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ADDC02803SC

28 V/66 W/100 W DC/DC Converters with Integral EMI Filter

Analog Devices 

ADDC02803SC/ADDC02805SA

Pin 6 (INHIBIT)

Pin 6 is active low and is referenced to the input return of the

converter. Connecting it to the input return will turn the con-

verter off. For normal operation, the inhibit pin is internally pulled

up to 12 V. Use of an open collector circuit is recommended.

When Pin 6 is disconnected from input return, the converter

will restart in the soft-start mode. Pin 6 must be kept low for at

least 2 milliseconds to initiate a full soft start. Shorter off times

will result in a partial soft start. Figure 31 shows the input

characteristics of Pin 6.

1.2

1.1

1.0

0.9

0.8

0.7

0.5

1.0

1.5

2.0

VIL – V

Figure 31. Input Characteristics of Pin 6 When Pulled Low

Pin 7 (SYNC)

Pin 7 can be used for connecting multiple converters to a mas-

ter clock. This master clock can be either an externally user-

supplied clock or a converter that has been modified and

designated as a master unit. Capacitive coupling of the clock

signal will ensure that if the master clock stops working the

individual units will continue to operate at their own internal

clock frequency, thereby eliminating a potential single point

failure. Capacitive coupling will also permit a wider duty cycle

to be used. Consult factory for more information. The SYNC

pin has an internal pull-down so it is not necessary to sink any

current when driving the pin low. Reference Figure 38 for a

fault tolerant, secondary side powered SYNC drive circuit.

For user-supplied master clocks with no external circuitry, the

following specifications must be met:

a. Frequency: 1.00 MHz min

b. Duty cycle: 7% min, 14% max

c. High state voltage high level: 4 V min to 7 V max

d. Low state voltage low level: 0 V min to 3.0 V max

Users should note that the SYNC pin is referenced to the input

return of the converter. If the user-supplied master clock is

generated on the output side of the converter, the signal should

be isolated.

Users should be careful about the frequency selected for the

external master clock. Higher switching frequencies will

reduce efficiency and may reduce the amount of output power

available at minimum input line. Consult factory for modi-

fied standard switching frequency to accommodate system

clock characteristics.

Pin 8 (ISHARE)

Pin 8 allows paralleled converters to share the total load cur-

rent, typically within ± 5% at full load. To use the current share

feature, connect all current share pins to each other and con-

nect the SENSE pins on each of the converters. The current

sharing function is sensitive to the differential voltage between the

input return pins of paralleled converters. The current sharing

function is also sensitive to noise, and care should be taken in

the routing of connections. Refer to Figure 37 for typical appli-

cation circuits using paralleled converters.

Pin 9 (TEMP)

Pin 9 can be used to indicate case temperature or to raise or

disable the temperature at which thermal shutdown occurs.

Typically, 3.90 V corresponds to +25°C, with a +13.1 mV/°C

change for every 1°C rise. The sensor IC (connected from Pin

9 to the input return [Pin 10]) has a 13.1 kΩ impedance.

The thermal shutdown feature has been set to shut down the

converter when the case temperature is nominally 110°C to

115°C. To raise the temperature at which shutdown occurs,

connect a resistor with the value shown in Figure 32 from Pin 9

to the input return (Pin 10). To completely disable the tempera-

ture shutdown feature, connect a 50 kΩ resistor from Pin 9 to

the input return (Pin 10).

1400

1200

1000

800

600

400

200

0

120

125

130

135

140

145

150

SHUTDOWN CASE TEMPERATURE – ؇C

Figure 32. External Resistor Value for Raising

Temperature Shutdown Point

INPUT VOLTAGE RANGE

The steady state operating input voltage range for the converter

is defined as 18 V to 40 V. The abnormal operating input

voltage range is defined as 16 V to 50 V. In accordance with

MIL-STD-704D, the converter can operate up to 50 V dc

input for transient conditions as long as 50 milliseconds, and it

can operate down to 16 V dc input for continuous operation

during emergency conditions. Figure 4 (typical low line dropout

vs. load) shows that the converter can work continuously down

to and below 16 V dc under reduced load conditions.

The ADDC02803SC and ADDC02805SA can be modified to

survive, but not work through, the upper limit input voltages

defined in MIL-STD-704A (aircraft) and MIL-STD-1275A

(military vehicles). MIL-STD-704A defines an 80 V surge that

lasts for 1 second before it falls below 50 V, while MIL-STD-

1275A defines a 100 V surge that lasts for 200 milliseconds

–10–

REV. A

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