Tripath Technology, Inc. - Preliminary Information
The equation for calculating RVNNSENSE is as follows:
VNN
R
VNN1 =
I
VNNSENSE
Set
.
R
VNN2 = 3 × RVNN1
IVPPSENSE or IVNNSENSE can be any of the currents shown in the Electrical Characteristics table for
VPPSENSE and VNNSENSE, respectively.
The two resistors, RVPP2 and RVNN2 compensate for the internal bias points. Thus, RVPP1 and RVNN1 can be
used for the direct calculation of the actual VPP and VNN trip voltages without considering the effect of
R
VPP2 and RVNN2.
Using the resistor values from above, the actual minimum over voltage turn off points will be:
VPP MIN_OV_TUR N_OFF = RVPP1 × IVPPSENSE (MIN_OV_TU RN_OFF)
VNN MIN_OV_TUR N_OFF = −(RVNN1 × IVNNSENSE (MIN_OV_TU RN_OFF)
)
The other three trip points can be calculated using the same formula but inserting the appropriate
IVPPSENSE (or IVNNSENSE) current value. As stated earlier, the usable supply range is the difference between
the minimum overvoltage turn off and maximum under voltage turn-off for both the VPP and VNN
supplies.
VPP RANGE = VPP MIN_OV_TUR N_OFF - VPP MAX_UV_TUR N_OFF
VNN RANGE = VNN MIN_OV_TUR N_OFF - VNN MAX_UV_TUR N_OFF
Output Transistor Selection
The key parameters to consider when selecting what n-channel and p-channel MOSFETs to use with the
TDA1400 are drain-source breakdown voltage (BVdss), gate charge (Qg), and on-resistance (RDS(ON)).
The BVdss rating of the MOSFET needs to be selected to accommodate the voltage swing between
V
SPOS and VSNEG 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 VPP to VNN 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 RDS(ON) are desired for the best amplifier performance.
Unfortunately, these are conflicting requirements since RDS(ON) is inversely proportional to Qg for a typical
MOSFET. The design trade-off is one of cost versus performance. A lower RDS(ON) means lower I2RDS(ON)
losses but the associated higher Qg translates into higher switching losses (losses = Qg x 10 x 1.2MHz).
A lower RDS(ON) also means a larger silicon die and higher cost. A higher RDS(ON) means lower cost and
lower switching losses but higher I2RDSON losses.
Gate Resistor Selection
The gate resistors, RG, are used to control MOSFET switching rise/fall times and thereby minimize
voltage overshoots. They also dissipate a portion of the power resulting from moving the gate charge
each time the MOSFET is switched. If RG is too small, excessive heat can be generated in the driver.
Large gate resistors lead to slower MOSFET switching edges which require a larger break-before-make
(BBM) delay.
21
TDA1400 – Rev. 0.65/KLi/02.06
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
