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 150K

Ω 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.

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 10K

Ω 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.

Rev. C 7/04

5

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=

(±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 >400K

Ω from pin 5 to 7 provides pull down for pin 5 if

there is no input setpoint programming resistor already in place.

210·10³

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.