TA0103A Datasheet, PDF (13/18 Pages) Tripath Technology Inc. – Stereo 250W (4Ω) Class-T Digital Audio Amplifier Driver using Digital Power Processing (DPPTM) Technology

Tripath Technology, Inc. - Technical Information
Generating the VN12 voltage per Figure 5 is NOT recommended. Most power supplies only sink
current from the negative terminal and will not be capable of sourcing the current required by
VN12. Furthermore, problems can arise since VN12 will not track movements in V_SNEG.
V_SPOS
V5
V_S
5V
P GND
A GND
-33V
V_S
VN12
V_SNEG
Setting Over-current Threshold
Figure 5
R_S and R_OCR determine the value of the over-current threshold, I_SC:
I_SC x R_S = (V_TOC x 9100)/(9100 + R_OCR)
where:
R_S and R_OCR are in Ω
I_SC = 3 x I_RMS = 3 x (P_OUT/R_L) ^0.5 (Over-current is typically set for 3 x RMS current)
V_TOC = Over-current sense threshold voltage (See Electrical Characteristics Table)
= 0.75V typically
when R_OCR = 0Ω, R_S = (0.75)/I_SC
Note that R_S will dissipate approximately (I_RMS)² x R_S of power. To set an I_SC of 30A, for example,
with R_OCR = 0Ω, means that R_S = 25mΩ and R_S must dissipate 2.5W on average. If R_OCR = 9.1KΩ,
then to set I_SC = 30A, R_S will be 12.5mΩ and will only have to dissipate 1.13W on average.
As high-wattage resistors are usually only available in a few low-resistance values (10mΩ, 25mΩ
and 50mΩ), R_OCR can be used to adjust for a particular over-current threshold using one of these
values for R_S.
Output Transistor Selection
The key parameters to consider when selecting a MOSFET to use with the TA0103A are drain-
source breakdown voltage (BVdss), gate charge (Qg), and on-resistance (R_DS(ON)).
The BVdss rating of the MOSFET needs to be selected to accommodate the voltage swing between
V_SPOS and V_SNEG as well as any voltage peaks caused by voltage ringing due to switching transients.
With a ‘good’ circuit board layout, a BVdss that is 50% higher than the V_SPOS and V_SNEG voltage
swing is a reasonable starting point. The BVdss rating should be verified by measuring the actual
voltages experienced by the MOSFET in the final circuit.
Ideally a low Qg (total gate charge) and low R_DS(ON) are desired for the best amplifier performance.
Unfortunately, these are conflicting requirements since R_DS(ON) is inversely proportional to Qg for a
typical MOSFET. The design trade-off is one of cost versus performance. A lower R_DS(ON) means
lower I²R_DS(ON) losses but the associated higher Qg translates into higher switching losses (losses =
Qg x 12 x 1.2MHz). A lower R_DS(ON) also means a larger silicon die and higher cost. A higher R_DS(ON)
means lower cost and lower switching losses but higher I²R_DSON losses.
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TA0103 – Rev 3.3/06.00