LM34923MMX National Semiconductor Corporation, LM34923MMX Datasheet - Page 12

• Manufacturer Part Number: LM34923MMX
• Description: 80v, 600 Ma Constant On-time Buck Switching Regulator
• Manufacturer: National Semiconductor Corporation

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Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V
reference. In normal operation (the output voltage is regulat-
ed), 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 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 260
ns.
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.85V
reference. If the voltage at FB rises above 2.85V 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.
FIGURE 2. Startup Sequence
12
On-Time Generator and Shutdown
The on-time for the LM34923 is determined by the R
and is inversely proportional to the input voltage (Vin), result-
ing in a nearly constant frequency as Vin is varied over its
range. The on-time equation for the LM34923 is:
R
V
This requirement limits the maximum frequency for each ap-
plication, depending on V
The LM34923 can be remotely disabled by taking the RT/SD
pin to ground. See
between 1.5 and 5.0 volts, depending on Vin and the value of
the R
IN
T
) greater than 200 ns, for proper current limit operation.
should be selected for a minimum on-time (at maximum
T
resistor.
Figure
IN
3. The voltage at the RT/SD pin is
and V
OUT
.
30141714
T
resistor,
(3)