ADDC02803SC/ADDC02805SA
BASIC OPERATION
one of the output pins of the converter, or remotely at the load.
A remote connection at the load can adjust for voltage drops of
as much as 0.25 V dc between the converter and the load.
The ADDC02803SC and ADDC02805SA converters use a
flyback topology with dual interleaved power trains operating
180° out of phase. Each power train switches at a fixed fre-
quency of 500 kHz, resulting in a 1 MHz fixed switching fre-
quency as seen at the input and output of the converter. In a
flyback topology, energy is stored in the inductor during one
half portion of the switching cycle and is then transferred to the
output filter during the next half portion. With two interleaved
power trains, energy is transferred to the output filter during
both halves of the switching cycle, resulting in smaller filters to
meet the required ripple.
Long remote sense leads can affect converter stability, although
this condition is rare. The impedance of the long power leads
between the converter and the remote sense point could affect
the converter’s unity gain crossover frequency and phase margin.
Consult factory if long remote sense leads are to be used.
Pin 3 (ADJUST)
An adjustment pin is provided so that the user can change the
nominal output voltage during the prototype stage. Since very
low temperature coefficient resistors are used to set the output
voltage and maintain tight regulation over temperature, using
standard external resistors to adjust the output voltage will
loosen output regulation over temperature. Furthermore, since
the status trip point is not changed when the output voltage is
adjusted using external resistors, the status line will no longer
trip at the standard levels of the newly adjusted output voltage.
If necessary, modified standard units can be ordered with the
necessary changes made inside the package at the factory. The
ADJUST function is sensitive to noise, and care should be taken
in the routing of connections.
A five pole differential input EMI filter, along with a common-
mode EMI capacitor and careful attention to layout parasitics, is
designed to meet all applicable requirements in MIL-STD-461D
when installed in a typical system setup. A more detailed dis-
cussion of CE102 and other EMI issues is included in the sec-
tion entitled EMI Considerations.
The converters use current mode control and employ a high
performance opto-isolator in its feedback path to maintain
isolation between input and output. The control circuit is
designed to give a nearly constant output current as the output
voltage drops from VO nom to VSC during a short circuit
condition. It does not let the current fold back below the
maximum rated output current. The output overvoltage
protection circuitry, which is independent of the normal
feedback loop, protects the load against a break in the remote
sense leads. Remote sense connections, which can be made at
the load, can adjust for voltage drops of as much as 0.25 V dc
between the converter and the load, thereby maintaining an
accurate voltage level at the load.
To make the output voltage higher, place a resistor from
ADJUST (Pin 3) to –SENSE (Pin 1). To make the output
voltage lower, place a resistor from ADJUST (Pin 3) to +SENSE
(Pin 2). Figures 24 and 25 show resistor values for a ±5% change
in output voltage:
8
7
6
5
4
3
2
1
An input overvoltage protection feature shuts down the con-
verter when the input voltage exceeds (nominally) 52.5 V dc.
An internal temperature sensor shuts down the unit and pre-
vents it from becoming too hot if the heat removal system fails.
The temperature sensed is the case temperature and is factory
set to trip at a nominal case temperature of 110°C to 115°C.
The shutdown temperature setting can be raised externally or
disabled by the user.
Each unit has an INHIBIT pin that can be used to turn off the
converter. This feature can be used to sequence the turn-on of
multiple converters and to reduce input power draw during
extended time in a no load condition.
99
98
97
96
95
OUTPUT VOLTAGE – %
Figure 24. External Resistor Value for Reducing Output
Voltage
A SYNC pin, referenced to the input return line (Pin 10),
is available to synchronize multiple units to one switching
frequency. This feature is particularly useful in eliminating
beat frequencies which may cause increased output ripple on
paralleled units. A current share pin (ISHARE) is available that
permits paralleled units to share current typically within 5% at
full load.
5
4
3
2
A low level dc auxiliary voltage supply referenced to the input
return line is provided for miscellaneous system use.
PIN CONNECTIONS
Pins 1 and 2 (؎SENSE)
1
0
Pins 1 and 2 must always be connected for proper operation,
although failure to make these connections will not be catastrophic
to the converter under normal operating conditions. Pin 1 must
always be connected to the output return and Pin 2 must always
be connected to +VOUT. These connections can be made at any
101
102
103
104
105
OUTPUT VOLTAGE – %
Figure 25. External Resistor for Increasing Output Voltage
REV. A
–8–
ADI [ ADI ]
