Datasheet

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