OKLP-X25-W12-C Datasheet, PDF (18/21 Pages) Murata Power Solutions Inc. – 25A Power Block Non-Isolated DC-DC Converter

In addition to the possibility of failed or delayed startup, increasing
external capacitance can cause sluggish transient response, possible
ringing and instability. The maximum output capacitive load specification is
really an indication of acceptable and stable startup performance but with
moderated transient response.
Designing for Transient Performance
When designing for a load transient, the output bulk capacitors and high
frequency bypass capacitors determine the response performance and
voltage deviation of the regulator. The most important parameters are the
magnitude of the load transient (ΔI) and the distributed bus impedance to
the load. The selection of the output capacitors is determined by the
allowable peak voltage deviation (ΔV). This limit should reflect the actual
requirements, and should not be specified lower than needed.
The distribution bus impedance seen by the load is the parameter that
determines the peak voltage deviation during a fast transient. The system
requires a low impedance bus over all frequencies with adequate bypass
capacitors to achieve fast slew rates. If the impedance of the network
that supplies the load remains below a maximum impedance, the voltage
deviation due to the transient will remain within allowable voltage deviation
requirements. It is simply Ohm’s Law: ΔV= ΔI x Z. Keep the magnitude of
Z below the maximum limit, and the transient voltage deviation will stay
within its limits.
Divide ΔV by the ΔI to determine the maximum allowable impedance,
Zmax. This is the impedance limit which must be maintained by the output
capacitor network for frequencies above which the regulator is effective. To
maintain low impedance from the regulator to the load, high frequency, low
value ceramic capacitors must be placed very close to the load to minimize
the effects of trace inductance while larger value ceramic capacitors can
be placed closer to the regulator.
Transient Design Example
Calculating the maximum allowable output impedance, given the following
requirements:
• VO = 2.5 V
• Output current step from 0.8 A to 12.5 A (ΔI = 11.7 A)
• Maximum allowable voltage deviation (ΔV) is 100 mV
• 20 A/μsec slew rate.
ΔV/ΔI = Maximum impedance
ΔV/ΔI = 100 mV/11.7 A = 8.55 mΩ
Selecting four 330 μF capacitors with an ESR of 25 mΩ would provide
an effective ESR of 6.25 mΩ and 1320 μF of total capacitance. Using these
capacitors, the actual amplitude of the transient deviation would be about
±73 mV (11.7 A × 6.25 mΩ). By maintaining the low impedance over the
complete frequency range, any high slew rate transient will be achieved.
Absolute Maximum Capacitor Limits
All regulators have an absolute maximum capacitance limit. MPS DC-DC
converter modules incorporate output short-circuit protection. During
startup, the regulator must charge the output capacitance in order to raise
the output voltage to its set-point and this current flow is in addition to any
load current that may be drawn by the application circuit. If there is too
much output capacitance, the current demanded from the regulator trips its
over-current protection circuit. Furthermore, each over-current trip will be
followed by further attempts by the regulator to restart. This can result in
the regulator entering a perpetual cycle of over-current shutdown.
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OKLP-X/25-W12-C
25A Power Block Non-Isolated DC-DC Converter
Data sheet tables give the maximum allowable output capacitance
for each module. If external capacitance is required for stable operation,
the minimum value will be listed in the datasheet. Recommended capaci-
tance is also listed in the datasheet for improved transient performance.
The recommended capacitance value will meet a typical ΔV spec at a 50%
transient load step.
Detailed analysis has been performed to allow capacitor limits to be
accurately defined. By following the capacitor recommendations in the data
sheet and selecting capacitors based on your actual operating conditions, a
reliable, low-cost power system can be designed.
Additional explanation can be found at the Murata Power Solutions web
page at www.murata-ps.com/data/apnotes/dcan-58.pdf, Application Note
D-CAN-58, Output Capacitive Load Considerations.
Power Block Performance
The OKLP-X/25 Power Block modules have been tested using controller
(PWM) IC’s under various transient load conditions. Contact Murata Power
Solutions for further details.
Power Block Pin Functional Descriptions
Pin #1 (Vin):
Input supply pin, with a range of 7 – 13.2Vin (see Table 1.1).
Pin #2 (Enable):
This provides the host or system controller to the option turn-on/off the
module. Alternatively, this input can also be tied to Vin or the +5V supply to
the gate driver, Pin 10 (up to 15V).
Pin #3 (+Cs), Positive DCR (current) sense and Pin #4 (-Cs), Negative DCR
(current) sense: For additional information, please reference the relevant
applications notes (www.zmdi.com/zspm1000) for the specific controller
(PWM) IC to be used. Contact Murata Power Solutions for further infor-
mation. A brief description of using current sense with the Power Block
follows.
Current Sensing
Many controller (PWM) IC’s implement average current sensing to provide
accurate current information over the switching period. A generated
schematic of the required current sensing circuitry is shown in figure 7
for the widely used DCR current-sensing method, which uses the parasitic
resistance of the inductor to acquire the current information. The principle
is based on a matched time-constant (i.e., RC=L/R (DCR) between the
inductor and the low-pass filter built from the 2.15KΩ resistor and an exter-
nal capacitor (not supplied with the OKLP-X/25 Power Block) across +Cs
and –Cs (220μf), where the inductor L and DCR (+5%) values are used.
Pin #5 (Vout): Output voltage supplied to the load. Note: There is minimal
input/output capacitance incorporated in the Power Block. Approximately
600μf is “typical” at 25A output current level, depending on the application.
Ref. to paragraph 2.4. Capacitor (OKLP-X25 datasheet). The user needs to
determine the appropriate amount of external capacitance for energy stor-
age, ripple voltage requirements, etc.
Pin #6 (Gnd)
Pin #7 (Temperature): Temperature Measurement
The OKLP-X/25 includes a Thermal Sensor in the module for temperature
sensing of the inductor. This element is used by the controller for tempera-
ture compensation, measuring the inductor temperature. This information
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