```Application Note
AN2097
Switch Mode Pump
By: Mohana Koteeswaran
Associated Project: No
Associated Part Family: CY8C25xxx, CY8C26xxx
Summary
TM
The PSoC
device Switch Mode Pump (SMP) includes a boost converter and a voltagecontroller loop that allows conversion of low voltage (e.g., battery) inputs to 3.3V and 5.0V
operating levels.
Introduction
Many applications operate from a low voltage
source, but still need a higher regulated voltage.
The SMP is a boost converter with flyback
topology that converts a low voltage to a higher
voltage. The control loop allows regulation to the
desired value.
When the switch is closed (storage phase), the
input voltage is applied to the inductor. The
inductor current (Iin) increases linearly as shown
in Equation (1):
I in (t ) =
This Application Note includes:
A brief tutorial on boost-converter
operation.
Implementation of the SMP in PSoC
Designer.
Performance of the SMP for 3.3V and
5V applications.
Board layout techniques.
Boost Converter Operation
The boost converter, shown in Figure 1, uses a
switching device to transfer power from a battery
through an inductor as an energy storage device
to the filter capacitor and load:
Vsmp
Vcc
V ⋅ t1
L
(1)
The diode prevents the filter capacitor from
discharging into the switch, Q1. The energy
stored in the inductor while the storage phase is
given by:
E=
1 2
LI in
2
(2)
When the switch is opened, the inductor current
continues to flow; this causes the voltage at node
Vsmp to “flyback” to a voltage higher than the
capacitor voltage. This triggers the diode to start
conducting which in turn allows the charge stored
in the inductor to be transferred into the filter
capacitor. Equation
(3) shows the transfer of
power in the boost converter:
Is
1
1
1
2
2
CV new
= LI in2 + CVold
2
2
2
Vin
(3)
Vsw
Voltage and current waveforms for the standard
form of the boost converter are shown in Figure
2:
Figure 1: Boost Converter Circuit
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S to ra g e
phase
The monitor compares the Vcc voltage with the
SMP trip voltage. When the trip voltage (Vref) is
larger than the Vcc, the comparator enables the
oscillator. The oscillator generates a pulse to turn
on and off the switch. The oscillator runs at a
nominal frequency of 1.3 MHz.
T ra n s fe r
phase
Vsw
I
s
Vcc
Vsm p
I
in
Figure 2: Voltage and Current Waveforms in a
Boost Converter
Unless the output voltage Vcc is controlled, this
boosting will go on indefinitely until something
breaks. A feedback circuit switches off the
oscillator driving the switching transistor to
implement this control.
Oscillator cutoff
T discharge
T charge
Ch1: VSMP Waveform
PSoC Implementation
Figure 5: Output Voltage Ripple and SMP Voltage
The PSoC implementation of the Switch Mode
Pump is shown in Figure 4. In the PSoC device,
the voltage control loop compares the voltage Vcc
with the SMP trip voltage. The SMP trip voltage
can be set either in the “Trip Voltage [LVD
(SMP)]” entry of Global Resources in the Device
Editor or by setting the VM [2:0] bits in the
Voltage Monitor Control Register (VLT_CR) in
the user’s code. The SMP must be enabled either
in the Device Editor or by writing 1 to the SMP bit
(bit 7) of the VLT_CR.
Figure 3: Global Resource Settings
OSC
+
-
Vcc
Vref
Figure 5 shows the output voltage (Vcc) and the
waveform at the SMP node (VSMP). The choice of
capacitor at the Vcc node determines the ripple at
the output voltage. The voltage at SMP node
shows the switching operation clearly. When the
voltage at Vcc drops below the set value (VREF),
the oscillator turns on and switching starts. The
period for which the switch is turned on
corresponds to the charge phase shown in Figure
4. The voltage during charge phase ramps up
due to the presence of the resistances in the
circuit. Then, the switch is turned off (discharge
phase) and VSMP flies up to a value greater than
the output voltage Vcc plus the drop across the
diode. When all of the energy in the inductor is
dumped, the inductor voltage will ring. Once the
voltage across the capacitor reaches the set
value of Vcc, the oscillator and the SMP voltage
remain constant.
Design Details
The parameters of interest in a SMP are the
maximum load current that can be delivered and
the efficiency. Resistances in the circuit limits the
inductor current and changes Equation (1) to:
SMP
I in =
PSoC
Microcontroller
Figure 4: SMP using Feedback Loop in PSoC
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Ch2: Vcc Waveform
(
Vin
1 − e −t / τ
Rt
)
(4)
where,
t = on time of the switch
L is the time constant of the circuit and
τ=
Rt
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Rt = Rsw + Rind
Schottky diodes are recommended because they
have a low forward voltage and fast switching
speed. The current rating of the diode should be
greater than twice the peak load current. The
breakdown voltage should be greater than Vcc.
