ATR28XXS_07 Datasheet, PDF (10/12 Pages) International Rectifier – HYBRID-HIGH RELIABILITY DC/DC CONVERTER

ATR28XXS Series
+Input
1
EMI
Filter
Block Diagram
5 +Vout
Inhibit
Input
2
Case
8
Sync
9
Primary
Housekeeping
Supply
Drive
Pulse Width
Modulator ^FB
CS
Error
Amp &
Ref
4
Output
Return
6 +Sense
3
Sense
Return
Input
Return
10
Application Information
Inhibit Function
Connecting the inhibit input (Pin 2) to input common (Pin 10)
will cause the converter to shut down. It is recommended
that the inhibit pin be driven by an open collector device
capable of sinking at least 400µA of current. The open
circuit voltage of the inhibit input is 11.5 ±1.0VDC.
EMI Filter
An Optional EMI filter (AFC461)is available that will reduce
the input ripple current to levels below the limits imposed by
MIL-STD-461B CEO3.
Output Voltage Adjustment
The output voltage of the ATR2805S converter can be
adjusted upward by connecting an appropriate resistor value
between the Positive Output (Pin 5) and the Positive Sense
(Pin 6) as shown in Table 1 below.
Resistance Pin 5 to 6 (Ω)
109
240
400
600
857
1200
Output Voltage Increase (V)
0.1
0.2
0.3
0.4
0.5
0.6
Table1 Output adjustment resistor values for ATR2805S
Device Synchronization
Whenever multiple DC/DC converters are utilized in a single
system, significant low frequency noise may be generated
due to slight difference in the switching frequencies of the
converters. Because of the low frequency nature of this
noise (typically less than 10KHz), it is difficult to filter out
and may interfere with proper operation of sensitive
systems. International Rectifier permits synchronization of
multiple ATR28XX converters to match switching frequency
of the converter to the frequency of the system clock, thus
eliminating this type of noise.
Thermal Management
Assuming that there is no forced air flow, the package
temperature rise above ambient (∆T) may be calculated
using the following expression:
∆T = 80 A ^–0.7 P ^0.85 (°C)
where A = the effective surface area in square inches
(Including heat sink if used),P_d = power dissipation in watts.
The total surface area of the ATR package is 7.34 square
inches. If a worse case full load efficiency of 76% is
assumed, then the case temperature rise of an ATR2805S
can be calculated as follows:
P
=
P_out
⎡
⎢⎣
1
Eff
− 1_{⎥⎦⎤}
=
30_{⎢⎣⎡}
1
76
− 1_{⎥⎦⎤} = 9.5W
∆T = 80 (7.34^)-0.7 (9.5)^0.85 = 134°C
Hence, if T_AMBIENT = +25°C, the DC/DC converter case
temperature will be approximately 159°C if no heat sink
or air flow is provided.
To calculate the heat sink area required to maintain a
specific case temperature rise, the above equation may
be manipulated as follows:
A_HEATSINK
=
⎡ _∆T
⎢
⎢⎣
80P_d
0.85
⎤ ^−1.43
⎥
⎥⎦
−
A_PKG
As an example, if a maximum case temperature rise of
50°C above ambient is required, then the required
effective heat sink area is:
( ) ^AHEATSINK
=
⎡
⎢
⎢⎣ 80
50
9.5
0.85
⎤ ^−1.43
⎥
⎥⎦
− 7.34
= 22.9in²
10
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