 Hz
or about 0.01 mF. When doing this, the output capacitor must be
increased to 3.3 mF to maintain stability. These changes reduce
the output noise from 430 mV to 100 mV 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.
Output Isolation
Heatsink Requirements
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.
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.
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
