ACT8937A
Rev 1, 22-Oct-12
LOW-NOISE, LOW-DROPOUT LINEAR REGULATORS
to that LDO's ON[ ] bit. The regulator accept rising or
falling edge of ON[ ] bit as on/off signal. To enable
the regulator, clear ON[ ] to 0 first then set to 1. To
disable the regulator, set ON[ ] to 1 first then clear it
to 0.
General Description
REG4, REG5, REG6, and REG7 are low-noise,
low-dropout linear regulators (LDOs) that supply up
to 150mA, 150mA, 250mA, and 250mA,
respectively. Each LDO has been optimized to
achieve low noise and high-PSRR, achieving more
than 65dB PSRR at frequencies up to 10kHz.
REG4, REG5, REG6, REG7 Turn-on Delay
Each of REG4, REG5, REG6 and REG7 features a
programmable Turn-on Delay which help ensure a
reliable qualification. This delay is programmed by
DELAY[2:0], as shown in Table 7.
Output Current Limit
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.
Output Discharge
Each of the ACT8937A’s 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.
Compensation
The LDOs are internally compensated and require
very little design effort, simply select input and
output capacitors according to the guidelines below.
Low-Power Mode
Each of ACT8937A's LDOs features a LOWIQ[-] bit
which, when set to 1, reduces the LDO's quiescent
current by about 16%, saving power and extending
battery lifetime.
Input Capacitor Selection
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.
OK[ ] and Output Fault Interrupt
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.
Output Capacitor Selection
Each LDO requires
a small ceramic output
capacitor for stability. Capacitance value is 1.5μF
for REG4 and REG5, 2.2μF for REG6 and REG7.
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.
If a LDO's nFLTMSK[-] bit is set to 1, the
ACT8937A 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.
PCB Layout Considerations
Configuration Options
The ACT8937A’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.
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 8.
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
Enable / Disable Control
During normal operation, each LDO may be
enabled or disabled via the I2C interface by writing
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