OKL2-T/20-W5 Series
Programmable Output 20-Amp iLGA SMT PoL
DC-DC Converter Series
TECHNICAL NOTES
Output Voltage Adustment
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 TIMES.
The output voltage may be adjusted over a limited range by connecting an
external trim resistor (Rtrim) between the Trim pin and Ground. The Rtrim
resistor must be a 1/10 Watt precision metal film type, 0.5% accuracy or
better with low temperature coefficient, 100 ppm/degC. or better. Mount
the resistor close to the converter with very short leads or use a surface
mount trim resistor.
Start-Up Time
Assuming that the output current is set at the rated maximum, the Vin to
Vout Start-Up Time (see Specifications) is the time interval between the
point when the ramping input voltage crosses the Start-Up Threshold and
the fully loaded regulated output voltage enters and remains within its
specified accuracy band. Actual measured times will vary with input source
impedance, external input capacitance, input voltage slew rate and final
value of the input voltage as it appears at the converter.
In the table below, the calculated resistance is given. Do not exceed the
specified limits of the output voltage or the converter’s maximum power
rating when applying these resistors. Also, avoid high noise at the Trim
input. However, to prevent instability, you should never connect any capaci-
tors to Trim.
OKL2-T/20-W5
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 On command to Vout regulated
Output Voltage
3.3 V
Calculated Rtrim (KΩ)
0.444
0.63
2.5 V
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
specified accuracy band. The specification assumes that the output is fully
loaded at maximum rated current. Similar conditions apply to the On to Vout
regulated specification such as external load capacitance and soft start
circuitry.
1.8 V
1.00
1.5 V
1.33
1.2 V
2.00
1.0 V
3.00
0.6 V
∞ (open)
Resistor Trim Equation, OKL2-T/20-W5 models:
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.
1.2
VOUT – 0.6
________
RTRIM (kΩ) =
Input Fusing
For best performance, we recommend installing a low-ESR capacitor
immediately adjacent to the converter’s input terminals. The capacitor
should be a ceramic type such as the Murata GRM32 series or a poly-
mer type. Initial suggested capacitor values are 44 μF, rated at twice the
expected maximum input voltage. Make sure that the input terminals do
not go below the under voltage shutdown voltage at all times. More input
bulk capacitance may be added in parallel (either electrolytic or tantalum)
if needed.
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.
Recommended Output Filtering
The converter will achieve its rated output ripple and noise with no
additional external capacitor. However, the user may install more exter-
nal output capacitance 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 94 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
capacitance listed in the specifications.
Input Under-Voltage Shutdown and Start-Up Threshold
Under normal start-up conditions, converters will not begin to regulate
properly until the ramping-up input voltage exceeds and remains at the
Start-Up Threshold Voltage (see Specifications). Once operating, convert-
ers 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
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MDC_OKL2-T/20-W5.B02 Page 13 of 17
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