Split Supply Operation

The LM2611 may be operated with separate power and bias

supplies. In the circuit shown in Figure 12.V

supply that the regulated voltage is derived from, and V

a low current supply used to bias the LM2611. Conditions for

the supplies are:

2.7V

≤ V

DD

≤ 14V

0V

≤ V

IN

≤ (36-IV

As the input voltage increases, the maximum output current

capacbility increases. Using a separate, higher voltage sup-

ply for power conversion enables the LM2611 to provide

higher output currents than it would with a single supply that

is limited in voltage by V

Shutdown/Soft Start

A soft start circuit is used in switching power supplies to limit

the input inrush current upon start-up. Without a soft-start

circuit, the inrush current can be several times the steady-

state load current, and thus apply unnecessary stress to the

input source. The LM2611 does not have soft-start circuitry,

but implementing the circuit in Figure 13 will lower the peak

inrush current. The SHDN pin is coupled to the output

through C

run states while the output slowly decreases to its steady-

state value. The energy required to reach steady-state is

spread over a longer time and the input current spikes

decrease (see Figure 14 and Figure 15).

20018114

FIGURE 12. LM2611 Operating with Separate Power and Biasing Supplies

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