ACT81460
Rev 1.0, 18-Dec-2018
85% Duty Cycle Operation
STEP-DOWN DC/DC REGULATORS
The maximum duty cycle is limited to 85%. The maxi-
mum duty cycle must be considered when calculating
the maximum allowable output voltage.
General Description
The ACT81460 contains two fully integrated step-down
converters. Buck1 and Buck2 both provide 0.4A outputs.
They operate with the Active-On-Time control loop,
which is a hybrid topology of a Constant ON-Time (COT)
synchronous step-down converter and a hysteretic con-
verter. This combination allows them to achieve very
low quiescent current during extremely light load oper-
ation. The regulators can achieve peak efficiencies of
up to 95%. They are internally compensated, requiring
only three external components (Cin, Cout, and L) for
operation. They allow the use of small external compo-
nents while emulating a constant frequency PWM mode
regulator during continuous mode high load current sit-
uations. Additionally, all regulators are available with a
variety of standard and custom output voltages and may
be software-controlled via the I2C interface for systems
that require advanced power management functions.
Synchronous Rectification
Each BUCK regulator features an integrated synchro-
nous rectifier (or low side FET), maximizing efficiency
and minimizing the total solution size and cost by elimi-
nating the need for external rectifiers.
Soft-Start
The BUCK regulators include a fixed 2.5V/ms soft-start
ramp which limits the rate of change of the output volt-
age, minimizing input inrush current and ensuring that
the output powers up in a monotonic manner that is in-
dependent of loading on the outputs. This circuitry is ef-
fective any time the regulator is enabled, as well as after
responding to a short circuit or other fault condition.
Output Voltage Setting
The ACT81460 buck regulators are highly configurable
and can be quickly and easily reconfigured via I2C. This
allows them to support changes in hardware
requirements without the need for PCB changes.
Examples of I2C functionality are given below:
The buck converters regulate to the voltage defined by
I2C register VSET0 in normal operation and by VSET1
in DVS mode. They can be programmed between 0.6V
and 3.6V in 50mV steps. The following equation calcu-
lates the output voltage based on the VSETx register
setting.
Real-time power good, OV, and current limit status
Ability to mask individual faults
Vbuck1 = 0.6V + VSETx * 0.05V
Where VSETx is the decimal equivalent of the value in
each regulator’s I2C VSETx register. The VSETx regis-
ters contain an unsigned 6-bit binary value. As an ex-
ample, if Buck 1’s VSET0 register contains 100110b (38
decimal), the output voltage is 2.5V.
Dynamically change output voltage
Low Power Mode On/Off control
Overcurrent Thresholds
Refer to theActive-Semi Application Note describing the
Register Map for full details on I2C functionality and
programming ranges.
Table 4 shows the output voltage in voltage as a func-
tion of the VSETx register settings.
Buck Output Voltage (V)
VSETx [5:3]
Operating Mode
The Buck regulators operate in a fixed frequency Active-
On-Time, fixed-frequency PWM mode at medium to
heavy loads. This results in a fixed frequency, PWM
mode with a constant on-time. The control loop auto-
matically adjusts the on-time with changes in input volt-
age, output voltage, and load to maintain a fixed switch-
ing frequency. At lighter loads, the inductor current goes
discontinuous and the control loop increases the off-
time. This reduces the switching frequency and mini-
mize switching losses. At extremely light loads, the IC
continues to decrease the off-time, further reducing
switching losses.
000
001
010
011
100
101
110
111
0.60
1.00
1.40
1.80
2.20
2.60
3.00
3.40
000
001
010
011
100
101
110
111
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.25
2.30
2.35
2.40
2.45
2.50
2.55
2.65
2.70
2.75
2.80
2.85
2.90
2.95
3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.45
3.50
3.55
3.60
3.60
3.60
3.60
Table 4. Vout vs VSETx Register Setting
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Copyright © 2018 Active-Semi, Inc.
36
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