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ACT8798

Six Channel Integrated Power Management IC for Handheld Portable Equipment

Active-Semi, Inc 

Active-Semi

ACT8798

Rev 1, 16-Nov-09

LOW-DROPOUT LINEAR REGULATORS

FUNCTIONAL DESCRIPTION

General Description

REG4, REG5, and REG6 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

output voltages that are set via the I2C serial

interface, increasing the ACT8798’s flexibility while

reducing total solution size and cost. Set the output

voltage by writing to the REG456CFG/VSETx[ ]

registers.

Output Current Capability

REG4, REG5, and REG6 each supply an output

current of 250mA. 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 30% under extreme

overload conditions.

Enabling and Disabling the LDOs

REG4 is enabled whenever either of the following

conditions are met:

1) nMSTR is driven low, or

2) PWRHLD is asserted high.

Furthermore, once these conditions are met REG5

and REG6 maybe independently enabled or

disabled via the I2C serial interface by writing the

appropriate REG56/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 15% below the

programmed regulation voltage, the corresponding

REG456CFG/OKx[ ] will clear to 0.

Reference Bypass Pin

The ACT8798 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 ACT8798’s LDOs features an optional,

independent output voltage discharge feature.

When this feature is enabled, the LDO output is

discharged to ground through a 650Ω resistance

when the LDO is shutdown. This feature may be

enabled or disabled via the I2C interface by writing

to the REG456CFG/DISx[ ] bits.

Output Capacitor Selection

REG4, REG5, and REG6 each require only a small

ceramic capacitor for stability. For best

performance, 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

PCB Layout Considerations The ACT8798’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

Innovative PowerTM

ActivePMUTM is a trademark of Active-Semi.

I2CTM is a trademark of Philips Electronics.

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Copyright © 2009 Active-Semi, Inc.

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