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# TLV711: what's the slew rate on output of LDO (TLV707 and TLV711)

Part Number: TLV711
Other Parts Discussed in Thread: TLV707,

Hi Team,

My customer want to know what's the slew rate of TLV707 and TLV711.

Is this able to calculate if the output cap is fixed?

How to adjust the slew rate?

Thanks

Shaq.

• Hi Shaq,

These devices have an internal current limit feature. Current limit specs are shown in the Electrical Characteristics and Typical Characteristics sections of the datasheet. During startup, the current that charges the output capacitor and supplies the load is limited to Icl:

Icl = Cout*dVout/dt + Vout/Rl

Solving this differential equation gives a function for Vout in terms of Icl, Cout, and Rl (load resistor):

Vout = Icl * Rl * {1 - exp[-t/(Cout*Rl)]}

To approximate the time required for the output to reach its final value, set Vout = Vout,nom and solve for t = tr:

tr = - Rl * Cout * ln[1 - Vout,nom/(Icl * Rl)]

As can be seen from this equation, increasing the output capacitor or the load current (decreasing Rl) will increase the rise time and therefore decrease the output slew rate. The output slew rate is calculated as:

Output Slew Rate = Vout,nom / tr

Thanks,

Gerard

• Hi Gerard

Due to our output equivalent resistance is < 1ohm,
tr = - Rl * Cout * ln[1 - Vout,nom/(Icl * Rl)]
It will made 1 - Vout,nom/(Icl * Rl) < 0

By the way, could we use the startup time to calculate the initial slew rate?
Vout,nom / tsart

Steven
• Hi Steven,

If Vout,nom exceeds Icl * Rl, your application is exceeding the maximum current of 200 mA for these devices. The current limit for an LDO will always be larger than the maximum output current:

Iout = Vout,nom / Rl < Icl

Rearranging this shows that the formula for tr is always valid:

Vout,nom / (Icl * Rl) < 1

Using the startup time is a valid way to calculate the average output slew rate. However, keep in mind that the startup time is measured from the moment the LDO is enabled. There is a time delay between the EN signal and the output voltage starting to rise. Because of this delay, the actual output voltage slew rate will be higher than Vout,nom / tstart.

Thanks,
Gerard