ACT8810
Active- Semi
Rev 4, 01-Oct-09
LOW-DROPOUT LINEAR REGULATORS
FUNCTIONAL DESCRIPTION
which filters noise from the reference, providing a
low noise voltage reference to the LDOs. Bypass
General Description
REG4 and REG5 are low-noise, low-dropout linear
regulators (LDOs) that are optimized for low noise
and high-PSRR operation, achieving more than
60dB PSRR at frequencies up to 10kHz.
REFBP to GA with a 0.01μF ceramic capacitor.
Optional LDO Output Discharge
Each of the ACT8810’s LDOs features an optional,
independent output voltage discharge feature.
When this feature is enabled, the LDO output is
discharged to ground through a 1kꢀ resistance
when the LDO is shutdown. This feature may be
enabled or disabled via the I2C interface by writing
to the REG45CFG/DISx[ ] bits.
LDO Output Voltage Programming
All LDOs feature independently-programmable
output voltages that are set via the I2C serial
interface, increasing the ACT8810’s flexibility while
reducing total solution size and cost. Set the output
voltage by writing to the REG45CFG/VSETx[ ]
registers.
Output Capacitor Selection
REG4 and REG5 each require only a small ceramic
capacitor for stability. For best performance, each
output capacitor should be connected directly
between the OUTx and GA pins as possible, with a
short and direct connection. To ensure best
performance for the device, the output capacitor
should have a minimum capacitance of 1μF, and
ESR value between 10mꢀ and 200mꢀ. High quality
ceramic capacitors such as X7R and X5R dielectric
types are strongly recommended.
Output Current Capability
REG4 and REG5 each supply an output current of
360mA. Excellent performance is achieved over this
load current range.
Output Current Limit
In order to ensure safe operation under over-load
conditions, each LDO features current-limit circuitry
with current fold-back. The current-limit circuitry
limits the current that can be drawn from the output,
providing protection in over-load conditions. For
additional protection under extreme over current
conditions, current-fold-back protection reduces the
current-limit by approximately 40% under extreme
overload conditions.
PCB Layout Considerations
The ACT8810’s LDOs provide good DC, AC, and
noise performance over a wide range of operating
conditions, and are relatively insensitive to layout
considerations. When designing a PCB, however,
careful layout is necessary to prevent other circuitry
from degrading LDO performance. A good design
places input and output capacitors as close to the
LDO inputs and output as possible, and utilizes a
star-ground configuration for all regulators to
prevent noise-coupling through ground. Output
traces should be routed to avoid close proximity to
noisy nodes, particularly the SW nodes of the
DC/DCs. REFBP is a filtered reference noise, and
internally has a direct connection to the linear
regulator controller. Any noise injected onto REFBP
will directly affect the outputs of the linear
regulators, and therefore special care should be
taken to ensure that no noise is injected to the
outputs via REFBP. As with the LDO output
capacitors, the REFBP bypass capacitor should be
placed as close to the IC as possible, with short,
direct connections to the star-ground. Avoid the use
of vias whenever possible. Noisy nodes, such as
from the DC/DCs, should be routed as far away
from REFBP as possible.
Enabling and Disabling the LDOs
All LDOs feature independent enable/disable
control via the I2C serial interface. Independently
enable or disable each output by writing to the
appropriate REG45CFG/ONx[ ] bit.
In addition REG4 or REG5 may be enable when
nPBIN is pushed low via 100kꢀ resistance. It
depends on sequence is set. See the Control
Sequence section for more information.
Power-OK
REG4 features power-OK status bit that can be
read by the system microprocessor via the I2C
interface. If an output voltage is lower than the
power-OK threshold, typically 12% below the
programmed regulation voltage, the corresponding
REG45CFG/OK[ ] will clear to 0.
Reference Bypass Pin
The ACT8810 contains a reference bypass pin
Innovative PowerTM
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