### DN429 - Tiny Amplifiers Drive Heavy Capacitive Loads at Speed

```Tiny Amplifiers Drive Heavy Capacitive Loads at Speed
Design Note 429
Keegan Leary and Brian Hamilton
Introduction
Parasitic capacitance lurks behind every corner of an
electronic circuit. FET gates, cabling, ground and power
planes all add to the Farad bottom line. When the capacitive load gets heavy in high speed circuits, careful op amp
selection is paramount for optimizing slew rate, current
output capability, power dissipation, and feedback loop
stability.
Demanding Circuit Requirements
For example, consider a 100MHz, 2VP-P sine wave signal
driving a 350pF capacitive load. The minimum required
slew rate without distortion for this scenario is:
SRMIN = 2πfVPK
SRMIN = 2π(100MHz)(1V)
V
≈ 630
µs
The slew rate sets the maximum output current—the
ampliﬁers are charging a capacitor, so the maximum
output current occurs at maximum slew.
I= C
dV
dt
⎛
V⎞
I = ( 350pF ) ⎜ 630 ⎟
⎝
µs ⎠
≈ 220mA
Maximum power dissipation is an important consideration. For an op amp operating from ±5V supplies, and
assuming the capacitive load starts at 0V and is charged
at maximum current, peak power is:
P = IV
P = (220mA)(5V)
≈ 1.1W
11/07/429
With a package that has a thermal resistance of 135°C/W,
this much continuous power would result in a 148°C rise
in die temperature. If the ambient temperature is 85°C,
this brings the die to a package-melting 233°C!
To isolate CLOAD from the ampliﬁer, a design could use
a series resistor, RS. This technique ultimately limits
bandwidth when the resistor or capacitive load gets very
large. The bandwidth reduction associated with this RC
time constant may limit performance. With a current feedback ampliﬁer, increasing the feedback resistor, RF, is an
alternative compensation method to reduce peaking.
Tiny Current Feedback Ampliﬁers
For the high speed, large capacitive load example above,
the 400MHz LT1395/LT1396/LT1397 family of current
feedback ampliﬁers certainly satisﬁes the slew rate requirement. The LT1395/LT1396/LT1397 can process large
signals with speed and 80mA minimum guaranteed output
current. However, for the example above, this ampliﬁer
family falls short of the 220mA requirement. In this case
one may not be enough, but four certainly are. Paralleling these ampliﬁers satisﬁes current requirements while
maintaining safe power dissipation and stability.
The LT1397 quad was designed to push big loads of
current while maintaining good thermal properties. The
copper underbelly of the tiny 4mm × 3mm DFN package
brings the thermal resistance down to 43°C/W, and a
die temperature rise above ambient of only 47°C for the
given example.
Component Selection and Testing
Without assembling the entire parallel conﬁguration, a
single-ampliﬁer test circuit can be constructed to check
results into the load capacitance divided by the number
of ampliﬁers to be used, CLOAD/4.
The remaining task is to select appropriate values of the
feedback resistor (RF) and series resistor (RS) to maximize
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
the –3dB bandwidth and sufﬁciently minimize the amount
of peaking in the frequency response. For both RF and
RS, smaller values result in both additional bandwidth
and increased peaking. RF has a practical lower limit of
about 255Ω. As load capacitance increases, RF and/or
RS values must increase to maintain stability.
Figure 2 shows measurement results using the 4-ampliﬁer circuit of Figure 1 with various RF/RS combinations
and 350pF of total load capacitance. Measurements were
performed at a gain of 1, so RG was not used.
The effectiveness of the 4-ampliﬁer circuit topology over
a single ampliﬁer can be seen in Figure 3. For a more
representative effect the load capacitance was tripled to
1000pF. The paralleled 4-ampliﬁer circuit is capable of
slewing 4V into 1000pF in under 10ns. This corresponds
to a slewing output current of 400mA. The single ampliﬁer current limits at about 140mA, reducing the slew rate
into this large capacitive load. The same 4V swing for the
single requires 28ns, almost three times longer than the
4-ampliﬁer conﬁguration.
Conclusion
Always consider using all of the ampliﬁers available in a tiny
power-enhanced package to provide the muscle needed
to rapidly slew heavy capacitive loads. Also consider
current feedback ampliﬁers such as the LT1397 to make
it easy to control a very wide bandwidth circuit.
5V
14
180
+
RS
170
1/4 LT1397
12
RS FOR 3dB PEAKING
160
–
RF
RG
5V
+
8
140
6
–3dB BANDWIDTH
130
4
120
RS
1/4 LT1397
110
–
–5V
10
150
RF
IN
OUT
RG
CBIG
RS (7)
BANDWIDTH
–5V
2
VS = ±5
100
225 305
355
0
455
405
505
555
605
RF
DN429 F02
5V
+
Figure 2. Selecting RF and RS to Drive 350pF When
Paralleling the Four Ampliﬁers of the LT1397
RS
1/4 LT1397
–
–5V
RF
RG
5V
+
RS
1/4 LT1397
DN429 F01
–
–5V
RF
RG
Figure 1. Using All Four Ampliﬁers of the LT1397
to Drive Large Capacitive Loads
Figure 3. Four Ampliﬁers Out-Race One Ampliﬁer When Driving
a 1000pF Capacitive Load. The Response Time of the Single
Ampliﬁer Lags the Quad by a Factor of Three.