SM72485 Datasheet, PDF (6/16 Pages) National Semiconductor (TI) – SolarMagic 100V, 150 mA Constant On-Time Buck Switching Regulator

Block Diagram
Functional Description
The SM72485 Step Down Switching Regulator features all
the functions needed to implement a low cost, efficient, Buck
bias power converter. This high voltage regulator contains a
100 V N-Channel Buck Switch, is easy to implement and is
provided in the MSOP-8 and the thermally enhanced LLP-8
package. The regulator is based on a control scheme using
an on-time inversely proportional to V_IN. The control scheme
requires no loop compensation. Current limit is implemented
with forced off-time, which is inversely proportional to V_OUT.
This scheme ensures short circuit control while providing min-
imum foldback.
The SM72485 can be applied in numerous applications to ef-
ficiently regulate down higher voltages. This regulator is well
suited for high voltage PV panel junction boxes, 48 Volt Tele-
com and the new 42V Automotive power bus ranges. Fea-
tures include: Thermal Shutdown, V_CC under-voltage lockout,
Gate drive under-voltage lockout, Max Duty Cycle limit timer,
intelligent current limit off timer, and a pre-charge switch.
Control Circuit Overview
The SM72485 is a Buck DC-DC regulator that uses a control
scheme in which the on-time varies inversely with line voltage
(V_IN). Control is based on a comparator and the on-time one-
shot, with the output voltage feedback (FB) compared to an
internal reference (2.5V). If the FB level is below the reference
the buck switch is turned on for a fixed time determined by the
line voltage and a programming resistor (R_T). Following the
ON period the switch will remain off for at least the minimum
off-timer period of 300ns. If FB is still below the reference at
that time the switch will turn on again for another on-time pe-
riod. This will continue until regulation is achieved.
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The SM72485 operates in discontinuous conduction mode at
light load currents, and continuous conduction mode at heavy
load current. In discontinuous conduction mode, current
through the output inductor starts at zero and ramps up to a
peak during the on-time, then ramps back to zero before the
end of the off-time. The next on-time period starts when the
voltage at FB falls below the internal reference - until then the
inductor current remains zero. In this mode the operating fre-
quency is lower than in continuous conduction mode, and
varies with load current. Therefore at light loads the conver-
sion efficiency is maintained, since the switching losses re-
duce with the reduction in load and frequency. The discon-
tinuous operating frequency can be calculated as follows:
where R_L = the load resistance
In continuous conduction mode, current flows continuously
through the inductor and never ramps down to zero. In this
mode the operating frequency is greater than the discontinu-
ous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode oper-
ating frequency can be calculated as follows:
(1)
The output voltage (V_OUT) is programmed by two external re-
sistors as shown in the Block Diagram. The regulation point
can be calculated as follows:
V_OUT = 2.5 x (R_FB1 + R_FB2) / R_FB1
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