APPLICATION NOTES

POWER SUPPLIES

The output voltage of the DHD2805S may be adjusted from

90% to 110% of nominal value by the use of a 50K

Ω 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.

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 10K

Ω 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 DHD2805S 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.

Rev. C 3/05

4

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=

(±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

Load fault conditions include short-circuit and severe over-

load conditions. The DHD2805S converter responds to load

faults by turning off all power conversion circuits for 250mS

and then attempting to restart for 10mS (typical). The net "on"

duty factor during a fault is very low resulting in low converter

dissipation and immunity from overheating at 125°C. Current

beyond rated can flow into the load at startup time. This al-

lows the converter to bring up capacitive and other difficult

load types more reliably than competing converters.