Regulation Comparator

The feedback voltage at FB is compared to an internal 2.5V

reference. In normal operation (the output voltage is regu-

lated), an on-time period is initiated when the voltage at FB

falls below 2.5V. The buck switch will stay on for the on-time,

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

period, the buck switch will stay off until the FB voltage again

falls below 2.5V. During start-up, the FB voltage will be below

2.5V at the end of each on-time, resulting in the minimum

off-time of 300 ns. Bias current at the FB pin is nominally 100

nA.

Over-Voltage Comparator

The feedback voltage at FB is compared to an internal

2.875V reference. If the voltage at FB rises above 2.875V

the on-time pulse 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 Generator and Shutdown

The on-time for the LM5008 is determined by the R

tor, and is inversely proportional to the input voltage (Vin),

resulting in a nearly constant frequency as Vin is varied over

its range. The on-time equation for the LM5008 is:

T

(2)

See Figure 3.R

(at maximum V

limit operation. This requirement limits the maximum fre-

quency for each application, depending on V

See Figure 4.

The LM5008 can be remotely disabled by taking the R

pin to ground. See Figure 11. The voltage at the R

is between 1.5 and 3.0 volts, depending on Vin and the value

of the R

20097906

FIGURE 10. Self Biased Configuration

20097907

FIGURE 11. Shutdown Implementation

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