Digital Potentiometers Enable Fast, Linear Adjustment of Switched Mode Power...

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Digital Potentiometers Enable Fast, Linear
Adjustment of Switched Mode Power Supplies
by Miguel Usach, Analog Devices, Inc.
VIN
The capability to finely adjust the output voltage in a power supply makes
it is possible to remove tolerances and drops in the power path, verify the
operation at the system limits, or implement simple dynamic voltage control
for microprocessors. This article explores several options for adjusting a
switched mode power supply (SMPS) and proposes a solution that uses
a switching regulator with a digital potentiometer as the feedback control
element, highlighting the design issues and the ways that they can be solved.
Finally, the AD5141 single channel, nonvolatile digiPOT is presented as a
simple way to overcome common limitations in this application.
OUTPUT FILTER
Switched mode power supply regulators provide higher efficiency than linear
regulators in high current systems, with typical efficiencies of greater than
90% for currents above 100 µA.
In a low dropout (LDO) regulator, the efficiency depends on the quiescent
current (Iq) and the forward voltage drop, with higher quiescent current
causing lower efficiency, as shown in Equation 1.
LDEFFICIENCY(%) =
VOUT IOUT
+ 100
VIN (IOUT+Iq )
(1)
Today’s LDOs have reasonably
low quiescent current, so Iq can be neglected
= Vsmall
( R1 + 1)to I . Then, the LDO efficiency is simply
FEEDBACK
(2)
if itVOUT
is very
compared
LOAD
R2
(VOUT/VIN) × 100. Because the LDO has no way to store significant amounts
of Runused
energy, power not delivered to the load is dissipated as heat(3)
AW = RAB − RWB
within the LDO. Typical LDO efficiencies are less than 83%.
AW × R
3
With
losses,
switching regulators are replacing linear regulators
R4 their
= Rlower
(4)
R
R3 as ATE, FPGAs, and instrumentation that require high
AB + such
in applications
current
dynamic
× R3 loads.
R5 =orRWB
(5)
R
AB + R3
It is often necessary for the system designer to adjust supply voltages, either
to Roptimize
their levels or to force them away from nominal values when
(6)
7 = R1 + R4
characterizing system performance under extreme conditions. This function
RAW = Rperformed
(7)
AB + RWB during in-circuit test (ICT), when a manufacturer wants
is typically
to guarantee that a product functions correctly at nominal supplies ± 10%,
for example.
This procedure, called margining, is done by deliberately changing the supply
RAW ≠within
RAB −
(8)
WB
voltage
theRexpected
range. In addition, the capability to finely adjust
the output voltage makes it possible to compensate for supply tolerance and
voltage drops in the power path.
Other applications, such as dynamic voltage control for a microprocessor,
must be able to change the voltage on-the-fly, reducing the voltage in low
power modes and increasing it in high performance modes.
An SMPS works similarly to an LDO, as shown in Figure 1. The output voltage
is compared with an internal reference, with the difference connected to the
pulse width modulator.
VOUT
PULSE WIDTH
MODULATOR
FEEDBACK
ERROR
AMPLIFIER
VREF
Figure 1. SMPS voltage control loop.
The pulse width modulator compares a ramp with the amplifier output, and
generates the PWM signal that controls the switches that deliver energy to
the load.
Adjusting the output voltage can be done by controlling the voltage at the
inverting amplifier pin.
This can be done externally, using a DAC, or a digital potentiometer. Some
regulators allow internal control of the feedback voltage using a serial interface such as PMBUS, I2C, or SPI. Table 1 compares all three methods in terms
of adjustment capability and power dissipation.
Table 1. Benchmark Analysis Summary—Adjustable SMPS
Method
Coarse
Adjustment
DAC
digiPOT
Internal Registers
Fine
Adjustment
Medium
High
High
High
Medium
Low
Power Supply Typical Power
Rails
Consumption
VMIN < 2.5 V
VMIN < 2.3 V
Not applicable
>100 µA
<20 µA
Low
Some digital potentiometers are available with nonvolatile memory, so the
output supply can be programmed in test. This easy to use feature provides
a substantial benefit as compared to the other two methods.
Linearizing the Transfer Equation
VOUT IOUT
LDEFFICIENCY
(%) =
+of100
Equation
2 describes
theVoutput
voltage
the SMPS based on the ratio of
IN (IOUT+Iq )
(1)
feedback resistors R1 and R2,
VOUT = VFEEDBACK ( R1 + 1)
R2
where VFEEDBACK is the internal reference voltage.
(2)
RAW = RAB − RWB
(3)
R4 = RAW × R3
RAB + R3
(4)
analog.com R5 = RWB × R3
RAB + R3
(5)
R7 = R1 + R4
(6)
VOUT IOUT
LDEFFICIENCY(%) =
+ 100
(1)
) a digital potentiometer, some issues
VINR1(Iand
Before directly replacing
R2q by
OUT+I
should be considered. Internally the digital potentiometer has two resistor
strings, RAW and RWB. R1
VOUT = VFEEDBACK ( + 1)
(2)
R2
Both string resistors are complementary,
RAW = RAB − RWB
DC-TO-DC
CONVERTER
VIN
INPUT
R1
(3)
where RAB is the end-to-end resistance or nominal value.