(5)
Where:
Rsw = 8 Ω, the switch resistance.
Rind = Inductor DC resistance.
The power in the inductor during the storage or
charging phase is:
Pch arg e =
1 2
LI in f
2
(6)
where f is the frequency of the oscillator.
Discharge time or transfer time is:
t disch arg e =
I in L
(Vout + Vdiode − Vin )
(7)
Applications
3.3V Operation
Output voltage of 3.3V can be obtained using a
nominal 1.5V single-cell battery. Using a
capacitor of 0.1µF, a Schottky diode of 1A current
rating, inductors of current rating more than 300
mA, and DC resistance less than 0.5 Ω, the SMP
starts regulating the output voltage to within 5%
of the set value at an input voltage of 1V. But as
the load is increased, the required minimum
voltage to maintain regulated output also goes
up.
The power delivered during discharge is:
1
I in fVin t disch arg e
2
= Pdisch arg e + Pch arg e
Pdisch arg e =
Total power Ptotal
(8)
(9)
The output current Iout is:
I out =
Ptotal
Vout + Vdiode
(10)
The efficiency is the ratio of power at the output
to the total power delivered by the battery given
as:
η=
Vout I out
(0.25Vin I in + Pdisch arg e )
(11)
The switch resistance and the inductor resistance
limit the efficiency of the SMP because the major
2
component of power loss is the I R loss. Care
must to taken to keep this resistance to a
minimum. Since the switch resistance is not
accessible, one of the critical parameters in
choosing the inductor is the DC resistance. The
DC saturation current of the inductor should be
chosen to be greater than the peak inductor
current.
Figure 6: Startup Voltage vs. Load Current
The start-up time of the SMP, defined as the time
taken for the output voltage to reach 5% of the
set SMP trip voltage, is less than 1 ms with no
load connected (Figure 7). The output voltage
increases quickly until it reaches the input value
and then slopes up to the set output value due to
the pumping action.
The output capacitor can cause significant ripple
due to its Equivalent Series Resistance (ESR). If
aluminum capacitors are chosen to reduce cost,
a ceramic capacitor should also be connected in
parallel in order to minimize ripple. The hold time
of the output voltage is shown in Figure 5 as the
oscillator cutoff period. This is determined by the
size of the capacitor used.
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Revision A
Ch1: Vcc
Ch2: Vbatt
Figure 7: Start-up Time of the SMP with No Load
-3-
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Note that the inductor current (Figure 8)
decreases as the inductor value is increased.
This can be seen from Equation (5). The input
current is so high because it is a function of the
efficiency obtained, the battery voltage, the
output voltage, and the load current as can be
I in _ average
(12)
Iout (mA)
31.00
70
60
50
30
L=1uH
L=2.2uH
L=4.7uH
L=10uH
41.00
80
40
Typically, input currents are larger than the
output current as shown in Equation (12). For
example, to drive a load of 10 mA at 3.3V with a
1.3V battery and get 3.3V Vcc with an efficiency of
mA.
51.00
L=1uH
L=2.2uH
L=4.7uH
L=10uH
90
Efficiency (%)
I ⋅V
= out out
Vin ⋅η
100
1
1.5
2
2.5
Vin (Volts)
3
3.5
Figure 9: Typical Efficiency Values at Room
Temperature
The maximum-load current that can be driven by
the SMP is a function of the input battery voltage
and the inductor used, as shown in Figure 10.
The output current delivered increases as the
inductor value is increased for larger battery
voltage. This is because the inductor current Iin is
inversely proportional to the inductance value,
2
thereby reducing the I R loss and increasing the
power delivered. Greater output power delivered
equates to larger load that can be driven.
21.00
11.00
100
90
80
70
60
50
40
30
20
10
0
1.00
1
2
3
4
Iout (mA)
Vin (Volts)
Figure 8: Inductor Current vs. Input Voltage with a
The efficiency of the SMP for 3.3V operation
increases with inductor value. This is because as
the inductor value increases, the input peak
2
current decreases. This makes the I R loss
incurred lower, thereby increasing the efficiency
of the system (Figure 9). The efficiency does not
be driven increases, the system draws a larger
current from the battery by increasing the
operating duty cycle of the oscillator.
Figures 9 and 10 show typical values for
efficiency and maximum-load current that the
SMP can drive. Appendix A gives worst-case
values of efficiency and load currents that the
SMP can drive from a sampling of parts.
