AN1337

AN1337
Optimizing Battery Life in DC Boost Converters Using MCP1640
Author:
Valentin C. Constantin,
Microchip Technology Inc.
INTRODUCTION
Microchip Technology Inc. has developed the
MCP1640/B/C/D
devices
for
battery-powered
applications. These devices possess all the modern
design features, such as high efficiency, low quiescent
current, compact size, and low number of external
components.
The MCP1640 is a synchronous step-up DC-DC
converter that provides up to 96% efficiency and runs
at 500 kHz frequency. The device offers easy-to-use
power supply solutions for applications powered by
one, two or three-cell alkaline, NiCd, NiMH, or singlecell Li-Ion/Li-Polymer batteries.
This application note details the practical
considerations for more efficient use of the MCP1640
device in applications. It also gives ideas on how to
increase battery life.
The typical peak current limit is 800 mA. It delivers
more than 100 mA load current at 1.2V input and 3.3V
output, or more than 300 mA at 5.0V output, when
supplied with 3.3V input. Detailed information will be
presented in the following sections.
Microchip offers the MCP1640 in four options, which
help users meet different system requirements. The
devices and their available options are shown in
Table 1.
TABLE 1:
Part
Number
PART NUMBER SELECTION
PWM/
PWM
PFM
True
Output
Disconnect
Bypass
MCP1640
X
—
X
—
MCP1640B
—
X
X
—
MCP1640C
X
—
X
MCP1640D
—
—
X
X
MCP1640/B/C/D FEATURES AND
OPTIONS
The MCP1640/B/C/D features include:
• Low start-up voltage (typically 0.65V, at 1 mA load
and 3.3V output) and continuous operating after
start-up, until 0.35V input voltage is reached
• Output voltage range, from 2V to 5.5V
• PWM/PFM mode operation automatically
selected (MCP1640/C)
• Low quiescent current (19 µA typical in PFM
mode)
• Shutdown current less than 1 µA
• Integrated synchronous switch
• Internal compensation
• Low noise, anti-ring control
• Inrush current limit and soft start
 2010 Microchip Technology Inc.
DS01337A-page 1
AN1337
Choosing Between PWM/PFM
and PWM-Only Mode
The MCP1640/B/C/D series operate in two modes:
• Pulse-Width Modulation (PWM, in continuous and
discontinuous mode), or Pulse Frequency Modulation (PFM) – for MCP1640 and MCP1640C
• PWM only – for MCP1640B and MCP1640D
16
14
VOUT = 2.0V
12
VOUT = 3.3V
VOUT = 5.0V
FIGURE 2:
Evaluation of the PWM and
PFM Modes – Load Step from 25 mA to 1 mA.
Figure 3 depicts the efficiency of the two modes: PFM/
PWM mode and PWM-only mode. It shows the main
disadvantage of not entering in PFM mode – lower
efficiency for light loads.
10
8
6
4
2
0
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
VIN (V)
FIGURE 1:
PFM to PWM Output
Threshold vs. Input Voltage.
Figure 2 demonstrates the difference between the
output voltage in PFM mode and PWM mode, which is
approximately 50 mV at 1.2V input and 3.3V output.
The load step is from 25 mA to 1 mA. As shown in
Figure 1, the threshold between modes (from PWM to
PFM) is approximately 6 mA.
DS01337A-page 2
Efficiency @ VOUT = 3.3V (%)
PFM/PWM IOUT Threshold
(mA)
The PFM mode starts when the output current reduces
below a predetermined threshold. During PFM mode, a
high peak current is used to pump up the output to the
threshold limit. If the output voltage reaches the
maximum limit, the switching pulses will stop and the
device enters in a low quiescent current, to minimize
the current drawn from the power source (battery). The
automatic switching from PWM to PFM mode is used
for light load conditions to maximize the efficiency over
a wide range of output current. PFM mode has one
disadvantage: higher output voltage ripple. While
working in PFM/PWM mode, the output voltage
increases to approximately 50 mV. The PFM to PWM
current threshold depends on the input voltage (see
Figure 1).
MCP1640B/D devices operate at a constant 500 kHz
switching frequency, lowering the output ripple voltage
when compared to the MCP1640/C devices, which
have the PWM/PFM mode option. Under light load
conditions and a typical minimum duty cycle of 100 ns,
the MCP1640B/D devices continue to switch at a
constant frequency. At lighter loads (below few mA),
the MCP1640B/D devices begin to skip pulses.
100
90
80
70
60
50
40
30
20
10
0
0.01
VIN = 2.5V
VIN = 0.8V
VIN = 1.2V
PWM / PFM
PWM ONLY
0.1
1
10
100
1000
IOUT (mA)
FIGURE 3:
(VOUT = 3.3V).
Efficiency vs. Load Current
In conclusion, when the output ripple is not a primary
design goal, but efficiency is a key feature of the project
(especially for light loads), the MCP1640/C devices are
strongly recommended, especially in battery-powered
systems. They will help to increase the battery lifetime
in portable applications.
