Core Power Control - Spectrum Digital TMS320VC5509A EVM PLUS Technical Reference

Spectrum Digital, Inc

2.6 Core Power Control

The C5509A EVM PLUS uses two transistors to modify the feedback to the TPS62000
regulator used to supply the DSP's core voltage. These two transistors form a voltage
divider on the feedback to allow the core voltage to switch from 1.6 volts to 1.4 volts to
1.2 volts.
Control of the feedback can be done in 2 ways. The default mechanism is with GPIO5
and GPIO6 of the DSP. The alternative method is to use the 2 bits VCORE_CTL1 and
VCORE_CTL0 in the CPLD MISC register. VCORE_SEL in the MISC register
determines which mode is used. At power up the register is set to "0" for GPIO mode
when VCORE_SEL is set to a "1" the bits 6 and 7 control the voltage control.
2.7 Current Shunts
The C5509A EVM PLUS has 3 shunt devices to convert the small currents of the core,
I/O and board currents to voltages. These voltages are then driven into an op-amp
which directly interfaces to the National Instruments I/O connector. The shunt
resistance, shunt gain, and op-amp gain are shown in the table below.
DSP Core
DSP I/O
EVM
To determine the formula for output voltage to the input current we calculate the value
in stages. An example is shown below.
The voltage going into the shunt resistor is derived from:
So for the core current of 1 MA. we have:
The internal resistance of the shunt current device is 1K ohm. The output is basically a
constant current source with a load resistance of 100K(see table above), with this value
gain is 100 regardless of the input shunt resistance. So for 1 MA. we have
.001 x .1 x 100 at the output of the current shunt amplifier. This is driven into a
non-inverting output amplifier with a gain of 3 so we have .001 x .1 x 100 x 3 for
.03 volts per milliampere.
2-10
Table 8: Current Shunts
Shunt Shunt Output
Resistance Resistance
0.1 50K
0.1 100K
0.025 100K
V = IR
V = .001 amp x
Op-Amp Total Volts
Gain Gain per MA
3 150 .03 volts
3 300 .03 volts
3 300 .0075 volts
.1 ohm = .0001
TMS320VC5509A EVM PLUS Technical Reference
Typical Typical
Current Output
150 MA
5 MA
400 MA