9.1.1.4 TurboTrans™ Selection

Utilizing TurboTrans requires connecting a resistor, R_TT, between the +Sense pin (pin 17) and the TurboTrans pin (pin 19). The value of the resistor directly corresponds to the amount of output capacitance required. All T2 products require a minimum value of output capacitance whether or not TurboTrans is utilized. For the PTH08T250W, the minimum required capacitance is 1000 μF. When using TurboTrans, capacitors with a capacitance × ESR product below 10,000 μF×mΩ are required. (Multiply the capacitance (in μF) by the ESR (in mΩ) to determine the capacitance × ESR product.) See the Capacitor Selection section of the datasheet for a variety of capacitors that meet this criteria.

Figure 15 thru Figure 18 show the amount of output capacitance required to meet a desired transient voltage deviation with and without TurboTrans for several capacitor types; Type B (e.g. polymer-tantalum) and Type C (e.g. OS-CON). To calculate the proper value of R_TT, first determine your required transient voltage deviation limits and magnitude of your transient load step. Next, determine what type of output capacitors to be used. (If more than one type of output capacitor is used, select the capacitor type that makes up the majority of your total output capacitance.) Knowing this information, use the chart in Figure 15 thru Figure 18 that corresponds to the capacitor type selected. To use the chart, begin by dividing the maximum voltage deviation limit (in mV) by the magnitude of your load step (in Amps). This gives a mV/A value. Find this value on the Y-axis of the appropriate chart. Read across the graph to the 'With TurboTrans' plot. From this point, read down to the X-axis which lists the minimum required capacitance, C_O, to meet that transient voltage deviation. The required R_TT resistor value can then be calculated using the equation or selected from the TurboTrans table. The TurboTrans tables include both the required output capacitance and the corresponding R_TT values to meet several values of transient voltage deviation for 25% (12.5 A), 50% (25 A), and 75% (37.5 A) output load steps.

The chart can also be used to determine the achievable transient voltage deviation for a given amount of output capacitance. Selecting the amount of output capacitance along the X-axis, reading up to the 'With TurboTrans' curve, and then over to the Y-axis, gives the transient voltage deviation limit for that value of output capacitance. The required R_TT resistor value can be calculated using the equation or selected from the TurboTrans table.

As an example, take a look at a 12-V application requiring a 60 mV deviation during an 15 A load transient. A majority of 470 μF, 10 mΩ ouput capacitors are used. Use the 12 V, Type B capacitor chart, Figure 15. Dividing 60 mV by 15 A gives 4 mV/A transient voltage deviation per amp of transient load step. Select 4 mV/A on the Y-axis and read across to the 'With TurboTrans' plot. Following this point down to the X-axis gives us a minimum required output capacitance of approximately 1500 μF. The required R_TT resistor value for 1500 μF can then be calculated or selected from Table 5. The required R_TT resistor is approximately 17.4 kΩ.

To see the benefit of TurboTrans, follow the 4 mV/A marking across to the 'Without TurboTrans' plot. Following that point down shows that you would need a minimum of 7500 μF of output capacitance to meet the same transient deviation limit. This is the benefit of TurboTrans.