OKI-T/36W-W40 Series
Selectable Output 36-Watt DOSA-SMT DC/DC Converters
TECHNICAL NOTES
Input Fusing
Certain applications and/or safety agencies may require fuses at the inputs of
power conversion components. Fuses should also be used when there is the
possibility of sustained input voltage reversal which is not current-limited. For
greatest safety, we recommend a fast blow fuse installed in the ungrounded
input supply line.
The installer must observe all relevant safety standards and regulations. For
safety agency approvals, install the converter in compliance with the end-user
safety standard.
Input Under-Voltage Shutdown and Start-Up Threshold
Under normal start-up conditions, converters will not begin to regulate properly
until the rising input voltage exceeds and remains at the Start-Up Threshold
Voltage (see Speci?cations). Once operating, converters will not turn off until
the input voltage drops below the Under-Voltage Shutdown Limit. Subsequent
restart will not occur until the input voltage rises again above the Start-Up
Threshold. This built-in hysteresis prevents any unstable on/off operation at a
single input voltage.
Users should be aware however of input sources near the Under-Voltage
Shutdown whose voltage decays as input current is consumed (such as
capacitor inputs), the converter shuts off and then restarts as the external
capacitor recharges. Such situations could oscillate. To prevent this, make
sure the operating input voltage is well above the UV Shutdown voltage AT ALL
be a ceramic type such as the Murata GRM32 series or a polymer type. Initial
suggested capacitor values are 10 to 22 μF, rated at twice the expected maxi-
mum input voltage. Make sure that the input terminals do not go below the
undervoltage shutdown voltage at all times. More input bulk capacitance may
be added in parallel (either electrolytic or tantalum) if needed.
Recommended Output Filtering
The converter will achieve its rated output ripple and noise with no additional
external capacitor. However, the user may install more external output capaci-
tance to reduce the ripple even further or for improved dynamic response.
Again, use low-ESR ceramic (Murata GRM32 series) or polymer capacitors.
Initial values of 10 to 47 μF may be tried, either single or multiple capacitors in
parallel. Mount these close to the converter. Measure the output ripple under
your load conditions.
Use only as much capacitance as required to achieve your ripple and noise
objectives. Excessive capacitance can make step load recovery sluggish or
possibly introduce instability. Do not exceed the maximum rated output capaci-
tance listed in the speci?cations.
Input Ripple Current and Output Noise
All models in this converter series are tested and speci?ed for input re?ected
ripple current and output noise using designated external input/output compo-
nents, circuits and layout as shown in the ?gures below. The Cbus and Lbus
components simulate a typical DC voltage bus. Please note that the values of
TO
TIMES.
Start-Up Time
Assuming that the output current is set at the rated maximum, the Vin to Vout
OSCILLOSCOPE
+
L BUS
CURRENT
PROBE
+VIN
Start-Up Time (see Speci?cations) is the time interval between the point when
the rising input voltage crosses the Start-Up Threshold and the fully loaded
V IN
+
C BUS
C IN
regulated output voltage enters and remains within its speci?ed accuracy band.
Actual measured times will vary with input source impedance, external input
capacitance, input voltage slew rate and ?nal value of the input voltage as it
appears at the converter.
These converters include a soft start circuit to moderate the duty cycle of its
PWM controller at power up, thereby limiting the input inrush current.
The On/Off Remote Control interval from inception to Vout regulated
assumes that the converter already has its input voltage stabilized above the
Start-Up Threshold before the On command. The interval is measured from the
On command until the output enters and remains within its speci?ed regulation
band. The speci?cation assumes that the output is fully loaded at maximum
rated current. Similar conditions apply to the On to Vout regulated speci?cation
such as external load capacitance and soft start circuitry.
Recommended Input Filtering
The user must assure that the input source has low AC impedance to provide
dynamic stability and that the input supply has little or no inductive content,
including long distributed wiring to a remote power supply. The converter will
operate with no additional external capacitance if these conditions are met.
-VIN
C IN = 2 x 100μF, ESR < 700mΩ @ 100kHz
C BUS = 1000μF, ESR < 100mΩ @ 100kHz
L BUS = 1μH
Figure 2. Measuring Input Ripple Current
Cin, Lbus and Cbus will vary according to the speci?c converter model.
Minimum Output Loading Requirements
All models regulate within speci?cation and are stable under no load to full
load conditions. Operation under no load might however slightly increase
output ripple and noise.
Thermal Shutdown
To prevent many over temperature problems and damage, these converters
include thermal shutdown circuitry. If environmental conditions cause the
temperature of the DC/DC’s to rise above the Operating Temperature Range
up to the shutdown temperature, an on-board electronic temperature sensor
For best performance, we recommend installing a low-ESR capacitor
immediately adjacent to the converter’s input terminals. The capacitor should
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MDC_OKI-T/36W-W40.A05 Page 6 of 11
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