Datasheet

LMZ35003

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SNVS988 –JULY 2013

Switching Frequency

Nominal switching frequency of the LMZ35003 is set from the factory at 400 kHz. This switching frequency is

optimum for output voltages below 5.0 V. For output voltages 5.0V and above, better operating performance can

be obtained raising the operating frequency. This is easily done by adding a resistor, R

in , from the RT/CLK

pin (Pin 31) to the AGND pin (Pin 30). Raising the operating frequency reduces output voltage ripple, lowers the

load current threshold where pulse skipping begins, and improves transient response.

The recommended switching frequency for all output voltages is listed in Table 3.

For the maximum recommended output voltage value of 15 V, the switching frequency computes to 1000 kHz or

1 MHz. Operation above 1 MHz is not recommended. Use Table 5 below to select the value of the timing resistor

for the given values of switching frequencies.

Table 5. Standard Resistor Values to set the Switching Frequency

(kHz) 400 500 600 700 800 900 1000

(kΩ) OPEN 1100 549 365 267 215 178

It is also possible to synchronize the switching frequency to an external clock signal. See the Synchronization

(CLK) section for further details.

While it is possible to set the operating frequency higher than 400 kHz when using the device at output voltages

of 5 V or less, minimum duty cycle and pulse skipping issues restrict the maximum recommended input voltage

under these conditions. The recommended operating conditions for the LMZ35003 can be summarized by

Figure 27. The graph shows the maximum input voltage vs. output voltage restriction for several operating

frequencies. The lower boundary of the graph shows the minimum input voltage as a function of the output

voltage.

Figure 27. Optimum Operating Range with Switching Frequency

Product Folder Links: LMZ35003