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TA0105A View Datasheet(PDF) - Tripath Technology Inc.

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TA0105A Datasheet PDF : 29 Pages
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Tripath Technology, Inc. - Technical Information
experienced by the MOSFET in the final circuit. Thus, for TA0105A “typical” applications a mosfet with
500V rating is required.
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 12 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
TA0105A.
Part Number
STW20NM50FD
STW20NM50
STW18NB40
Manufacturer
ST Microelectronics
ST Microelectronics
ST Microelectronics
BVDSS (V)
500
500
400
ID (A)
20
20
18.4
Qg (nC)
38
40
60
RDS(on) ()
0.22
0.22
0.19
PD (W)
214
214
190
Package
TO247
TO247
TO247
GATE RESISTOR / GATE DIODE 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.
In addition, it is strongly recommended to use a schottky or ultra-fast PN junction diode in parallel with the
gate resistor as shown in the Application/Test Schematic. This diode serves to “speed up” the turn-off of
the output devices further reducing cross conduction and minimizing output stage idle current.
A typical gate resistor value for the mosfets recommended above is 33ohms. This resistor value
assumes the use of a 1A 40V(or greater) schottky diode such as an IRF 11DQ04 or General
Semiconductor SS16. Ultra fast recovery diodes will also work adequately for the gate diode, DG.
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.
BBM1
0
0
1
1
BBM0
0
1
0
1
Delay
145 ns
105 ns
65 ns
25 ns
Table 1: BBM Delay
The tradeoff involved in making this setting is that as the delay is reduced, distortion levels improve but
shoot-through and power dissipation increase. The actual amount of BBM required is dependent upon
components such as MOSFET type and gate resistor value as well as circuit board layout. The BBM
value selected should be verified in the actual application circuit board. It should also be verified under
maximum temperature and power conditions since shoot-through in the output MOSFETs can increase
under these conditions, possibly requiring a higher BBM setting than at room temperature.
23
TA0105A – RW/ Rev. 2.2/05.05
 

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