Datasheet

OP184/OP284/OP484
Rev. J | Page 18 of 24
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 =
L
SENSE
I
R1
R
R2 ×






×
For the element values shown, the transfer characteristic of the
monitor output is 2.5 V/A.
00293-055
R
SENSE
0.1Ω
I
L
8
1
4
3
3V
3V
G
S
D
2
M1
SI9433
MONITOR
OUTPUT
3V
1/2
OP284
R1
100Ω
R2
2.49kΩ
0.1µF
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 (R
S
, C
S
), as shown in
Figure 56, connected from the output of the device to ground.
This network operates in parallel with the load capacitor, C
L
, to
provide the necessary phase lag compensation. The value of the
resistor and capacitor is best determined empirically.
00293-056
R
S
50Ω
0.1µF
C
L
1nF
C
S
100nF
5V
V
IN
100mV p-p
V
OUT
1/2
OP284
Figure 56. Snubber Network Compensates for Capacitive Load
The first step is to determine the value of Resistor R
S
. 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, C
S
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.
00293-057
2µs
100
90
10
0%
50mV
1nF LOAD
ONLY
SNUBBER
IN
CIRCUIT
DLY
5.49µs
50mV
B
W
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 (C
L
) Snubber Network (R
S
, C
S
)
1 nF 50 Ω, 100 nF
10 nF 20 Ω, 1 µF
100 nF 5 Ω, 10 µF