ACT8810 datasheet(29/54 Pages) ACTIVE-SEMI

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STEP-DOWN DC/DC CONVERTERS

ACT8810

Rev 9, 15-Nov-12

Innovative Power

- 29 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of NXP.

(2)

−

Inductor Selection

REG1, REG2 and REG3 utilize current-mode control

and a proprietary internal compensation scheme to

simultaneously

simplify

external

component

selection and optimize transient performance over

their full operating range.

REG1, REG2 and REG3 of the device were 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 to the REGx/VSETx[_] and 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 8 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 8:

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)

ACT8810'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

between two sets of voltages, such as to implement

'normal' and 'standby' modes in a system utilizing

the ACT8810 to implement an advanced power

management architecture.

PCB Layout Considerations

High switching frequencies and large peak currents

make PC board layout an important part of step-

down DC/DC converter design. A good design

minimizes excessive EMI on the feedback paths

and voltage gradients in the ground plane, both of

which can result in instability or regulation errors.

Step-down DC/DCs exhibit discontinuous input

current, so the input capacitors should be placed as

FUNCTIONAL DESCRIPTION CONT’D

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

FBx

OUTx

(1)

RFB1

RFB2

OUTx

ACT8810

FBx

CFF

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