ACT8828 datasheet(29/49 Pages) ACTIVE-SEMI

29 / 49 page

STEP-DOWN DC/DC CONVERTERS

ACT8828

Rev 3, 24-Sep-09

Active-Semi

Innovative Power

- 29 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of Philips Electronics.

(2)

−

FUNCTIONAL DESCRIPTION CONT’D

RFB1

RFB2

OUTx

ACT8828

FBx

CFF

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

FBx

OUTx

(1)

Output Capacitor Selection

For most applications, 22μF ceramic output

capacitors are recommended for REG1 and 10μF

ceramic output capacitors are recommended for

REG2, REG3. Although the these regulators were

designed to take advantage of the benefits of

ceramic capacitors, namely small size and very-low

ESR, low-ESR tantalum capacitors can provide

acceptable results as well.

Inductor Selection

REG1, REG2 and REG3 utilize current-mode

control and a proprietary internal compensation

scheme

simultaneously

simplify

external

component

selection

and

optimize

transient

performance over their full operating range.

The REG1, REG2 and REG3 of these devices was

optimized for operation with and 3.3μH inductor,

although inductors in the 2.2μH to 4.7μH range can

be used.

Choose an inductor with a low DC-resistance, and

avoid inductor saturation by choosing inductors with

DC ratings that exceed the maximum output current

of the application by at least 30%.

Output Voltage Programming

By default, REG1, REG2 and REG3 each power up

and regulate to their default output voltage, as

defined in the Ordering Information section. Once

the system is enabled, each regulator’s output

voltage may be modified through either the I2C

interface or the Voltage Selection (VSEL) pin.

Programming via the I

2C Interface

Following startup, REG1, REG2, and REG3 may be

independently programmed to different values by

writing

the

REGx/VSETx[_ ]

an d

REGx/VRANGE[_] registers via the I2C interface.

To program each regulator, first select the desired

output voltage range via the REGx/VRANGE[ ] bit.

Each regulator supports two overlapping ranges;

set REGx/VRANGE[_] to 0 for voltages below

2.245V, set REGx/VRANGE[_] to 1 for voltages

above 1.25V.

Once the desired range has been selected,

program the output to a voltage within that range by

setting the REGx/VSETx bits. For more information

about the output voltage setting options, refer to

Tables 4, 7, and 10, for REG1, REG2, and REG3,

respectively.

Programming with Adjustable Option

Figure 6 shows the feedback network necessary to

set the output voltage when using the adjustable

output voltage option. Select components as

follows: Set RFB2 = 51kΩ, then calculate RFB1 using

the following equation:

Where VFBx is 0.625V when REGx × VRANGE[ ] = 0

and 1.25V when REGx × VRANGE[ ] = 1

Figure 6:

Output Voltage Programming

Finally choose CFF using the following equation:

Where RFB1 = 47kΩ, use 47pF. When using

Adjustable Option, OUTx pins works as FBx

function.

Output Voltage Selection Pin (VSEL)

ACT8828's VSEL pin provides a simple means of

alternating between two preset output voltage

settings, such as may be needed for dynamic

voltage selection (DVS). The operation of this pin is

as follows: when VSEL is driven to GA or a logic

low, the output voltages of REG1, REG2, and

REG3 are each defined by their VSET0[ ] register.

when VSEL is driven to VSYS or a logic high, the

output voltages of REG1, REG2, and REG3 are

each defined by their VSET1[ ] register.

By default, each regulator's VSET0[ ] and VSET1[ ]

registers are both programmed to the same voltage,

as defined in the Ordering Information section. As a

result, toggling VSET under default conditions has

no affect. However, by re-programming one or more

regulator's VSET0[ ] and/or VSET1[ ] registers, one

can easily toggle these regulators' output voltages

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