VCA8500_0803 Datasheet, PDF (31/40 Pages) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

VCA8500
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POWER SUPPLIES
The VCA8500 operates on two supply rails, a +3.3V
and a +5V supply. At initial power-up, the part
operates in the TGC mode, with the registers in the
default configurations (see Table 2).
In TGC mode, only the VCA (attenuator) draws a
small current (typically 1.5mA) from the +5V supply.
Switching into the CW mode, the internal V/I
amplifiers are then powered from the +5V rails as
well, raising the operating current on the +5V rail. At
the same time, the post-gain amplifiers (PGA) are
powered down, reducing the current consumption on
the +3.3V rail (refer to the Electrical Characteristics
table for details).
All supply rails for the VCA8500 should be clean,
low-noise, analog supplies. This consideration
includes the +3.3V digital supply DVDD (pin 59) that
connects to the internal logic blocks of the VCA8500.
It is recommended to tie the DVDD pin to the same
+3.3V analog supply as the AVDD1 pins, rather than
a different +3.3V rail that may also power other logic
devices in the system. Transients and noise
generated by those devices can couple into the
VCA8500 and degrade performance.
While the VCA8500 uses a thermally-enhanced QFN
package that includes a PowerPAD on its backside,
the primary function of the PowerPAD is to provide a
solid ground reference point. Care should be taken to
use this package pad during the PCB layout phase as
the main ground return point.
POWER-DOWN MODES
The VCA8500 features two power-down modes—a
standby mode and a shutdown mode. The standby
mode function allows the VCA8500 to be rapidly
placed in a low-power state. When in this mode, most
amplifiers in the signal path are powered-down, while
the internal references remain active. This state
ensures that the external bypass capacitors retain the
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
respective charges, minimizing the wake-up response
time. As an example, Figure 59 illustrates the standby
power-up/down response when the PD pin is toggled
with a period of approximately 7.7ms (130Hz) with a
50% duty cycle. Here, the wake-up response time is
approximately 40µs, while the power-down time is
instantaneous (≤ 0.2µs). Factors such as the control
voltage setting, input signal level, PD pin toggle time,
and duty cycle primarily affect the wake-up response
time. Therefore, the user should evaluate the
VCA8500 performance under the desired system
conditions.
When in standby mode, the part typically dissipates
only 104mW, representing an 80% power reduction
compared to the normal operating mode. This
function is controlled through the PD pin (pin 49),
which is designed to interface to +3.3V low-voltage
logic. For normal operation, the PD pin should be tied
to a logic low ('0'); pulling this pin high ('1') places the
VCA into standby mode.
To achieve the lowest power dissipation of only
19mW, the VCA8500 can be placed in shutdown
mode. This mode is controlled through the serial
interface by setting bit D2 (PWR) of the control
register to '1'. When in shutdown mode, all circuits
(including references) within the VCA8500 are
powered-down, causing the bypass capacitors to be
discharged. Consequently, the wake-up time depends
largely on the time needed to charge the bypass
capacitors back up. Another factor is the elapsed time
the VCA8500 spends in shutdown mode.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): VCA8500
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