TA0104A Tripath Technology Inc., TA0104A Datasheet - Page 12

• Manufacturer Part Number: TA0104A
• Manufacturer: Tripath Technology Inc.

Setting Over-current Threshold
R
V
when R
Note that R
R
set I
As high-wattage resistors are usually only available in a few low-resistance values (10m:, 25m: and
50m:), R
for R
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
The BVdss rating of the MOSFET needs to be selected to accommodate the voltage swing between
V
With a ‘good’ circuit board layout, a BVdss that is 50% higher than the V
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
Unfortunately, these are conflicting requirements since R
typical MOSFET. The design trade-off is one of cost versus performance. A lower R
lower I
Qg x 12 x 1.2MHz). A lower R
means lower cost and lower switching losses but higher I
The following table lists BVdss, Qg and R
TA0104A:
12
= 0.75V typically
Mfg. Part Number
TOC
SPOS
S
OCR
ST STW34NB20
ST STW38NB20
ST STP19NB20
and R
SC
S
= Over-current sense threshold voltage (See Electrical Characteristics Table)
= 0:, means that R
.
and V
2
= 30A, R
OCR
R
I
where:
R
I
SC
SC
OCR
DS(ON)
S
OCR
and R
x R
= 3 x I
S
= 0:, R
SNEG
determine the value of the over-current threshold, I
will dissipate approximately (I
can be used to adjust for a particular over-current threshold using one of these values
S
losses but the associated higher Qg translates into higher switching losses (losses =
S
= (V
as well as any voltage peaks caused by voltage ringing due to switching transients.
will be 12.5m: and will only have to dissipate 1.13W on average.
OCR
RMS
S
TOC
= (0.75)/I
are in :
= 3 x (P
x 9100)/(9100 + R
S
BVdss
200
200
200
= 25m: and R
OUT
SC
DS(ON)
/R
L
)
0.5
also means a larger silicon die and higher cost. A higher R
(nanoCoulombs)
(Over-current is typically set for 3 x RMS current)
DS(ON)
S
Qg (Max)
OCR
RMS
must dissipate 2.5W on average. If R
)
)
80
95
40
2
for MOSFETs that Tripath has used with the
x R
DS(ON)
S
of power. To set an I
are desired for the best amplifier performance.
2
R
DS(ON)
DSON
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
R
SC
DS(ON)
losses.
is inversely proportional to Qg for a
(Ohms)
:
0.075
0.065
0.18
DS(ON)
(Max)
SPOS
).
SC
of 30A, for example, with
and V
TA104A – Rev. 3.1/06.00
OCR
SNEG
= 9.1K:, then to
voltage swing
DS(ON)
means
DS(ON)