MPDTH***10 Series
Adjustable Non-isolated 12/15-Amp
POLA DC/DC Converters
APPLICATION NOTES
[1] The pre-bias voltage must be lower than the expected final set-point
output voltage. If the pre-bias voltage remains higher than the set-point
and is capable of back-driving excess current, possible output damage may
occur. If the pre-bias voltage has high impedance or has very limited current
capability, normal startup will proceed.
Tracking Input
When enabled, the tracking function allows the output to follow an external
analog signal presented at the Tracking input. This pin becomes active
approximately 50 mS after the input voltage has been applied and the con-
verter reaches regulation. External track inputs must be referred to Ground.
Several converters may be Tracked in parallel to generate outputs which rise
and fall together.
[2] Tracking operation must not be used during pre-bias startup. The Track-
ing input should be either open or connected to Vin so that normal soft start
will proceed. After successful startup of approximately 50 mSec, Tracking
may be resumed.
The Tracking input range is zero Volts up to the nominal set-point output
voltage. If the Tracking input rises above the Vout nominal voltage, Vout will
remain at the set point after the power up ramp. If Track is unused, it should
be connected to +Vin.
[3] Carefully examine any output circuits with large external output capaci-
tors and low-ESR. If there is no bleeder circuit or other method to reduce the
stored voltage AND the cap voltage is higher than the expected set-point,
operation may fail or the output may be damaged. Smaller capacitors with
lower energy storage should not interfere with startup. See the recom-
mended Connection Diagram. Use only as much output capacitance as
needed.
Please note that because of the Under Voltage input lockout feature (UVLO),
the output cannot follow Track In during power up. Also, the Track feature is
disabled whenever the converter is in Inhibit (OFF) mode.
Over-Current Output Protection
Also be careful immediately turning the converter back on after the Inhibit
control turns the converter off. Any energy stored in larger output capacitors
may cause the problems described above.
If excess current is drawn from the converter, the output will shut down
and then periodically attempt to restart with narrow full power cycles. If the
excess load is reduced to within the rated maximum load, the converter
will immediately restart and operate normally. If the load is still too great,
another power-down cycle will begin. This is called “hiccup” operation. It
significantly reduces the output power to avoid damage. The converter will
quickly and automatically recover once the load is reduced.
Margin Up/Down Controls
Ground each Margin control to increase or decrease the output voltage by
5% from its setpoint value. If not needed, leave the margin controls uncon-
nected (open).
Pre-Bias Operation
The margin controls are analog inputs and are not logic compatible. If a
switch is not used to ground, a small signal transistor (such as an n-channel
MOSFET with low off-state leakage) will work if biased fully into either pinchoff/
cutoff or saturation. Sink current is approximately -8 mA when ON and the
open circuit voltage is approximately 0.8 Volts when OFF. Do not ground
both margin controls simultaneously.
The MPDTH_WAH/WAS series of converters use synchronous rectification in
their output circuits. Normally, synchronous rectification may cause excess
current being sunk back in to the converter if there is higher external output
voltage present. If such prebias voltage is present on the output at the time
of startup, the converter will operate in a manner which avoids excessive
reverse current being sunk back into the output. This pre-bias protection
will continue until the converter achieves normal regulation at its set-point
output voltage.
For output changes less than 5%, insert an additional resistor in series
with the switch or transistor to ground. The margin adjustment equation is:
R-up or R-down (in KΩ) = (499 / delta%) – 99.8
In order to achieve successful pre-bias voltage protection, several conditions
must be met:
Where “delta%” is the desired change in percent of Vout up or down relative
to the set-point output. Be sure to use low tempco precision resistors.
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03/26/09
Murata Power Solutions, Inc. makes no representation that the use of its products in the circuits described herein, or the use of other
technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained herein do not imply
the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without
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MPDTH_10 Series.A04 Page 6 of 6
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