tda2500 Tripath Technology Inc., tda2500 Datasheet - Page 22

• Manufacturer Part Number: tda2500
• Description: Stereo Class-t Digital Audio Amplifier Driver Using Digital Power Processing Dpp
• Manufacturer: Tripath Technology Inc.

OU TPUT TRAN SISTOR SELECTION
The key parameters to consider when selecting what MOSFET to use with the TDA2500 are drain-source
breakdown voltage (BVdss), gate charge (Qg), and on-resistance (R
The BVdss rating of the MOSFET needs to be selected to accommodate the voltage swing between
V
a ‘good’ circuit board layout, a BVdss that is 20% higher than the VPP and VNN voltage swing is a
reasonable starting point.
experienced by the MOSFET in the final circuit. Thus, for TDA2500 “typical” applications a mosfet with
200V rating is required.
Ideally a low Qg (total gate charge) and low R
Unfortunately, these are conflicting requirements since R
MOSFET. The design trade-off is one of cost versus performance. A lower R
losses but the associated higher Qg translates into higher switching losses (losses = Qg x 12 x 700kHz).
A lower R
lower switching losses but higher I
The following table lists BVdss, Qg and R
TDA2500.
GA T E RESIST OR SEL EC T ION
The gate resistors, R
voltage overshoots. They also dissipate a portion of the power resulting from moving the gate charge
each time the MOSFET is switched. If R
Large gate resistors lead to slower MOSFET switching, which requires a larger break-before-make (BBM)
delay.
In addition, a Schottky or ultra-fast PN junction diode can be used in parallel with the gate resistor. The
anode of the diode is connected to the MOSFET gate. 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 3.3 – 5.6ohms. The value of the gate
resistor needs to be lowered as the gate charge of the output fets is increased so as to maintain a
reasonable idle current. As mentioned earlier, the use of gate diodes will further reduce the idle current
for a given value of gate
BR EAK-B EFOR E-MAK E (BB M) TIMING CON TROL
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.
22
STW20NM50FD
SPOS
Part Number
STW34NB20
STW50NB20
STW18NB40
and V
DS(ON)
SNEG
also means a larger silicon die and higher cost. A higher R
as well as any voltage peaks caused by voltage ringing due to switching transients. With
ST Microelectronics
ST Microelectronics
ST Microelectronics
ST Microelectronics
Manufacturer
G
, are used to control MOSFET switching rise/fall times and thereby minimize
The BVdss rating should be verified by measuring the actual voltages
2
R
DSON
BV
200
200
500
400
DSS
G
losses.
is too small, excessive heat can be generated in the driver.
(V)
DS(ON)
I
18.4
D
DS(ON)
34
50
20
(A)
for MOSFETs that Tripath has used with the
are desired for the best amplifier performance.
DS(ON)
T r i p a t h T e c h n o l o g y , I n c . - T e c h n i c a l I n f o r m a t i o n
Q
g
60
84
38
60
(nC)
is inversely proportional to Qg for a typical
R
DS(ON)
DS(on)
0.062
0.047
0.22
0.19
TDA2500 – KL/ Rev. 0.9/05.05
).
(:)
DS(ON)
DS(ON)
P
D
180
280
214
190
(W)
means lower cost and
means lower I
Package
TO247
TO247
TO247
TO247
2
R
DS(ON)