ACT8847
Rev 5, 15-Nov-12
gradients in the ground plane, both of which can
result in instability or regulation errors.
Enable / Disable Control
During normal operation, each buck may be enabled
or disabled via the I2C interface by writing to that
regulator's ON[ ] bit.
Step-down DC/DCs exhibit discontinuous input
current, so the input capacitors should be placed as
close as possible to the IC, and avoiding the use of
via if possible. The inductor, input filter capacitor, and
output filter capacitor should be connected as close
together as possible, with short, direct, and wide
traces. The ground nodes for each regulator's power
loop should be connected at a single point in a star-
ground configuration, and this point should be
connected to the backside ground plane with multiple
via. The output node for each regulator should be
connected to its corresponding OUTx pin through the
shortest possible route, while keeping sufficient
distance from switching nodes to prevent noise
injection. Finally, the exposed pad should be directly
connected to the backside ground plane using
multiple via to achieve low electrical and thermal
resistance.
OK[ ] and Output Fault Interrupt
Each DC/DC features a 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 7% below the programmed
regulation voltage, that regulator's OK[ ] bit will be 0.
If a DC/DC's nFLTMSK[-] bit is set to 1, the ACT8847
will interrupt the processor if that DC/DC's output
voltage falls below the power-OK threshold. In this
case, nIRQ will assert low and remain asserted until
either the regulator is turned off or back in regulation,
and the OK[ ] bit has been read via I2C.
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
Table 5:
REGx/VSET[ ] Output Voltage Setting
REGx/VSET[5:3]
REGx/VSET[2:0]
000
001
010
011
100
101
110
111
000
001
010
011
100
101
110
111
0.600
0.625
0.650
0.675
0.700
0.725
0.750
0.775
0.800
0.825
0.850
0.875
0.900
0.925
0.950
0.975
1.000
1.025
1.050
1.075
1.100
1.125
1.150
1.175
1.200
1.250
1.300
1.350
1.400
1.450
1.500
1.550
1.600
1.650
1.700
1.750
1.800
1.850
1.900
1.950
2.000
2.050
2.100
2.150
2.200
2.250
2.300
2.350
2.400
2.500
2.600
2.700
2.800
2.900
3.000
3.100
3.200
3.300
3.400
3.500
3.600
3.700
3.800
3.900
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