LMZ34002_14 Datasheet, PDF (17/32 Pages) Texas Instruments – Using the LMZ34002: Negative Output Voltage Simple Switcher® Power Module Evaluation Module

LMZ34002
www.ti.com
SNVS989A – JULY 2013 – REVISED SEPT 2013
Power-Up Characteristics
When configured as shown in the application schematics, the LMZ34002 produces a regulated output voltage
following the application of a valid input voltage. During the power-up, internal soft-start circuitry slows the rate
that the output voltage rises, thereby limiting the amount of in-rush current that can be drawn from the input
source. The soft-start circuitry introduces a short time delay from the point that a valid input voltage is
recognized. Figure 37 shows the start-up waveforms for a LMZ34002, operating from a 12 V input and the output
voltage adjusted to –5 V. The waveform were measured with a 1.5-A constant current load.
Figure 37. Start-Up Sequence
Light-Load Behavior
The LMZ34002 is a non-synchronous converter. One of the characteristics of non-synchronous operation is that
as the output load current decreases, a point is reached where the energy delivered by a single switching pulse
is more than the load can absorb. This energy causes the output voltage to rise slightly. This rise in output
voltage is sensed by the feedback loop and the device responds by skipping one or more switching cycles until
the output voltages falls back to the set point. At very light loads or no load, many switching cycles are skipped.
The observed effect during this pulse skipping mode of operation is an increase in the peak to peak ripple
voltage, and a decrease in the ripple frequency. The amount of load current when pulse skipping begins is a
function of the input voltage, the output voltage, and the switching frequency.
No-Load Operation
When operating at no load or very light load and the input voltage is removed, the output voltage discharges very
slowly. If the input voltage is re-applied before the output voltage discharges, the slow-start circuit does not
activate and the amount of inrush current is extremely large and may cause an over-current condition. To avoid
this condition the output voltage must be allowed to discharge before re-applying the input voltage. Applying a
50-mA to 100-mA minimum load helps discharge the output voltage. Additionally, monitoring the input voltage
with a supervisor and shuting-down using the INH control (see Output On/Off Inhibit (INH)) activates the internal
slow-start circuit.
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