TLE2024-Q1: Question about Output resister value

Hello Satoshi-san,

The TLE2024-Q1 does not have any internal resistors in the output stage. It is simply the Q39 NPN, Q40 PNP, emitter follower structure shown in the datasheet equivalent schematic on Page 3.

If you are concerned about the open-loop output impedance (Zo), it is typically low (ten's of ohms) and primarily resistive over the usable bandwidth for an op amp using an emitter follower output stage design. If you intend to drive an RC low pass filter with the TLE2024-Q1 output the resistive portion of the RC filter acts as an output isolation resistor (Riso) and the phase margin degradation will be considerably less than if the output had to drive the filter capacitor directly.

Regards, Thomas

Precision Amplifiers Applications Engineering

Thomas-san

Thank you for reply,
Customer is designing TLE2024-Q1 on voltage follower circuit.
Vcc is 0V to 5V single supply, customer accept low voltage saturation.

Measurement result is two pattern below for difference RC filter condition;
①R=2.4kΩ, C=47nF ⇒ Transient is clear(no problem and matched customer's calculation)
②R=68Ω, C=47nF ⇒ Transient is delay and differ to customer's calculation
About ②, output side is connecting MCU with internal 50kΩ pulldown.
Should ② set high value resister regardless of output side pulldown?

Additional question, please let me know about test condition of Rs=50Ω, datasheet Page-5.
Where Rs connecting on reference circuit?

Best regards,
Satoshi

Hi Satoshi,

Rs=50R is an input resistance. It's connected from the input to GND.

Kai

Satoshi-san,

For the condition: ②R=68Ω, C=47nF ⇒ Transient is delay and differ to customer's calculation

About ②, output side is connecting MCU with internal 50kΩ pulldown.

Should ② set high value resister regardless of output side pulldown?

During a fast transient event the op amp output will attempt to slew to the necessary output voltage level. While slewing the output stage must supply whatever current is necessary to charge that capacitance. A 47 nF capacitance is a huge capacitance load. If an operational amplifier such as the TLE2024-Q1 is expected to charge the capacitor the required output current will follow the i = C dv/dt relationship. Any resistance in series with the capacitor will limit how quickly, and how much current will be available, to charge the capacitor.

In conditions 1 and 2 there is a series output resistor, 2.4 kΩ and 68Ω, respectively. For condition 1 the resistance is high and the output current demanded from the TLE2024-Q1 will remain low - likely no more than a few milliamperes. The result is a long charge time for the load capacitor. When the resistor is changed to 68Ω in condition 2, the demanded output current would be much, much higher.

If you review the TLE2024-Q1 datasheet Figures 20 and 22 shown below, you can see its output current source/sink capability for a +5 V supply. Typically, it can only sink about 2.5 mA and source about -5 mA, before starting to saturate. It does not have a high output current drive capacity and therefore cannot charge a very high capacitance such as 47 nF quickly. Essentially, the output transistor's saturation resistance limits how quickly the capacitor can be charged. I suspect the current limiting is the reason the transient response it not as expected for the condition 2, 68Ω resistance.

Reducing the load capacitance by a significant amount should help. Otherwise, I suggest using a fast, high output current capable op amp instead of the TE2024-Q1.

Regards, Thomas

Precision Amplifiers Applications Engineering