ACT8847
Rev 5, 15-Nov-12
LOW-NOISE, LOW-DROPOUT LINEAR REGULATORS
General Description
Output Discharge
Each of the LDOs features an optional output
discharge function, which discharges the output to
ground through a 1.5kꢀ resistance when the LDO is
disabled. This feature may be enabled or disabled
by setting DIS[-]; set DIS[-] to 1 to enable this
function, clear DIS[-] to 0 to disable it.
ACT8847 features eight low-noise, low-dropout
linear regulators (LDOs) that supply up to 350mA.
Three of these LDOs (REG10, REG11, and
REG12) supports extended input voltage range
down to 1.7V. Each LDO has been optimized to
achieve low noise and high-PSRR.
OK[ ] and Output Fault Interrupt
Output Current Limit
Each LDO features a power-OK status bit that can
be read by the system microprocessor via the
interface. If an output voltage is lower than the
power-OK threshold, typically 11% below the
programmed regulation voltage, the value of that
regulator's OK[-] bit will be 0.
Each LDO contains current-limit circuitry featuring a
current-limit fold-back function. During normal and
moderate overload conditions, the regulators can
support more than their rated output currents.
During extreme overload conditions, however, the
current limit is reduced by approximately 30%,
reducing power dissipation within the IC.
If a LDO's nFLTMSK[-] bit is set to 1, the ACT8847
will interrupt the processor if that LDO's output
voltage falls below the power-OK threshold. In this
case, nIRQ will assert low and remain asserted until
either the regulator is turned off or back in
regulation, and the OK[-] bit has been read via I2C.
Compensation
The LDOs are internally compensated and require
very little design effort, simply select input and
output capacitors according to the guidelines below.
Input Capacitor Selection
PCB Layout Considerations
Each LDO requires a small ceramic input capacitor
to supply current to support fast transients at the
input of the LDO. Bypassing each INL pin to GA
with 1μF. High quality ceramic capacitors such as
X7R and X5R dielectric types are strongly
recommended.
The ACT8847’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.
Output Capacitor Selection
Each LDO requires a small 2.2μF ceramic output
capacitor for stability . For best performance, each
output capacitor should be connected directly
between the output and GA pins, as close to the
output as possible, and with a short, direct
connection. High quality ceramic capacitors such as
X7R and X5R dielectric types are strongly
recommended.
REFBP is a noise-filtered reference, 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 via whenever
possible. Noisy nodes, such as from the DC/DCs,
should be routed as far away from REFBP as
possible.
Configuration Options
Output Voltage Programming
By default, each LDO powers up and regulates to
its default output voltage. Once the system is
enabled, each output voltage may be independently
programmed to a different value by writing to the
regulator's VSET[-] register via the I2C serial
interface as shown in Table 5.
Enable / Disable Control
During normal operation, each LDO may be
enabled or disabled via the I2C interface by writing
to that LDO's ON[ ] bit.
Innovative PowerTM
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