Start Up Considerations

When power is first applied to the DC/DC converter, operation is different

than when the converter is running and stabilized. There is some risk of start

up difficulties if you do not observe several application features. Lower input

voltage converters may have more problems here since they tend to have

higher input currents. Operation is most critical with any combination of the

following external factors:

1 - Low initial input line voltage and/or poor regulation of the input source.

2 – Full output load current on lower output voltage converters.

3 – Slow slew rate of input voltage.

4 – Longer distance to input voltage source and/or higher external input

source impedance.

5 - Limited or insufficient ground plane. External wiring that is too small.

6 – Too small external input capacitance. Too high ESR.

7 – High output capacitance causing a start up charge overcurrent surge.

8 – Output loads with excessive inductive reactance or constant current

characteristics.

If the input voltage is already at the low limit before power is applied, the start

up surge current may instantaneously reduce the voltage at the input termi-

nals to below the specified minimum voltage. Even if this voltage depression

is very brief, this may interfere with the on-board controller and possibly

cause a failed start. Or the converter may start but the input current load will

now drive the input voltage below its running low limit and the converter will

shut down.

If you measure the input voltage before start up with a Digital Voltmeter

(DVM), the voltage may appear to be adequate. Limited external capacitance

and/or too high a source impedance may cause a short downward spike at

power up, causing an instantaneous voltage drop. Use an oscilloscope not a

DVM to observe this spike. The converter’s soft-start controller is sensitive to

input voltage. What matters here is the actual voltage at the input terminals

at all times.

Symptoms of start-up difficulties may include failed started, output oscillation

or brief start up then overcurrent shutdown. Since the input voltage is never

absolutely constant, the converter may start up at some times and not at

others.

Solutions

To improve start up, review the conditions above. One of the better solutions

is to place a moderate size capacitor very close to the input terminals. You

may need two parallel capacitors. A larger electrolytic or tantalum cap sup-

plies the surge current and a smaller parallel low-ESR ceramic cap gives low

AC impedance. Too large an electrolytic capacitor may have higher internal

impedance (ESR) and/or lower the start up slew rate enough to upset the

DC/DC’s controller. Make sure the capacitors can tolerate reflected switching

current pulses from the converter.

The capacitors will not help if the input source has poor regulation. A

converter which starts successfully at 3.3 Volts will turn off if the input voltage

decays to below the input voltage theshold, regardless of external capaci-

tance.

Increase the input start up voltage if possible to raise the downward voltage

spike. Also, make sure that the input voltage ramps up in a reasonably short

time (less than a few milliseconds). If possible, move the input source closer

to the converter to reduce ohmic losses in the input wiring. Remember that

the input current is carried both by the wiring and the ground plane return.

Make sure the ground plane uses adequate thickness copper. Run additional

bus wire if necessary.

Any added output capacitor should use just enough capacitance (and no

more) to reduce output noise at the load and to avoid marginal threshold

noise problems with external logic. An output cap will also “decouple”

inductive reactance in the load. Certain kinds of electronic loads include

“constant current” characteristics which destabilize the output with insufficient

capacitance. If the wiring to the eventual load is long, consider placing this

decoupling cap at the load. Use the Remote Sense input to avoid ohmic

voltage drop errors.

An elegant solution to start up problems is to apply the input voltage with the

Remote On/Off control first in the off setting (for those converters with an

On/Off Control). After the specified start-up delay (usually under 20 mSec),

turn on the converter. The controller will have already been stabilized. The

short delay will not be noticed in most applications. Be aware of applications

which need “power management” (phased start up).

Finally, it is challenging to model some application circuits with absolute fidel-

ity. How low is the resistance of your ground plane? What is the inductance

(and distributed capacitance) of external wiring? Even a detailed mathemati-

cal model may not get all aspects of your circuit. Therefore it is difficult to

give cap values which serve all applications. Some experimentation may be

required.

LSM-10A D3 Models

10 Amp DC/DC’s in SMT Packages

www.murata-ps.com

MDC_LSM-10A D3.B01 Page 7 of 13