THS4541-Q1 Datasheet, PDF (32/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
www.ti.com
8.7 Driving Capacitive Loads
A very common requirement is driving the capacitive load of an ADC or some other next stage device. Directly
driving a capacitive load with a closed-loop amplifier such as the THS4541-Q1 can lead to an unstable response,
as shown in the step response plots into a capacitive load (see Figure 8 and Figure 26). One typical remedy for
this instability is to add two small series resistors (Ro in Figure 71) at the outputs of the THS4541-Q1. Figure 6
and Figure 24 provide parametric plots of recommended Ro values versus differential capacitive load values and
gain.
50- Input Match,
Gain of 2 V/V from Rt,
Single-Ended Source to
Differential Output
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
402
50-
Source
Rg1
191
Rt
Vocm
60.2
Rg2
191
Vcc
±
+
FDA
±
+
PD
Vcc
Ro1
17
Ro2
17
Cload
22 pF
Rload
500
Output
Measurement
Point
Rf2
402
Figure 71. Including Ro when Driving Capacitive Loads
Operating at higher gains requires lower Ro values to achieve a ±0.5-dB flat response for the same capacitive
load. Some direct parasitic loading is acceptable with no series Ro that increases with gain setting, as illustrated
in Figure 6 and Figure 24 where the Ro value is 0 Ω. Even when these plots suggest no series Ro is required,
good practice is to include a place for the Ro elements in the board layout (0-Ω load initially) for later adjustment,
in case the response appears unacceptable. The TINA simulation model does a good job of predicting this effect
and showing the impact for different choices of capacitive load isolating resistors (Ro).
8.8 Thermal Analysis
The relatively low internal quiescent power dissipation for the THS4541-Q1, combined with the excellent thermal
impedance of the 16-pin VQFN (RGT) package, limits the possibility of excessively-high, internal-junction
temperatures.
To estimate the internal junction temperature (T_J), an estimate of the maximum internal power dissipation (P_D) is
first required. There are two pieces to the internal power dissipation: quiescent current power and the power
used in the output stage to deliver load current. To simplify the latter, the worst-case, output-stage power is
driving a DC differential voltage across a load using half the total supply voltage. As an example:
1. Assume a worst-case, 5% high 5-V supply. This 5.25-V supply with a maximum I_CC of 11 mA gives a
quiescent power term = 58 mW.
2. Assume a 100-Ω differential load with a static 2.5-V differential voltage established across it. This 25 mA of
DC load current generates a maximum output stage power of (5.25 V – 2.5 V) × 25 mA = 69 mW.
3. From this total worst-case internal P_D = 127 mW, multiply times the 52°C/W thermal impedance to get a 7°C
rise from ambient.
Even for this extreme condition and the maximum rated ambient temperature of 125°C, the junction temperature
is a maximum 132°C (less than the rated absolute maximum of 150°C). Follow this same calculation sequence
for the exact application and package selected to predict the maximum T_J.
32
Submit Documentation Feedback
Product Folder Links: THS4541-Q1
Copyright © 2015, Texas Instruments Incorporated