Scope
Volt-meter
Volt-meter
Current-meter
80 Volt, 5 Amp
Power Supply
with Current
Meter
200 Watt, 60 Amp
Electronic Load
-
+
+
-
+
Evaluation Board
ON/OFF
(SHUTDOWN)
IN
OUT
Jumper
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Loading
6
Loading
An appropriate electronic load, with specified operation down to 3.0V minimum, is desirable. The
resistance of a maximum load is 0.11
Ω
. The high output current requires thick cables! If resistor banks are
used there are certain precautions to be taken. The wattage and current ratings must be adequate for a
30A, 100W supply. Monitor both current and voltage at all times. Ensure there is sufficient cooling
provided for the load.
7
Air Flow
Full power loading should never be attempted without providing the specified 200 CFM of air flow over the
evaluation board. A stand-alone fan should be provided.
8
Powering Up
Using the provided shutdown pin will allow powering up the source supply with the current level set low. It
is suggested that the load be kept low during the first power up. Set the current limit of the source supply
to provide about 1.5 times the wattage of the load. As you remove the connection from the shutdown pin
to ground, immediately check for 3.3 volts at the output.
A most common occurrence, that will prove unnerving, is when the current limit set on the source supply is
insufficient for the load. The result is similar to having the high source impedance referred to earlier. The
interaction of the source supply folding back and the evaluation board going into undervoltage shutdown
will start an oscillation, or chatter, that may have undesirable consequences.
A quick efficiency check is the best way to confirm that everything is operating properly. If something is
amiss you can be reasonably sure that it will affect the efficiency adversely. Few parameters can be
incorrect in a switching power supply without creating losses and potentially damaging heat.
9
Over Current Protection
The evaluation board is configured with hiccup over-current protection. In the event of an output overload
(approximately 33A) the unit will discharge the softstart capacitor, which disables the power stage. After a
delay the softstart is released. The shutdown, delay and slow recharge time of the softstart capacitor
protects the unit, especially during short circuit event where the stress is highest.
Figure 2. Typical Evaluation Setup
10
Performance Characteristics
10.1 Turn-On Waveforms
When applying power to the LM5026 evaluation board a certain sequence of events occurs. Soft-start
capacitor values and other components allow for a minimal output voltage for a short time until the
feedback loop can stabilize without overshoot.
Figure 3
shows the output voltage during a typical start-up
with a 48V input and a load of 5A. There is no overshoot during startup.
3
SNVA117A – September 2005 – Revised May 2013
AN-1387 LM5026 Evaluation Board
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