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Powering and Loading Considerations

3.2 Powering Up

It is suggested that the load power be kept low during the first power up. Set the current limit of the input

source to provide about 1.5 times the anticipated wattage of the load. Once the device is powered up,

immediately check for 3.3 volts at the output.

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.

3.3 Over Current Protection

The evaluation board is configured with cycle-by-cycle over-current protection. This function is completely

contained in the LM20333. The peak current is limited to approximately 5.2A.

Table 1. Connection Descriptions

Terminal Description

Silkscreen

VIN This terminal is the input voltage to the device. The evaluation board will operation over the input voltage

range of 4.5V to 25V. This voltage is limited due to the voltage rating of the input capacitors.

GND These terminals are the ground connections to the device. There are three different GND connections on the

PCB. J2 should be used for the input supply, J4 should be used for the load, and J8 should be used for low

power signal connections such as ENABLE.

VOUT This terminal connects to the output voltage of the power supply and should be connected to the load.

ENABLE This terminal connects to the enable pin of the device. This terminal can be left floating or driven externally.

If driven externally, a voltage typically less than 1.2V will disable the device. The operating voltage for this

pin should not exceed 5.5V. The absolute maximum voltage rating on this pin is 6V.

SS/TRK This terminal provides access to the SS/TRK pin of the device. Connections to this terminal are not needed

for most applications. The feedback pin of the device will track the voltage on the SS/TRK pin if it is driven

with an external voltage source that is below the 0.8V reference. The voltage on this pin should not exceed

5.5V during normal operation. The absolute maximum voltage rating on this pin is 6V.

PGOOD This terminal connects to the power good output of the device. There is an option to connect this pin to

VOUT or VCC through a 10 kΩ pull-up resistor. The voltage on this pin should not exceed 5.5V during

normal operation and has an absolute maximum voltage rating of 6V.

JP1 This terminal block selects the pull-up voltage for the PGOOD pin to be either VCC or VOUT. An external

voltage source for PGOOD may be supplied by connecting to the center terminal. The voltage applied to

PGOOD should not exceed 5.5V during normal operation and has an absolute maximum voltage rating of

6V.

SW This terminal allows easy probing of the switch node. Do not apply any external voltage source to this pin.

TP1 (VOUT) This is a oscilloscope probe connector point used to measure the output ripple and transient response. It is

design to work with most Tektronix oscilloscope probes and is the recommended measurement point for

evaluating AC performance.

SYNC This terminal connects to the SYNC pin of the device. If this pin is left open the switching frequency will

default to approximately 200kHz. The voltage on this pin should not exceed 5.5V during normal operation

and has an absolute maximum voltage rating of 6V.

SNVA325B–May 2008–Revised May 2013 AN-1791 LM20333 Evaluation Board

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