 2010 Microchip Technology Inc.
AN1337
Choosing Between True Output
Disconnect and Input Bypass
When starting to design with the MCP1640 device, the
engineer has to select a shutdown state. Depending on
the selected shutdown option, the output is completely
isolated from the input, or the input is bypassed to the
output. The device will be in Shutdown mode if EN pin
is low.
The MCP1640 and MCP1640B devices incorporate a
True Output Disconnect feature. The output is
disconnected from the input by turning off the
integrated P-Channel switch (Figure 4) and removing
the switch bulk diode connection (turning off the
additional P-Channel transistor). During this mode, the
current consumed from the input (battery) is less than
1 µA.
Shutdown
Control
The MCP1640C and MCP1640D devices incorporate
the Input Bypass shutdown option. If the device is shut
down, the output will be connected to the input through
the internal P-Channel MOSFET. In this mode, the
current drawn from the input is also less than 1 µA.
During shutdown, additional current flow is consumed
by the external resistor divider. The loss of the
feedback (FB) current is avoided by disconnecting the
feedback resistors during shutdown. The regulated
feedback loop is not used during Shutdown mode. It is
recommended to use high value resistors (of
approximately hundred kohms) in the feedback voltage
sense network, to keep the biasing current low (this
does not influence the frequency response).
The Input Bypass mode is used when the input voltage
is almost equal with the necessary output voltage, or is
high enough for the load to operate in Sleep or low
quiescent current mode. When regulated output
voltage is necessary, the shutdown control will enable
the boost converter.
D
L
VIN
CIN
NSW
VOUT
SW
PSW
COUT
Rectifier
Control
ROUT
Switch
Control
FIGURE 4:
Simplified Current Flow
Schematic of MCP1640 Boost Converter.
The output voltage is held up by the external COUT
capacitor, because the True Output Disconnect feature
does not discharge it.
 2010 Microchip Technology Inc.
DS01337A-page 3
AN1337
THE MCP1640 APPLICATIONS
L1
4.7 µH
600
VOUT = 5.0V
500
IOUT (mA)
This section describes the practical aspects and
considerations when working with the MCP1640. An
example of a 3.3V @ 100 mA application schematic is
shown in Figure 5.
VOUT = 3.3V
400
VOUT = 2.0V
300
200
100
0
VIN
0.9V to 1.7V
SW V
OUT
VIN
Alkaline
+
CIN
4.7 to 10 µF
-
FIGURE 5:
Schematic.
VFB
EN
GND
VOUT
3.3V @ 100 mA
RT
536k
COUT
10 µF
RB
309k
3.3V @ 100 mA Application
0.5
The MCP1640 can also operate below 2.0V output
voltage, with some limitations. Detailed information for
applications with VOUT = 1.8V can be found in AN1311
[2].
The maximum device output current is dependent upon
the input and output voltage. For example, to ensure a
100 mA load current for VOUT = 3.3V, a minimum of
0.9V input voltage (VIN) is necessary. If an application
is powered by one Li-Ion battery (VIN from 3.0V to
4.2V), the maximum load current the MCP1640 can
deliver is 300 mA.
DS01337A-page 4
1.5
2
2.5
3
3.5
4
4.5
5
VIN (V)
FIGURE 6:
Input Voltage.
Maximum Output Current vs.
Figure 7 illustrates the No Load Input Current for both
modulation options: MCP1640/C (PWM/PFM) and
MCP1640B/D (PWM-only). This parameter depends
on the input voltage, and is much lower in PWM/PFM
mode. By pulling the EN pin low, the current drawn from
the input source will be less than 1 µA (in Shutdown
mode). This helps to increase battery lifetime.
Maximum Output Current and Voltage
Range
10000
VOUT=3.3V
PWM ONLY
1000
IIN (µA)
The MCP1640 converter starts from 0.65V input, and
will continuously operate down to 0.35V. The maximum
output voltage is 5.5V and the minimum is 2.0V, with
VIN < VOUT. For alkaline battery-powered applications,
it is recommended that the battery discharge is
terminated at 0.6V to 0.7V, to prevent the rupturing of
the cell. For rechargeable chemistries, follow the
manufacturers’ recommended cutoff voltage.
1
PWM / PFM
100
10
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
VIN (V)
FIGURE 7:
Input Voltage.
No Load Input Current vs.
 2010 Microchip Technology Inc.
AN1337
Components – Input and Output
Capacitors, Boost Inductor
and Feedback Resistors
This section describes the recommended components
to use with MCP1640 devices.
