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OPA379 Datasheet, PDF (9/12 Pages) Burr-Brown (TI) – 1.8V, 2.9uA, 90kHz, Rail-to-Rail I/O OPERATIONAL AMPLIFIERS
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LOW-SIDE CURRENT MONITOR
The micropower OPA379 is well suited for current
monitoring circuits in applications such as a voltage
regulator with fold-back current limiting, or a high-current
power supply with crowbar protection. Figure 5 shows the
OPA379 monitoring the current in a power-supply return
path using a 0.1Ω shunt resistor. The NPN transistor, Q1
(2N2222 or equivalent) is used to generate equal voltages
at the inverting and noninverting inputs. Therefore, the
voltage drops across R1 and RS are equal, and the current
flowing through Q1 is directly proportional to the current
flowing through RS. As the load current increases, the
current through Q1 increases, the voltage drop across R2
increases, and this decreases the output voltage, VOUT, as
shown in Equation (5):
ǒ Ǔ VOUT + GND *
R2
R1
RS
IL
ǒ Ǔ + 0V *
2.49kW
100W
0.1W
IL
+ * 2.49W IL
(5)
OPA379
OPA2379
OPA4379
SBOS347A − NOVEMBER 2005 − REVISED DECEMBER 2005
WINDOW COMPARATOR
Figure 6 shows the OPA2379 used as a window
comparator. The threshold limits are set by VH and VL, with
VH > VL . When VIN < VH, the output of A1 will be low. When
VIN >VL, the output of A2 will be low. Therefore, both op
amp outputs will be at 0V as long as VIN is between VH and
VL. This results in no current flowing through either diode,
Q1 in cutoff, with the base voltage at 0V, and VOUT forced
high.
If VIN falls below VL, the output of A2 will be high, current
will flow through D2, and VOUT will be low. Likewise, if VIN
rises above VH, the output of A1 will be high, current will
flow through D1, and VOUT will be low.
The window comparator threshold voltages are set as
follows:
VH
+
R1
R2
)
R2
(6)
VL
+
R3
R4
)
R4
(7)
5V
R2
2.49kΩ
VOUT
Q1
R1
100Ω
RS
0.1Ω
5V
OPA379
Return to Ground
IL
Figure 5. Low-Side Current Monitor
3V
R1
R2
3V
VH
A1
1/2
OPA2379
D1(2)
RIN
2kΩ(1)
VIN
10kΩ
3V
R3
3V
A2
1/2
VL OPA2379
D2(2)
3V
5.1kΩ
VOUT
Q1(3)
5.1kΩ
R4
NOTES: (1) RIN protects A1 and A2 from possible excess current flow.
(2) IN4446 or equivalent diodes.
(3) 2N2222 or equivalent NPN transistor.
Figure 6. OPA2379 as a Window Comparator
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