ACT8810
Active- Semi
Rev 4, 01-Oct-09
STEP-DOWN DC/DC CONVERTERS
FUNCTIONAL DESCRIPTION CONT’D
Figure 8:
Output Voltage Programming
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.
OUTx
CFF
RFB1
ACT8810
FBx
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.
RFB2
Finally choose CFF using the following equation:
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%.
2.2 ×10 −6
CFF
=
(2)
RFB1
Where RFB1 = 47kꢀ, use 47pF.
Output Voltage Programming
When using Adjustable Option, OUTx pins works as
FBx function.
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.
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.
Programming via the I2C 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.
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.
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 = 51kꢀ, then calculate RFB1 using
the following equation:
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.
⎛
⎞
VOUTx
VFBx
(1)
⎜
⎜
⎟
⎟
RFB1 = RFB 2
−1
⎝
⎠
Where VFBx is 0.625V when REGx × VRANGE[ ] = 0
and 1.25V when REGx × VRANGE[ ] = 1
Step-down DC/DCs exhibit discontinuous input
current, so the input capacitors should be placed as
Innovative PowerTM
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