R4 = RAW × R3
(4)
ReplacingRRAB1 and
+ RR3 2 with RAW and RWB results in a logarithmic transfer function. The nonlinear relation between the digital code and the output voltage
×R
R5 = RWB
decreases
the low 3end resolution. Figure 2 shows an example for a 16-tap(5)
RAB + R3
digital potentiometer.
R7 = R20
(6)
1 + R4
RAW = RAB + RWB
(7)
VOUT
OUTPUT
FEEDBACK
digiPOT
R3
R2
GND
Figure 4. Potentiometer mode.
In rheostat mode, the series resistance must be high enough to render the
tolerance of the digital potentiometer negligible, that is R2 ≥ 10 × RAB.
In potentiometer mode, the parallel resistor must be small enough, that is
RAB
.
10
16
R3 ≤
12
Linearizing the potentiometer using a series-parallel combination could be
quite complex, as shown in the equivalent circuit of Figure 5,
RAW ≠ RAB − RWB
VOUT
(8)
R1
8
RAW
4
R4
0
4
8
RDAC DECIMAL CODE
12
R8
VOUT = VFEEDBACK ( RR12 + 1)
R2
This problem can be overcome in several ways; the more common are to use
the digital potentiometer in rheostat mode, or to place resistors in series with
the potentiometer.
(1)
R5
15
Figure 2. Logarithmic transfer function.
R7
R
R3
6
VOUT IOUT
+ 100
VIN (IOUT+Iq )
RWB
LDEFFICIENCY(%) =
0
R1
R2
(2)
Figure 5. Final Y-∆ transform.
RAW = RAB − RWB
(3)
R4 = RAW × R3
RAB + R3
(4)
Due to the resistor tolerance, using a digital potentiometer in conjunction
with external resistors can cause mismatch problems. Precision devices
might have 1% resistor tolerance, but the vast majority of digital potentiometers can only achieve 20% resistor tolerance.
R5 = RWB × R3
RAB + R3
(5)
R7 = R1 + R4
(6)
In this case, reducing the mismatch is possible by using a series/parallel
resistance combination, as shown in Figure 3 and Figure 4. As a downside,
the dynamic range is reduced as well.
RAW = RAB + RWB
(7)
Minimizing the Tolerance
DC-TO-DC
CONVERTER
VIN
INPUT
VOUT
OUTPUT
R1
FEEDBACK
R3
GND
R2
where:
the feedback input pin typically has high impedance, so the effect of R6 can
be made negligible.
Increasing the Bandwidth
RAW ≠ RAB − RWB
(8)
The switching regulator operates at high frequency, typically above 1 MHz,
allowing the use of small external components. In worst-case scenarios, it
must power dynamic loads, so the feedback resistor network must provide
enough bandwidth to accurately track the output voltage. Due to the parasitic
internal switch capacitance, the digital potentiometer acts as a low-pass filter.
If the feedback network does not have enough bandwidth, the output voltage
will oscillate, as shown in Figure 6.
Figure 3. Rheostat and serial resistor.
| Digital Potentiometers Enable Fast, Linear Adjustment of Switched Mode Power Supplies
2
SW
SW
VOUT
VOUT
IOUT
IOUT
Figure 6. Discrete feedback resistance vs. digital potentiometer with limited bandwidth.
A simple way to overcome this limitation is to place a capacitor in parallel
VOUTfeedback
IOUT
between
the output
and the
LDEFFICIENCY
(%) =
+network
100 (as shown in Figure 7), (1)
V
IN (IOUT+Iq )
reducing the high frequency impedance, and minimizing the oscillation time.
VOUT = VFEEDBACK ( R1 DC-TO-DC
+ 1)
R2CONVERTER
VIN
RAW = RAB − RWB
R4 = RAW × R3
RAB + R3
(2)
R7
C1
R8
GND
R7 = R1 + R4
A Simpler
R = RSolution
+ R Without Compromise
AB
(3)
IOUT
Figure 7. Parallel capacitor reduces high frequency impedance, minimizes oscillation.
AW
VOUT
VOUT
INPUT OUTPUT
FEEDBACK
R5 = RWB × R3
RAB + R3
SW
WB
(4)
(5)
(6)
(7)
ADI’s new AD5141 digiPOT overcomes the problems presented by other digital potentiometers. Its patented linear gain setting mode allows independent
control of each string resistor, so
RAW ≠ RAB − RWB
(8)
enabling this mode, no external resistors are needed. The resistor tolerance
becomes negligible, and the overall error of the transfer function is only due
to the internal string mismatch, which is typically less than 1%.
Each string resistor has an associated EEPROM location, so an independent
value for each string can be loaded upon power-up. In addition, the device
provides up to 3 MHz bandwidth for a fast feedback loop as shown in
Figure 8.
Figure 8. AD5141 (10 k Ω) version in linear gain setting mode.
Conclusion
Switched mode power supply regulators are commonly used in high current
applications due to their high efficiency. This article describes several ways
that can be used to digitally control the output voltage.
Due to the inherent benefits obtained by powering up a system in a predefined
output state, a solution that uses digital potentiometers with internal nonvolatile memory is desirable. The main trade-offs faced by designers include
providing enough resolution, accuracy, and bandwidth to achieve outstanding
performance. The AD5141 digiPOT enables designers to provide an optimum
solution without compromises.
For more information on any of the above mentioned products visit
www.analog.com.
Online Support Community
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