T E C H N I C A L I N F O R M A T I O N
One apparent method to generate the VN10 supply voltage is to use a negative IC regulator to drop
PGND down to 10V (relative to VNN). This method will not work since negative regulators only sink
current into the regulator output and will not be capable of sourcing the current required by VN10.
Furthermore, problems can arise since VN10 will not track movements in VNN.
Output Transistor Selection
The key parameters to consider when selecting what MOSFET to use with the TA3020 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 and 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.
The following table lists BVdss, Qg and RDS(ON) for MOSFETs that Tripath has used with the
TA3020:
Manufacturer
Manufacturer’s
Part Number
STW34NB20
STP19NB20
IRFB41N15D
IRFB31N20D
FQA34N20
BVdss
Qg
RDS(ON) (Max)
(Ohms)
0.075
(nanoCoulombs)
ST Microelectronics
ST Microelectronics
International Rectifier
International Rectifier
Fairchild
200
200
150
200
200
60
29
67
70
60
0.18
0.045
0.082
0.075
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, which requires a larger break-
before-make (BBM) delay.
Break-Before-Make (BBM) Timing Control
The half-bridge power MOSFETs require a deadtime between when one transistor is turned off and
the other is turned on (break-before-make) in order to minimize shoot through currents. BBM0 and
BBM1 are logic inputs (connected to logic high or pulled down to logic low) that control the break-
before-make timing of the output transistors according to the following table.
24
TA3020, Rev 2.1, 01.01
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
