OP284_15 Datasheet, PDF (18/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

OP184/OP284/OP484
HIGH-SIDE CURRENT MONITOR
In the design of power supply control circuits, a great deal of design
effort is focused on ensuring the long-term reliability of a pass
transistor over a wide range of load current conditions. As a result,
monitoring and limiting device power dissipation is of prime
importance in these designs. The circuit shown in Figure 55 is
an example of a 3 V, single-supply, high-side current monitor that
can be incorporated into the design of a voltage regulator with
fold-back current limiting or a high current power supply with
crowbar protection. This design uses an OP284 rail-to-rail input
voltage range to sense the voltage drop across a 0.1 Ω current shunt.
A P-channel MOSFET, used as the feedback element in the circuit,
converts the differential input voltage of the op amp into a current.
This current is applied to R2 to generate a voltage that is a linear
representation of the load current. The transfer equation for the
current monitor is given by
Monitor Output =
R2 ×
 ^RSENSE
 R1


×
I
L
For the element values shown, the transfer characteristic of the
monitor output is 2.5 V/A.
R_SENSE
0.1Ω
3V
I_L
3V
R1
100Ω
3V
0.1µF
3
8
1/2
OP284
1
2
4
MONITOR
OUTPUT
M1 ^S
G
SI9433
D
R2
2.49kΩ
Figure 55. High-Side Load Current Monitor
CAPACITIVE LOAD DRIVE CAPABILITY
The OP284 exhibits excellent capacitive load driving capabilities.
It can drive up to 1 nF, as shown in Figure 30. Even though the
device is stable, a capacitive load does not come without penalty in
bandwidth. The bandwidth is reduced to less than 1 MHz for loads
greater than 2 nF. A snubber network on the output does not
increase the bandwidth, but it does significantly reduce the amount
of overshoot for a given capacitive load.
A snubber consists of a series R-C network (RS, CS), as shown in
Figure 56, connected from the output of the device to ground.
This network operates in parallel with the load capacitor, CL, to
provide the necessary phase lag compensation. The value of the
resistor and capacitor is best determined empirically.
5V
0.1µF
V_IN
100mV p-p
1/2
OP284
R_S
50Ω
C_S
100nF
V_OUT
C_L
1nF
Figure 56. Snubber Network Compensates for Capacitive Load
The first step is to determine the value of Resistor RS. A good
starting value is 100 Ω (typically, the optimum value is less than
100 Ω). This value is reduced until the small-signal transient
response is optimized. Next, CS is determined; 10 μF is a good
starting point. This value is reduced to the smallest value for
acceptable performance (typically, 1 μF). For the case of a 10 nF
load capacitor on the OP284, the optimal snubber network is
a 20 Ω in series with 1 μF. The benefit is immediately apparent,
as shown in the scope photo in Figure 57. The top trace was taken
with a 1 nF load, and the bottom trace was taken with the 50 Ω,
100 nF snubber network in place. The amount of overshoot and
ringing is dramatically reduced. Table 7 shows a few sample
snubber networks for large load capacitors.
100
90
1nF LOAD
ONLY
DLY
5.49µs
SNUBBER
IN ¹⁰
CIRCUIT _0%
50mV 50mV ^BW
2µs
Figure 57. Overshoot and Ringing Are Reduced by Adding a Snubber
Network in Parallel with the 1 nF Load
Table 7. Snubber Networks for Large Capacitive Loads
Load Capacitance (CL)
Snubber Network (RS, CS)
1 nF
50 Ω, 100 nF
10 nF
20 Ω, 1 µF
100 nF
5 Ω, 10 µF
Rev. J | Page 18 of 24