Datasheet

TurboTrans™ Technology
TurboTrans™ Selection
PTH08T255W
www.ti.com ........................................................................................................................................................................................... SLTS290 – NOVEMBER 2008
TurboTrans technology is a feature introduced in the T2 generation of the PTH/PTV family of power modules.
TurboTrans optimizes the transient response of the regulator with added external capacitance using a single
external resistor. Benefits of this technology include reduced output capacitance, minimized output voltage
deviation following a load transient, and enhanced stability when using ultra-low ESR output capacitors. The
amount of output capacitance required to meet a target output voltage deviation is reduced with TurboTrans
activated. Likewise, for a given amount of output capacitance, with TurboTrans engaged, the amplitude of the
voltage deviation following a load transient is reduced. Applications requiring tight transient voltage tolerances
and minimized capacitor footprint area benefits greatly from this technology.
Utilizing TurboTrans requires connecting a resistor, R
TT
, between the +Sense pin (pin17) and the TurboTrans pin
(pin19). 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
PTH08T255W, the minimum required capacitance is 1000 µ F. When using TurboTrans, capacitors with a
capacitance × ESR product below 10,000 µ F × m Ω are required. (Multiply the capacitance (in µ F) by the ESR (in
m Ω ) to determine the capacitance × ESR product.) See the Capacitor Selection section of the datasheet for a
variety of capacitors that meet this criteria.
Figure 7 and Figure 8 show the amount of output capacitance required to meet a desired transient voltage
deviation with and without TurboTrans for several capacitor types; TypeB (e.g. polymer-tantalum) and TypeC
(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 7 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%(10A), 50%(20A), and 75%(30A) 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, let's look at a 12-V application requiring a 80 mV deviation during an 20A load transient. A
majority of 560 µ F, 10m Ω ouput capacitors are used. Use the 12 V, Type C capacitor chart, Figure 8 . Dividing
80mV by 20A gives 4mV/A transient voltage deviation per amp of transient load step. Select 4mV/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 1700 µ F. The required R
TT
resistor value for 1700 µ F can then be
calculated or selected from Table 5 . The required R
TT
resistor is approximately 31.6k Ω .
To see the benefit of TurboTrans, follow the 4mV/A marking across to the 'Without TurboTrans' plot. Following
that point down shows that you would need a minimum of 5500 µ F of output capacitance to meet the same
transient deviation limit. This is the benefit of TurboTrans.
Copyright © 2008, Texas Instruments Incorporated 13
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