APPLICATION NOTES
POWER SUPPLIES
The output voltage of the DAC2812S and DAC2815S may
be adjusted from 90% to 110% of nominal value by the use of
a 150KΩ potentiometer as shown. Adjustment beyond this
range is possible however certain characteristics of the con-
verter such as but not limited to input voltage range, efficiency,
ripple and temperature performance will change. Characteriza-
tion by the user is recommended in such applications.
LOAD FAULT RESPONSE
The MSK families of DC-DC converters share load fault phi-
losophies. Load fault conditions include short-circuit and se-
vere overload conditions. The DAC2800S converter series re-
sponds 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 4) may be driven by external circuitry refer-
enced to pin 2 (-output) if desired. Grounding pin 4 causes
voltage to increase (25% typically) while driving pin 4 above
1.3V causes output voltage to decrease. Pin 4 may be driven
negative without damage, however the resultant increase in
converter output voltage should be considered. Pin 4 may be
driven through 10KΩ or more if connection of the comp func-
tion is also required.
The comp function of pin 4 allows load transient response to
be tailored to suit specific application requirements. This fea-
ture may be utilized by connecting a 10 or less nF capacitor
between pins 4 and 1.
Note: The DAC2812S and DAC2815S use pin 4 while the
DHC2803S and DHC2805S use pin 3.
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 pin 7 (-input).
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. C 7/04
MSK [ M.S. KENNEDY CORPORATION ]
