APPLICATION NOTES
POWER SUPPLIES
The output voltage of the DHC2800S may be adjusted from
90% to 110% of nominal value by the use of a 50KΩ potenti-
ometer as shown. Adjustment beyond this range is possible
however certain characteristics of the converter such as but
not limited to input voltage range, efficiency, ripple and tem-
perature performance will change. Characterization by the user
is recommended in such applications.
LOAD FAULT RESPONSE
The DHC2800S series of dc-dc converters share load fault
philosophies. Load fault conditions include short-circuit and
severe overload conditions. The DHC2800S converter series
responds to load faults by turning off all power conversion cir-
cuits for 250mS and then attempting to restart for 10mS (typi-
cal). The net "on" duty factor during a fault is very low result-
ing in low converter dissipation and immunity from overheating
at 125°C. Current beyond rated can flow into the load at
startup time. This allows the converter to bring up capacitive
and other difficult load types more reliably than competing con-
verters.
Adjust/comp (pin 3) may be driven by external circuitry refer-
enced to pin 2 (-output) if desired. Grounding pin 3 causes
voltage to increase (25% typically) while driving pin 3 above
1.3V causes output voltage to decrease. Pin 3 may be driven
through 10KΩ or more if connection of the comp function is
also required.
The comp function of pin 3 allows capacitive loads as large
as 1000µF to be accommodated on the DHC2805S without
loss of stability. This feature may be utilized by connecting a
10nF capacitor between pins 3 and 1. This is generally recom-
mended when low ESR load capacitances of 100µF or greater
are used.
SHUTDOWN PLUS
Pin 5 is used for remote shutdown, output fault detection,
and/or setting the input voltage point at which the converter
will turn on as shown in the typical application diagram. No
connection to pin 5 is necessary for normal operation of the
converter. Pin 5 is referenced to the -input (pin 7).
Shutdown may be implemented by simply connecting pin 5
to an open collector logic output or switch rated at 2.5mA,
25Vdc or higher.
Input voltage turn on point is programmed with a single resis-
tor from pin 5 to 7. An input turn on/off hysteresis (typically
3.5% of Vin) will be observed. This should be considered when
making or verifying set point adjustment. The value of the
setpoint resistor may be determined by the following:
R= 210·10³
E
TO
-9.5
(±10% accuracy at 25°C)
Set point temperature coefficient is typically +400ppm/°C.
Output fault monitoring is accomplished by observing pin 5
with a high impedance monitoring circuit. Pin 5 voltage drops
from over 10V to below 1V when a load fault causes the con-
verters fault protection circuitry to activate. It will remain low
for at least 100mS and return high. If the load fault is still
present pin 5 will return low and the cycle will repeat. A resis-
tor >400KΩ from pin 5 to 7 provides pull down for pin 5 if
there is no input setpoint programming resistor already in place.
5
Rev. F 9/05
MSK [ M.S. KENNEDY CORPORATION ]
