Hardware Overview
4
SLOU441 – July 2016
Copyright © 2016, Texas Instruments Incorporated
TPA3255EVM
2.2
TPA3255EVM Frequency Adjust
The TPA3255EVM offers hardware trimmed oscillator frequency by external control of the FREQ_ADJ pin.
The Frequency adjust can be used to reduce interference problems while using a radio receiver tuned
within the AM band, the switching frequency can be changed from nominal to lower values. These values
should be chosen such that the nominal and the lower value switching frequencies together results in the
fewest cases of interference throughout the AM band. The oscillator frequency can be selected by the
value of the FREQ_ADJ resistor connected to GND in master mode according to
Table 1. Frequency Adjust Master Mode Selection
Master Mode
Resistor to GND
Nominal
30 k
Ω
AM1
20 k
Ω
AM2
10 k
Ω
For slave-mode operation, turn off the oscillator by pulling the FREQ_ADJ pin to DVDD. This configures
the OSC_I/O pins as inputs to be slaved from an external differential clock. In a master/slave system inter-
channel delay is automatically set up between the switching phases of the audio channels, which can be
illustrated by no idle channels switching at the same time. This will not influence the audio output, but only
the switch timing to minimize noise coupling between audio channels through the power supply. This will
optimize audio performance and result in better operating conditions for the power supply. The inter-
channel delay will be set up for a slave device depending on the polarity of the OSC_I/O connection such
that slave mode 1 is selected by connecting the OSC_I/O of the master device with the OSC_I/O of the
slave device with the same polarity (+ to + and – to –), while slave mode 2 is selected by connecting the
OSC_I/O's with the inverse polarity (+ to – and – to +).
2.3
TPA3255EVM Single-Ended and Differential Input
The TPA3255EVM supports both differential and single-ended inputs. For single-ended inputs, J4 and/or
J19 jumpers are set to the SE position, so that the TPA3255EVM uses the NE5532 to convert the single-
ended input signal to differential to properly drive the differential inputs of the TPA3255. The input RCA
jack, J3, is used to provide INA inputs and RCA jack J14 is used to provide INB inputs. RCA jack J18 is
used to provide INC inputs and RCA jack J15 is used to provide IND inputs with differential inputs.
For differential input operation, J4 and/or J19 jumpers are set to the DIFF position, and the TPA3255EVM
uses the NE5532 to buffer the differential input signal to the differential inputs of the TPA3255. The input
RCA jack, J3, is used to provide INA, RCA jack J14 provides INB, RCA jack J18 provides INC, and RCA
jack J15 provides IND with differential inputs.
NOTE:
Single-ended input settings on the TPA3255EVM should only be used for channels with
output configuration BTL or PBTL, not SE. For SE output configuration J4 and/or J19
jumpers for that channel must be set to the DIFF position, so the input signal INx is mapped
directly to OUTx.
