ACT8740 datasheet(32/40 Pages) ACTIVE-SEMI

32 / 40 page

LOW-NOISE, LOW-DROPOUT, LINEAR REGULATORS

ACT8740

Rev PrB, 25-Feb-08

Innovative Products. Active Solutions.

- 32 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

General Description

REG3, 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.

LDO Output Voltage Programming

All LDOs feature independently-programmable out-

put voltages that are set via the I2C serial interface,

increasing the ACT8740’s flexibility while reducing

total solution size and cost. Set the output voltage

by writing to the REG345CFG/VSETx[ ] registers.

Output Current Capability

REG3, REG4, and REG5 each supply 250mA of

load current. Excellent performance is achieved

over each regulator's entire load current ranges.

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 30% under extreme

overload conditions.

Enabling and Disabling the LDOs

All LDOs feature independent enable/disable con-

trol via the I2C serial interface. Independently en-

able or disable each output by writing to the appro-

priate REG345CFG/ONx[ ] bit.

Power-OK

Each of the LDOs 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 6% below the pro-

grammed regulation voltage, the corresponding

REG345CFG/OKx[ ] will clear to 0.

Reference Bypass Pin

The ACT8740 contains a reference bypass pin

which filters noise from the reference, providing a

low-noise voltage reference to the LDOs. Bypass

REF to G with a 0.01µF ceramic capacitor.

Optional LDO Output Discharge

Each of the ACT8740’s LDOs features an optional,

independent output voltage discharge feature.

When this feature is enabled, the LDO output is

discharged to ground through a 1kΩ resistance

when the LDO is shutdown. This feature may be

enabled or disabled via the I2C interface by writing

to the REG345CFG/DISx[ ] bits.

Output Capacitor Selection

REG3, REG4, and REG5 each require only a small

ceramic capacitor for stability. For best perform-

ance, each output capacitor should be connected

directly between the OUTx and G 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 10mΩ and 200mΩ. High

quality ceramic capacitors such as X7R and X5R

dielectric types are strongly recommended.

PCB Layout Considerations

The ACT8740’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 regu-

lators to prevent noise-coupling through ground.

Output traces should be routed to avoid close prox-

imity 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 con-

troller. 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 by-

pass 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 possi-

ble. Noisy nodes, such as from the DC/DCs, should

be routed as far away from REFBP as possible.

FUNCTIONAL DESCRIPTION

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