Dynamic control of the remote on/off function is best accomplished with
a mechanical relay or an open-collector/open-drain drive circuit (optically
isolated if appropriate). The drive circuit should be able to sink appropriate
current (see Performance Specs) when activated and withstand appropriate
voltage when deactivated.
Applying an external voltage to pin 4 when no input power is applied to
the converter can cause permanent damage to the converter.
Start-Up Time
The VIN to VOUT start-up time is the interval of time where the input volt-
age crosses the turn-on threshold point, and the fully loaded output voltage
enters and remains within its specifi ed accuracy band. Actual measured
times will vary with input source impedance, external input/output capaci-
tance, and load. The UWR 26-40 Watt implements a soft start circuit that
limits the duty cycle of the PWM controller at power up, thereby limiting the
Input Inrush current.
The On/Off Control to V
OUT start-up time assumes the converter has its
nominal input voltage applied but is turned off via the On/Off Control pin.
The specifi cation defi nes the interval between the time at which the con-
verter is turned on and the fully loaded output voltage enters and remains
within its specifi ed accuracy band. Similar to the V
IN to VOUT start-up, the
On/Off Control to V
OUT start-up time is also governed by the internal soft
start circuitry and external load capacitance.
Input Overvoltage/Undervoltage Shutdown and Start-Up Threshold
Under normal start-up conditions, devices will not begin to regulate
until the ramping-up input voltage exceeds the Start-Up Threshold Voltage
(35V for "D48" models). Once operating, devices will not turn off until the
input voltage drops below the Undervoltage Shutdown limit (34V for "D48"
models). Subsequent re-start will not occur until the input is brought back
up to the Start-Up Threshold. This built in hysteresis prevents any unstable
on/off situations from occurring at a single input voltage.
Input voltages exceeding the input overvoltage shutdown specifi cation
listed in the Performance/Functional Specifi cations will cause the device
to shutdown. A built-in hysteresis of 0.6 to 1.6 Volts for all models will not
allow the converter to restart until the input voltage is suffi ciently reduced.
Current Limiting
When output power increases above the rated output current, (see Cur-
rent Limit in Performance/Functional Specifi cations) the DC/DC converter
will go into a current limiting mode. In this condition the output voltage
will decrease proportionately with increases in output current, thereby
maintaining a somewhat constant power dissipation. This is commonly
referred to as power limiting. Current limit inception is defi ned as the point
where the full-power output voltage falls below the specifi ed tolerance.
See Performance/Functional Specifi cations. If the load current being drawn
from the converter is signifi cant enough, the unit will go into a short circuit
condition. See "Short Circuit Condition."
Short Circuit Condition
When a converter is in current limit mode the output voltages will drop
as the output current demand increases. If the output voltage drops too
low, the magnetically coupled voltage used to develop primary side volt-
ages will also drop, thereby shutting down the PWM controller.
Following a time-out period the PWM will restart causing the output volt-
ages to begin ramping to their appropriate values. If the short-circuit condi-
tion persists, another shutdown cycle will be initiated. This on/off cycling
is referred to as "hiccup" mode. The hiccup cycling reduces the average
output current, thereby preventing internal temperatures from rising to
excessive levels. The UWR 26-40 Watt A-Series is capable of enduring an
indefi nite short circuit output condition.
Thermal Shutdown
These A-Series converters are equipped with Thermal Shutdown
Circuitry. If environmental conditions cause the internal temperature of
the DC/DC converter to rise above the designed operating temperature, a
precision temperature sensor will power down the unit. When the internal
temperature decreases below the threshold of the temperature sensor the
unit will self start. See Performance/Functional Specifi cations.
Output Overvoltage Protection
The output voltage is monitored for an overvoltage condition via
magnetic coupling to the primary side. If the output voltage rises to a fault
condition, which could be damaging to the load circuitry (see Performance
Specifi cations), the sensing circuitry will power down the PWM controller
causing the output voltage to decrease. Following a time-out period the
PWM will restart, causing the output voltage to ramp to its appropriate
value. If the fault condition persists, and the output voltages again climb
to excessive levels, the overvoltage circuitry will initiate another shutdown
cycle. This on/off cycling is referred to as "hiccup" mode.
4
2
1
ON/OFF
CONTROL
+VIN
–VIN
Figure 3. Driving the Negative Logic On/Off Control Pin
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UWR 26-40W Series
Single Output, High-Density, 3.3VOUT and 5 VOUT, 26-40 Watt, DC/DC Converters
MDC_UWR26-40W.D01 Page 5 of 10
