Dynamic control of the On/Off function should be able to sink appropriate

signal current when brought low and withstand appropriate voltage when

brought high. Be aware too that there is a finite time in milliseconds (see

Specifications) between the time of On/Off Control activation and stable,

regulated output. This time will vary slightly with output load type and current

and input conditions.

Output Capacitive Load

These converters do not require external capacitance added to achieve rated

specifications. Users should only consider adding capacitance to reduce

switching noise and/or to handle spike current load steps. Install only enough

capacitance to achieve noise objectives. Excess external capacitance may

cause regulation problems, degraded transient response and possible oscilla-

tion or instability.

Remote Sense Input

The Sense input is normally connected at the load for the respective Sense

polarity (+Sense to the +Vout load). The sense input compensates for voltage

drops along the output wiring such as moderate IR drops and the current

carrying capacity of PC board etch. This output drop (the difference between

Sense and Vout when measured at the converter) should not exceed 0.5V. Use

heavier connections if this drop is excessive. The sense input also improves the

stability of the converter and load system by optimizing the control loop phase

margin.

If the Sense function is not used for remote regulation, the user should con-

nect the Sense to their respective Vout at the converter pins.

Sense lines on the PCB should run adjacent to DC signals, preferably

Ground. Any long, distributed wiring and/or significant inductance introduced

into the Sense control loop can adversely affect overall system stability. If in

doubt, test your applications by observing the converter’s output transient

response during step loads. There should not be any appreciable ringing or

oscillation.

Do not exceed maximum power ratings. Excessive voltage differences

between Vout and Sense together with trim adjustment of the output can cause

the overvoltage protection circuit to activate and shut down the output.

Power derating of the converter is based on the combination of maximum

output current and the highest output voltage at the ouput pins. Therefore the

designer must insure:

(Vout at pins) x (Iout) ≤ (Max. rated output power)

Sequence/Tracking Input (Optional)

After external input power is applied and the converter stabilizes, a high

impedance Sequence/Tracking input pin accepts an external analog volt-

age referred to -Vin. The output power voltage will then track this Sequence/

Tracking input at a one-to-one ratio up to the nominal set point voltage for that

converter. This Sequencing input may be ramped, delayed, stepped or other-

wise phased as needed for the output power, all fully controlled by the user’s

external circuits. As a direct input to the converter’s feedback loop, response to

the Sequence/Tracking input is very fast (milliseconds).

Operation

To use the Sequence/Tracking pin after power start-up stabilizes, apply a

rising external voltage to the Sequence/Tracking input. As the voltage rises, the

output voltage will track the Sequence/Tracking input (gain = 1). The output

voltage will stop rising when it reaches the normal set point for the converter.

The Sequence/Tracking input may optionally continue to rise without any effect

on the output. Keep the Sequence/Tracking input voltage below the converter’s

input supply voltage.

Use a similar strategy on power down. The output voltage will stay constant

until the Sequence/Tracking input falls below the set point.

Guidelines for Sequence/Tracking Applications

[1] Leave the converter’s On/Off Enable control in the On setting. Normally,

you should just leave the On/Off pin open.

[2] Allow the converter to stabilize (typically less than 20 mS after +Vin

power on) before raising the Sequence/Tracking input. Also, if you wish to have

a ramped power down, leave +Vin powered all during the down ramp. Do not

simply shut off power.

[3] If you do not plan to use the Sequence/Tracking pin, leave it open.

[4] Observe the Output slew rate relative to the Sequence/Tracking input.

A rough guide is 2 Volts per millisecond maximum slew rate. If you exceed

this slew rate on the Sequence/Tracking pin, the converter will simply ramp

up at it’s maximum output slew rate (and will not necessarily track the faster

Sequence/Tracking input).

The reason to carefully consider the slew rate limitation is in case you want

two different POL’s to precisely track each other.

[5] Be aware of the input characteristics of the Sequence/Tracking pin. The

high input impedance affects the time constant of any small external ramp

capacitor. And the bias current will slowly charge up any external caps over

time if they are not grounded.

[6] Allow the converter to eventually achieve its full rated setpoint output

voltage. Do not remain in ramp up/down mode indefinitely. The converter is

characterized and meets all its specifications only at the setpoint voltage (plus

or minus any trim voltage).

[7] The Sequence/Tracking is a sensitive input into the feedback control loop

of the converter. Avoid noise and long leads on this input. Keep all wiring very

short. Use shielding if necessary.

Pre-Biased Startup

Some sections have external power already partially applied (possibly because

of earlier power sequencing) before POL power up. Or leakage power is pres-

ent so that the DC/DC converter must power up into an existing output voltage.

This power may either be stored in an external bypass capacitor or supplied by

an active source. These converters include a pre-bias startup mode to prevent

initialization problems.

This “pre-biased” condition can also occur with some types of program-

mable logic or because of blocking diode leakage or small currents passed

through forward biased ESD diodes. This feature is variously called “mono-

tonic” because the voltage does not decay or produce a negative transient

once the input power is applied and startup begins.

Sequence/Tracking operation is not available during pre-bias startup.

Programmable DOSA-SMT 10/16-Amp DC/DC Converters

www.murata-ps.com

email: sales@murata-ps.com

22 Mar 2010

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