SM72485 Datasheet, PDF (8/16 Pages) National Semiconductor (TI) – SolarMagic 100V, 150 mA Constant On-Time Buck Switching Regulator

FIGURE 2. Startup Sequence
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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 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 SM72485 is determined by the R_T resistor,
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 SM72485 is:
T_ON = 1.385 x 10^-10 x R_T / V_IN
(2)
R_T should be selected for a minimum on-time (at maximum
V_IN) greater than 400 ns, for proper current limit operation.
This requirement limits the maximum frequency for each ap-
plication, depending on V_IN and V_OUT.
The SM72485 can be remotely disabled by taking the R_T/SD
pin to ground. See Figure 3. The voltage at the R_T/SD pin is
between 1.5 and 3.0 volts, depending on Vin and the value of
the R_T resistor.
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