Tripath Technology, Inc. - Technical Information
and the resultant supply ranges will be:
VPP RANGE = 36.03 − 23.64V = 12.39V
VNN RANGE = −36.60 − − 22.41V = -14.19V
It should also be noted that the tolerance of the RVPPSENSE (or RVNNSENSE) resistors will effect the trip
voltages and thus, the usable supply range. To minimize the additional variance Tripath recommends
1% tolerance resistors.
As a matter of completeness, the formulas below include the effect of resistor tolerance assuming a
known value of RVPPSENSE or RVNNSENSE
.
VPP MIN_OV_TUR N_OFF = (RVPPSENSE × ITRIP )÷ (1+ TOL /100 ) + 2.5V
VPP MAX_UV_TUR N_OFF = (RVPPSENSE × ITRIP )× (1+ TOL /100 ) + 2.5V
VNN MIN_OV_TUR N_OFF = 1.25 − (RVNNSENSE × ITRIP )÷ (1+ TOL /100 )
VNN MAX_UV_TUR N_OFF = 1.25 − (RVNNSENSE × ITRIP )× (1+ TOL /100 )
Using a value of 243kΩ for RVPPSENSE and a value of 249kΩ for RVNNSENSE, assuming 5% tolerance,
along with the appropriate value of ITRIP, the trip voltages and supply ranges can be calculated.
VPP MIN_OV_TUR N_OFF = (243k Ω × 138 µA)÷ (1+ 5 /100 ) + 2.5V
=
34.44V
VPP MAX_UV_TUR N_OFF = (243k Ω × 87µA )× (1+ 5 /100 ) + 2.5V = 24.70V
VPP RANGE = 34.44 − 24.70V = 9.74V
VNN MIN_OV_TUR N_OFF = 1.25 − (249kΩ × 152µA )÷ (1+ 5 /100 ) = −34.80V
VNN MAX_UV_TUR N_OFF = 1.25 − (249kΩ × 95µA )× (1+ 5 /100 ) = −23.59V
VNN RANGE = −34.80 − − 23.59V = -11.21V
Thus, by using 5% resistors, the supply range for the VPP has been reduced by 2.65V while the VNN
range has been reduced by approximately 3.0V (as compared to resistors with no tolerance
variation). In actuality, if a 5% resistor was to be used, then the initial value of RVPPSENSE and
RVNNSENSE would have had to be adjusted such that the minimum over voltage turn off points would
have never been less than +/-36V as defined by the supply voltage and tolerance specification.
It should be noted that the values for VVPPSENSE and VVNNSENSE shown in the Electrical Characteristics
table were calculated using a value of 249kΩ for both RVPPSENSE and RVNNSENSE. In addition, for the
maximum and minimum values, as opposed to the typical ones, a 1% tolerance resistor value around
249kΩ was chosen to show the effect on supply range. Thus, the minimum and maximum values
would be “worst case” assuming a supply voltage of 5V for the input section of the TA2022.
The entire discussion thus far has been for the “one resistor” sense circuit. This configuration
requires a single resistor from either VPPSENSE or VNNSENSE to the respective power supply.
While the simplest configuration, in terms of external components, there are some drawbacks to this
configuration. The first drawback is that the range for VPPRANGE and VNNRANGE are asymmetric due
to the different internal bias voltages of VPPSENSE and VNNSENSE. A second issue is that current
through RVPPSENSE or RVNNSENSE will change if the V5 voltage is not exactly 5V, since the bias voltages
of pin 18 and pin 19 are set by resistor dividers between V5 and AGND.
With an additional resistor per supply sense pin (2 resistors per VPPSENSE or VNNSENSE), the
drawbacks of the “one resistor” sense circuit can be eliminated. In addition, the calculations of the
sense resistors are actually more straightforward for the “two resistor” sense circuit as opposed to the
“one resistor” scheme. Figure 10 shows the proper connection for the “two resistor” sense circuit for
both the VPPSENSE and VNNSENSE pins.
26
TA2022 – KLI/1.2/07-04
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
