THS3215 Datasheet, PDF (62/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 2016
www.ti.com
The ferrite bead acts to break the feedback loop from the output stage load currents that re-enter the D2S and
midscale buffer stages. Operate the two positive supply pins and the two negative supply pins at the same
voltage. Using separate sources on the two pins risks forward-biasing the on-chip parallel diodes that connect
the two supply inputs together. +VCC1 (pin 13) and +VCC2 (pin 16) have two parallel diodes that are off if the
voltage at the two pins are equal. The same is true for –VCC1 (pin 8) and –VCC2 (pin 5).
The THS3215 provides considerable flexibility in the supply voltage settings. The overriding consideration is
always satisfying the required headroom to the supplies on all the I/O paths. The logic controls on PATHSEL (pin
4) and DISABLE (pin 10) are intended to operate ground referenced regardless of supplies used. The ground
connection on pin 7 is used to set the reference.
Power savings are certainly possible by operating with only the minimum required supplies for the intended
swings at each of the pins. For instance, consider an example design operating with a current-sinking DAC with
the input common-mode voltage at 3 V, with an output swing at the D2S output of ±1 V. Looking at just the D2S
under these conditions, the minimum positive supply is 3 V_CM + the maximum input headroom of 1.5 V to the
positive supply + the input signal swing of 0.25 V, resulting in a minimum 4.75-V supply for this operation. The
±1-V output at VO1 (pin 6) along with the D2S output headroom sets the minimum negative supply voltage. The
maximum 1.5-V headroom gives a possible minimum negative supply of –2.75 V. However, the minimum
operating total of 8 V increases the negative supply to –3.5 V.
If the ±1-V swing is then amplified by the OPS, the output swing and headroom requirements set the minimum
operating supply. For instance, if the OPS is operating at a gain of 2.5 V/V, the ±2.5-V output requires a
maximum headroom of 1.6 V to either supply. Achieving a 1.6-V headroom requires a minimum balanced supply
of ±4.1 V. However, the input stage overrides the positive side because the required minimum is 4.75 V, while
the negative increases to –4.1 V. This example of absolute minimum supplies saves power. Using a typical 35-
mA quiescent current for all stages, going to the minimum 8.5 V total across the device, uses 310 mW of
quiescent power versus the 420 mW if a simple ±6-V supply is applied. However, ac performance degrades with
the lower headroom. For more power-sensitive applications, consider adjusting the supplies to the minimum
required on each side.
11.1 Thermal Considerations
The internal power for the THS3215 is a combination of its quiescent power and load power. The quiescent
power is simply the total supply voltage times the supply current. This current is trimmed to reduce power
dissipation variation and minimize variations in the ac performance. At a ±7.5-V supply, the maximum supply
current of 36.5 mA dissipates 548 mW of quiescent power. The worst-case load power occurs if the output is at
½ the single-sided supply voltage driving a dc load. Placing a ±3.75-V dc output into 100 Ω adds another 37.5
mA × 3.75 V = 140 mW of internal power. This total of approximately 688 mW of internal dissipation requires the
thermal pad be connected to a good heat-spreading ground plane to hold the internal junction temperatures
below the rated maximum of 150°C.
The thermal impedance is approximately 45 °C/W with the thermal pad connected. For 688 mW of internal power
dissipation there is a 31°C (approximate) rise in the junction temperature from ambient. Designing for the
intended 85°C maximum ambient temperature results in a maximum junction temperature of 116°C.
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