OKY
-T/10 & T/16-D12 Series
Programmable DOSA-SMT 10/16-Amp DC/DC Converters
Dynamic control of the On/Off function must 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 your noise and surge response objectives. Excess
external capacitance may cause regulation problems and possible oscillation
or instability. Proper wiring of the Sense inputs will improve these factors under
capacitive load.
The maximum rated output capacitance and ESR specification is given for a
capacitor installed immediately adjacent to the converter. Any extended output
wiring, smaller wire gauge or less ground plane may tolerate somewhat higher
capacitance. Also, capacitors with higher ESR may use a larger capacitance.
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.
www.murata-ps.com
20 Oct 2009
email: sales@murata-ps.com
MDC_OKY_T10T16.D12.A05_long
Page 8 of 17
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