Figure 3. Measuring Output Ripple/Noise (PARD)
C1
C1 = 1µF
C2 = 10µF
LOAD 2-3 INCHES (51-76mm) FROM MODULE
C2 R
LOAD
7
8
4
5
SCOPE
+VOUT
–VOUT
+SENSE
–SENSE
In critical applications, output ripple/noise (also referred to as periodic and
random deviations or PARD) can be reduced below specified limits using filter-
ing techniques, the simplest of which is the installation of additional external
output capacitors. Output capacitors function as true filter elements and
should be selected for bulk capacitance, low ESR, and appropriate frequency
response.
All external capacitors should have appropriate voltage ratings and be
located as close to the converter as possible. Temperature variations for all
relevant parameters should be taken into consideration. OS-CONTM organic
semiconductor capacitors (www.sanyo.com) can be especially effective for
further reduction of ripple/noise. The most effective combination of external I/O
capacitors will be a function of line voltage and source impedance, as well as
particular load and layout conditions.
Start-Up Threshold and Undervoltage Shutdown
Under normal start-up conditions, the UVQ Series will not begin to regulate
properly until the ramping input voltage exceeds the Start-Up Threshold.
Once operating, devices will turn off when the applied voltage drops below
the Undervoltage Shutdown point. Devices will remain off as long as the
undervoltage condition continues. Units will automatically re-start when the
applied voltage is brought back above the Start-Up Threshold. The hysteresis
built into this function avoids an indeterminate on/off condition at a single input
voltage. See Performance/Functional Specifications table for actual limits.
Pre-Bias Protection
For applications where a pre-bias potential can be present at the output of the
power module it is recommended that either blocking diodes are added in series
with the Vout power lines or, a preferred solution is to use an OR-ing FET control-
ler like the LM5050-1 High-Side & LM5051 Low-Side OR-ing FET Controller from
TI. Starting the module into a pre-bias condition can cause permanent damage
to the module.
Start-Up Time
The VIN to VOUT Start-Up Time is the interval between the point at which a
ramping input voltage crosses the Start-Up Threshold voltage and the point at
which the fully loaded output voltage enters and remains within its specified
±1% accuracy band. Actual measured times will vary with input source imped-
ance, external input capacitance, and the slew rate and final value of the input
voltage as it appears to the converter. The On/Off to VOUT start-up time assumes
that the converter is turned off via the Remote On/Off Control with the nominal
input voltage already applied.
On/Off Control
The primary-side, Remote On/Off Control function (pin 2) can be specified to
operate with either positive or negative logic. Positive-logic devices ("P"suffix)
are enabled when pin 2 is left open or is pulled high. Positive-logic devices are
disabled when pin 2 is pulled low. Negative-logic devices are off when pin 2 is
high/open and on when pin 2 is pulled low. See Figure 4.
Dynamic control of the remote on/off function is best accomplished with a
mechanical relay or an open-collector/open-drain drive circuit (optically iso-
lated if appropriate). The drive circuit should be able to sink appropriate current
(see Performance Specifications) when activated and withstand appropriate
voltage when deactivated.
Current Limiting (Power limit with current mode control)
As power demand increases on the output and enters the specified “limit
inception range” (current in voltage mode and power in current mode) limiting
circuitry activates in the DC-DC converter to limit/restrict the maximum current
or total power available. In voltage mode, current limit can have a “constant or
foldback” characteristic. In current mode, once the current reaches a certain
range the output voltage will start to decrease while the output current con-
tinues to increase, thereby maintaining constant power, until a maximum peak
current is reached and the converter enters a “hic-up” (on off cycling) mode of
operation until the load is reduced below the threshold level, whereupon it will
return to a normal mode of operation. Current limit inception is defined as the
point where the output voltage has decreased by a pre-specified percentage
(usually a 2% decrease from nominal).
Short Circuit Condition (Current mode control)
The short circuit condition is an extension of the “Current Limiting” condition.
When the monitored peak current signal reaches a certain range, the PWM
controller’s outputs are shut off thereby turning the converter “off.” This is
followed by an extended time out period. This period can vary depending on
other conditions such as the input voltage level. Following this time out period,
the PWM controller will attempt to re-start the converter by initiating a “normal
start cycle” which includes softstart. If the “fault condition” persists, another
“hic-up” cycle is initiated. This “cycle” can and will continue indefinitely until
such time as the “fault condition” is removed, at which time the converter
will resume “normal operation.” Operating in the “hic-up” mode during a fault
condition is advantageous in that average input and output power levels are
held low preventing excessive internal increases in temperature.
2
3
1+Vcc
REF
+ VIN EQUIVALENT CIRCUIT FOR
POSITIVE AND NEGATIVE
LOGIC MODELS
CONTROL
–VIN
O N /O F F
C O N TR O L
COMMON
Figure 4. Driving the Remote On/Off Control Pin
MDC_UVQ Models.F07 Page 20 of 25
UVQ Series
Low Profile, Isolated Quarter Brick
2.5–40 Amp DC-DC Converters
www.murata-ps.com/support