TA0104A Datasheet, PDF (12/18 Pages) Tripath Technology Inc. – STEREO 500W (4Ω) CLASS-T DIGITAL AUDIO AMPLIFIER DRIVER USING DIGITAL POWER PROCESSING (DPPTM) TECHNOLOGY

Tripath Technology, Inc. - Technical Information
Setting Over-current Threshold
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 TA0104A 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.
The following table lists BVdss, Qg and R_DS(ON) for MOSFETs that Tripath has used with the
TA0104A:
Mfg. Part Number
ST STW34NB20
ST STW38NB20
ST STP19NB20
BVdss
200
200
200
Qg (Max)
(nanoCoulombs)
80
95
40
R_DS(ON) (Max)
(Ohms)
0.075
0.065
0.18
12
TA104A – Rev. 3.1/06.00