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WEBENCH Schematic Editor
Hands On Problems
Configuring Input and Load Transient Simulations
1
Hands-on Exercise: Perform Load
Transient Simulation In Editor/Simulator
Design Problem:
Goals:
Create the following design:
Device: LM2735X
Vin: 4.5-5.5V
Vout: 12V
Iout: 0.5A
Edit the design using the WEBENCH
Editor/Simulator and create a load
transient simulation using the
following load parameters:
Initial current (I1): 0.25A
Pulsed current (I2): 0.5A
Initial delay time (Td): 3.1msec
Rise time (Tr): 1usec
Fall time (Tf): 1usec
Pulse width (Pw): 0.5msec
Pulse period (Per): 1msec
Utilize WEBENCH Power Designer to
create a boost design
Understand how to configure a load
transient simulation in the WEBENCH
Editor/Simulator
Extra credit: Add a 2nd load source
and view the simulation results.
2
Why does the editor use startup as
default?
• Edited schematics always begin with a generic startup simulation
– This is to enable the most versatile schematic with no initial conditions
– In the normal WEBENCH Simulator, non startup simulations utilize initial
conditions on Vout and other nodes to speed up stabilization time. But this
is not possible in the editor since we do not know what the schematic
topology will be after editing.
• To do a transient simulation, add a pulsed (or other transient) source to
the desired node
• Note that sometimes a simulation model may not start up normally with
a fixed current source
– May need to add a small resistor in parallel
• Can use voltage sources in series or current sources in parallel
– Current sources in series may generate convergence errors
3
How to proceed
• Create the LM2735 design and click the Edit button in the top
navigation bar to edit the schematic.
• Once in the editor, click on the Simulation button to run a simulation in
the editor/simulator to see the behavior with a resistive load (startup).
• Note the time it takes to start up.
Original Schematic with Resistive Load
Time to start up
5
Next steps
• Click the Edit button to begin the editing operations.
– Delete the resistive load
– Click on the Place Part menu and select Pulse Current
Source to add it to the schematic
– Rotate the new source so the polarity is correct (+ up)
• Go to the simulator page and enter
the transient parameters including
a 3.1msec delay to allow the
system to start up
• Run a simulation to see the
behavior with a pulsed current
source (load transient).
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Initial current (I1): 0.25A
Pulsed current (I2): 0.5A
Initial delay time (Td): 3.1msec
Rise time (Tr): 1usec
Fall time (Tf): 1usec
Pulse width (Pw): 0.5msec
Pulse period (Per): 1msec
Edited Load Transient Schematic
Delay time to
allow startup
before pulse
7
Extra Credit: Edited Schematic With 2
Loads In Parallel
Added 0V voltage
source to probe
total load current
2 load sources in
parallel
8
Next steps
• Click the Edit button to begin the editing operations.
– Click on the Place Part menu and select Pulse Current
Source to add it to the schematic along with the
connecting wires
• Go to the simulator page and enter
the transient parameters for the
2nd source including a 3.2msec
delay to allow the system to start
up
• Also update the parameters for the
New Pulsed Current Source:
original source
Initial current (I1): 0.1A
•
Original Pulsed Current Source:
Initial current (I1): 0.25A
Pulsed current (I2): 0.05A
Pulsed current (I2): 0.5A
Run a simulation to see the Initial delay time (Td): 3.2msec Initial delay time (Td): 3.1msec
Rise time (Tr): 1usec
behavior with a pulsed current Rise time (Tr): 1usec
Fall time (Tf): 1usec
Fall time (Tf): 1usec
source (load transient).
Pulse width (Pw): 250usec
Pulse width (Pw): 750usec
Pulse period (Per): 500usec
Pulse period (Per): 1500usec
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Waveforms with 2 Loads In Parallel
10
Hands-on Exercise: Perform Input
Transient Simulation In Editor/Simulator
Design Problem:
Goals:
Create the following design:
Device: LM25011
Vin: 14-22V
Vout: 3.3V
Iout: 2A
Edit the design using the WEBENCH
Editor/Simulator and create an input
transient simulation using the going
from 18V down to 14V:
Initial voltage (V1): 0V
Pulsed voltage (V2): -4V
Initial delay time (Td): 3.1msec
Rise time (Tr): 1usec
Fall time (Tf): 1usec
Pulse width (Pw): 0.5msec
Pulse period (Per): 1msec
Understand how to configure an input
transient simulation in the WEBENCH
Editor/Simulator
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Configuring Input Transient Simulations
• Input transients require that the startup input voltage source be
supplemented by a pulsed voltage source.
• This pulsed source needs to be configured in series with the startup
source and should have a delay time so the pulse does not occur until
after the part has been started up.
12
How to proceed
• Create the LM25011 design and click the Edit button in the top
navigation bar to edit the schematic.
• Once in the editor, click on the Simulation button to run a simulation in
the editor/simulator to see the behavior with a startup voltage source.
• Note the time it takes to start up.
• Add the additional pulsed voltage source in series with the existing
startup source.
• Go to the simulator page
– Double click on the new source and configure the simulation parameters
(see next pages)
– Run a new simulation and note the behavior of the input and output
voltages.
Initial configuration uses a startup voltage
source on the input
Time to start up
Original startup source
parameters:
Initial voltage (V1): 0V
Pulsed voltage (V2): 18V
Initial delay time (Td): 0.1msec
Rise time (Tr): 10usec
14
Add Additional Pulsed Voltage Source
Add
pulse
voltage
source
New pulsed voltage source parameters (in
series with existing startup source)
Initial voltage (V1): 0V
Pulsed voltage (V2): -4V
Initial delay time (Td): 2.7msec
Rise time (Tr): 50nsec
Fall time (Tf): 50nsec
Pulse width (Pw): 0.1msec
Pulse period (Per): 0.2msec
Pulsed Vin
after startup
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Simulation Results
Delay time to allow
startup before pulse
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