500µs/div

Output Voltage

2V/div

PG

EN

Input Current

0.5A/div

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Output Current (A)

C001

LMZ20501

www.ti.com

SNVS874C – AUGUST 2012 – REVISED APRIL 2015

Feature Description (continued)

Figure 9. Typical Current Limit Profile

8.3.4 Start-Up

Start-up and shutdown of the LMZ20501 is controlled by the EN input. The characteristics of this input are found

in the Electrical Characteristics table. A valid input voltage must be present on VIN before the enable control is

float.

The LMZ20501 features a current limit based soft-start, that prevents large inrush currents and output overshoots

as the regulator is starting up. The peak inductor current is stepped-up in a staircase fashion during the soft start

period. A typical start-up event is shown in Figure 10:

8.3.5 Drop-Out Behavior

When the input voltage is close to the output voltage the regulator will operate at very large duty cycles. Normal

time delays of the internal circuits prevents the attainment of controlled duty cycles near 100%. In this condition

the LMZ20501 will skip switching cycles in order to maintain regulation with the highest possible input-to-output

ratio. Some increase in output voltage ripple may appear as the regulator skips cycles. As the input voltage gets

closer to the output voltage, the regulator will eventually reach 100% duty cycle, with the high side switch turned

on. The output will then follow the input voltage minus the drop across the high side switch and inductor

resistance. Figure 11 and Figure 12 show typical drop-out behavior for output voltages of 2.5 V and 3.3 V.

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