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L=1uH
L=2.2uH
L=4.7uH
L=10uH
1
1.5
2
2.5
Vin (Volts)
3
3.5
Figure 10: Typical Values of Maximum Load Current
Therefore, to choose the inductor value that
works best for your application, look at the
maximum output current graph (Figure 10) first to
see what inductor range will be needed to drive
the desired load. Then, looking at Figure 9, the
efficiency of the system while using a particular
inductor can be determined.
Figure 11 shows how efficiency varies with
temperature. The efficiency of the system is
higher at cold temperatures because the losses
due to the on-resistance of the FET are lowered
at low temperatures.
Revision A
-4-
70
120
65
100
60
80
Iout (mA)
Efficiency (%)
AN2097
55
50
Cold
45
Ambient
60
40
20
0
Hot
40
L=1uH
L=2.2uH
L=4.7uH
L=10uH
1.5
2.5
35
1.0
1.5
2.0
2.5
Vin (Volts)
3.0
3.5
Efficiency (%)
Efficiency (%)
Hot
60
55
50
1
2
3
Vin (Volts)
4
5
Figure 14: Typical Efficiency Curves at Different
Temperatures
Since it’s a power-supply board, one must be
careful to use short traces so as to avoid parasitic
inductances. A clean ground is essential to get
the best performance. If the battery is connected
circuit, thereby behaving like a higher inductor.
Connecting a sufficiently big capacitor at the
input node will negate this effect.
70
60
50
40
30
2.5
Ambient
65
For a 5V operation, efficiency shows the same
behavior with temperature as a 3.3V operation.
L=1uH
L=2.2uH
L=4.7uH
L=10uH
1.5
Cold
70
5V Operation
Figure 12 shows typical efficiency values
obtained with a multi-cell input to get a Vcc of 5V.
Comparing this with Figure 8, it can be seen that
using the SMP for 5V operation has more or less
the same efficiency as 3.3V operation. Minimum
values of efficiency for 5V operation are shown in
Appendix A.
80
5.5
75
All the above data were obtained using a 10 µF
output capacitor. A Schottky diode of 1A current
rating and a 10 µF bypass capacitor at the
battery input and inductor DC resistances less
than 0.5 Ω were also used.
90
4.5
Figure 13: Typical Values of Maximum Load Current
Figure 11: Typical Efficiency Curves at Different
Temperatures for a 2.2 µH Inductor
100
3.5
Vin (Volts)
3.5
Vin (Volts)
4.5
Conclusion
5.5
The methodology for building a Switch Mode
Pump using the PSoC device has been shown
here with three external components and the
performance documented.
Figure 12: Typical Efficiency Values
Note that a larger load can be driven for low
voltages with 3.3V operation as compared to 5V
operation. Whereas the battery voltage is
increased, the 3.3V operation can drive larger
current that can be driven using various
inductors.
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Appendix A
80
See the following minimum values of efficiency
and maximum output current for both 3.3V and
5V operation obtained from a sampling of parts:
50
100
Efficiency (%)
80
Iout (mA)
0
1
2
Vin (Volts)
3
4
70
Figure A3: Minimum Values of maximum load
current for 3.3V Operation
60
50
40
100
30
1.5
2
2.5
Vin (Volts)
3
L=1uH
L=2.2uH
L=4.7uH
L=10uH
90
3.5
Efficiency (%)
1
Figure A1: Minimum Values of Efficiency for 3.3V
Operation
80
70
60
50
40
120
30
L=1uH
L=2.2uH
L=4.7uH
L=10uH
100
Iout (mA)
40
30
20
10
L=1uH
L=2.2uH
L=4.7uH
L=10uH
90
L=1uH
L=2.2uH
L=4.7uH
L=10uH
70
60
80
1.5
2.5
3.5
Vin (Volts)
4.5
5.5
Figure A4: Minimum Values of Efficiency for 5V
Operation
60
40
20
Appendix B
0
1.5
2.5
3.5
Vin (Volts)
4.5
5.5
Figure A2: Minimum Values of Maximum Load
Current for 3.3V Operation
Part numbers used to obtain the above data are:
Schottky Diode Through Hole
1A, 20V Micro commercial components - 1N5817
Future Active – MCCN2381
Inductors Surface Mount
1uH – Panasonic - ELJ-EA1R0MF
Digikey - PCD1417CT-ND
2.2uH – Panasonic - ELJ-EA2R2MF
Digikey - PCD1419CT-ND
4uH – Panasonic - ELJ-PA4R7MF
Digikey - PCD14CT-ND
10uH – Panasonic - ELJ-PA100KF
Digikey - PCD1484ct-ND
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Cypress MicroSystems, Inc.
22027 17th Avenue S.E. Suite 201
Bothell, WA 98021
Phone: 877.751.6100
Fax: 425.939.0999
http://www.cypressmicro.com/ / http://www.cypress.com/aboutus/sales_locations.cfm / [email protected]