Comparator directly controls power-MOSFET gate
Circuit switching point is independent of unregulated FET voltage.
Peter Demchenko, Vilnius, Lithuania; Edited by Paul Rako and Fran Granville -- EDN, November 3, 2011
It is common practice to power a MOSFET with a comparator and with an unregulated voltage and to power the comparator driving it from a regulated one (Figure 1). Many loads are insensitive to driving voltage, so it would be a waste of money and power to use a regulated supply to drive the FET. It is also common practice to add resistors R1 and R2 to the comparator to put hysteresis in the operation, making the circuit less susceptible to noise, especially with slowly changing signals.
This circuit’s comparator changes
with changes in the unregulated power
supply. You can correct this problem
by adding diode D2 and resistor R5 to
the circuit (Figure 2). This approach
isolates the hysteresis circuit from the
unregulated output and instead drives
it from the same regulated supply that
drives the comparator. When the comparator
is on, it drives the FET just as
the original circuit does, pulling the
P-channel FET gate toward ground. In
both cases, you connect zener diode D1
to the FET gate to avoid exceeding the
gate-to-source voltage. The improvements
in the circuit in Figure 2 become apparent when the comparator turns off.
In either case, R4 pulls the comparator’s
open-collector output up to the positive
power supply. In Figure 2, however, the
diode isolates the hysteresis circuit from
the power supply so that R4 pulls up R5
to the regulated 15V, no matter how the
power supply changes.
With a legacy comparator such
as Texas Instruments’ LM193, the
common mode of the inputs must
stay well below the power-supply rail
(Figure 3). The circuit requires 1.5V
head room at 25°C and 2V head room
over temperature. Thus, for the circuits
in figures 1 and 2, you cannot set the
threshold voltage higher than 13V. If
your circuit requires a threshold voltage
closer to the power rail, consider using
newer parts with rail-to-rail inputs. You
must use an open-collector or open-drain
comparator for this hysteresis-isolation
circuit to work. It would be
incompatible with a totem-pole-output
IC.
Talkback
-
Ed,
Good catch, my appology to author.
Vladimir Doubovis - 2011-14-11 10:26:12 PST -
I have to disagree with Vladimir on point 2. When the comparator is off D2 will be reverse biased which will prevent the 15 Volt supply from pulling the gate of the MOSFET below the unregulated supply.
Ed Wozniak - 2011-14-11 04:46:52 PST -
1. The circuit Fig.1 is OK because the collector of output transistor of LM193 is isolated from the voltage regulator LM7815. So if it is OFF, the gate of MOSFET will be through R4+R3 connected to 20V rail.
2. But for circuitry on Fig.2 the voltage regulator is a matter. By adding D2, the author creates the path to 15V power supply output, so some portion of voltage difference between Vin-15V will be applied to MOSFET gate and in case low threshold MOSFET it could continue to conduct.
3. So the author’s proposal does not give an improvement and could make it worse.
Vladimir Doubovis - 2011-11-11 12:06:05 PST
