LM5010 datasheet(11/19 Pages) NSC

11 / 19 page

Start-up Regulator (V

CC)

(Continued)

Regulation Comparator

The feedback voltage at FB is compared to the voltage at the

Softstart pin (2.5V, ±2%). In normal operation (the output

voltage is regulated) an on-time period is initiated when the

voltage at FB falls below 2.5V. The buck switch stays on for

the on-time causing the FB voltage to rise above 2.5V. After

the on-time period the buck switch stays off until the FB

voltage falls below 2.5V. Bias current at the FB pin is less

than 5 nA over temperature.

Over-Voltage Comparator

The feedback voltage at FB is compared to an internal 2.9V

reference. If the voltage at FB rises above 2.9V the on-time

is immediately terminated. This condition can occur if the

input voltage, or the output load, change suddenly. The buck

switch will not turn on again until the voltage at FB falls below

2.5V.

ON-Time Control

The on-time of the internal switch (see Figure 5) is deter-

mined by the R

ON resistor and the input voltage (VIN), cal-

culated from the following:

(5)

The inverse relationship of t

ON vs. VIN results in a nearly

constant frequency as V

IN is varied. If the application re-

quires a high frequency the minimum value for t

ON, and

consequently R

ON, is limited by the off-time (265 ns, ±15%)

which limits the maximum duty cycle at minimum V

IN. The

tolerance for Equation 5 is ±25%. Frequencies in excess of

1 MHz are possible with the LM5010.

Shutdown

The LM5010 can be remotely shut down by taking the

ON/SD pin below 0.65V. See Figure 11. In this mode the

softstart pin is internally grounded, the on-timer is disabled,

and the input current at V

IN is reduced (Figure 7). Releasing

the R

ON/SD pin allows normal operation to resume. When

the switch is open, the nominal voltage at R

ON/SD is shown

in Figure 6.

20119916

FIGURE 10. Self Biased Configuration

20119918

FIGURE 11. Shutdown Implementation

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