The typical input capacitance is 4.7 µF. If the device is
located far from the input source (battery), additional
capacitance can be added. For higher output current
battery powered applications, a 10 µF input capacitor is
recommended. For low output current applications that
operate in PWM mode only (MCP1640B/D), lower input
capacitance can be used. Figures 8, 9 and 10
demonstrate how the MCP1640B device works with a
0.1 µF input capacitor for different load currents (no
load, 5 mA and 15 mA). The input ripple is large, but
the system is stable. This low-cost solution can be used
for low duty cycle (short on time) applications.
FIGURE 8:
MCP1640B Working with
0.1 µF Input Capacitor, No Load and VIN = 1.2V.
FIGURE 10:
MCP1640B Working with
0.1 µF Input Capacitor, IOUT = 15 mA.
A 10 µF output capacitor is recommended for most
applications. To avoid instability, ceramic output
capacitors with 4.7 µF can be used with some
restrictions. The output voltage ripple will also be
affected by the reduction of the output capacitance.
AN1311 [2] describes the system stability using a
4.7 µF output capacitor, and also includes additional
information on the boost inductance and output
capacitance limits used with MCP1640.
The boost converter efficiency depends on the input/
output voltage and load current. The majority of losses
come from the internal switch resistance. For low input/
output voltage applications, the efficiency is lower than
in high input/output voltage applications. The boost
inductor resistance also impacts the efficiency. Larger
size inductors have lower resistance, resulting in higher
efficiency. This implies a trade-off between size, cost
and performance. The inductor represents a decisive
factor in the application design.
FIGURE 9:
MCP1640B Working with
0.1 µF Input Capacitor, IOUT = 5 mA.
 2010 Microchip Technology Inc.
DS01337A-page 5
AN1337
Figure 11 demonstrates the influence of size and RDC
(DC series resistance of inductors) in the design, for
two inductor types:
• 4.7 µH, RDC = 0.04, ISAT = 1.8A, 6x6x3 mm
• 4.7 µH, RDC = 0.256, ISAT = 0.7A, 3x3x1 mm
The lower the inductor RDC, the higher the efficiency.
90
Efficiency (%)
80
70
VOUT=3.3V
60
VIN=1.2V
50
VIN=0.8V
40
30
20
0.01
_____ 4.7uH, RDC=0.04 , ISAT=1.8A, 6x6x3mm Inductor
- - - - 4.7uH, RDC=0.265 , ISAT=0.7A, 3x3x1mm
0.1
1
10
100
1000
IOUT (mA)
The output capacitor does not affect only the output
voltage ripple. Efficiency is also affected by the
capacitor's equivalent series resistance. The resistive
loss depends on the selected capacitor type (ceramic,
aluminum or tantalum dielectric). The best choice is the
ceramic capacitor, which has the lower DC equivalent
resistance, ESR (less than 10 m). Aluminum types
have a few ohms of resistance. Figure 12 illustrates
how the efficiency and the maximum output current are
affected by different output capacitor types, when VOUT
= 3.3V and VIN = 1.2V.
Using high ESR capacitor types result in poor
efficiency. When running with a 10 µF ceramic output
capacitor, the MCP1640 generates a maximum of
150 mA at 3.3V output and 1.2V input. If the ceramic
capacitor is replaced with a 10 µF aluminum capacitor,
the maximum output current reached by the MCP1640
is approximately 65 mA. Figure 12 also shows a 15 µF
low ESR tantalum capacitor that performs with similar
efficiency to a 10 µF ceramic capacitor.
FIGURE 11:
Efficiency @ VOUT = 3.3V
vs. Output Current for Two Inductor Types
(with Different RDC and ISAT).
DS01337A-page 6
VIN=1.2V
V OUT=3.3V
80
Efficiency (%)
The boost inductor value can vary from 2.2 µH to
10 µH. An inductance value of 4.7 µH is recommended
to achieve a good balance between inductor size,
converter load transient response and noise. The
MCP1640 Data Sheet [1] describes several inductors
that can be used (see Section 5 in the Data Sheet).
Application Note AN1311 [2] also describes several
conditions, where inductors smaller or larger than
4.7 µH are used. Note that for boost converters, the
inductor’s current can be much higher than the output
current. When choosing the inductor current, look for
the saturation current parameter to be higher than the
peak input current. Saturation current typically
specifies a point where the inductance decreases a
percentage of the rated value. This percentage is
between 10% to 40%. As inductance decreases, the
inductor ripple current increases. Reaching the current
peak limit should be avoided.
90
70
60
50
____
____
____
____
40
10
10
10
15
µF, 35V, SMD, Aluminium Electrolytic Capacitor, low ESR
µF, 16V, THT, Tantalum Capacitor
µF, 10V, Ceramic Capacitor, X7R, 0805
µF, 10V, SMD, Tantalum Capacitor, A type
30
0.1
1
10
IOUT (mA)
100
1000
FIGURE 12:
Efficiency @ VOUT = 3.3V
vs. Output Current for Different Output Capacitors
Types.
 2010 Microchip Technology Inc.
AN1337
As mentioned previously, the output voltage range for
the MCP1640 is from 2.0V to 5.5V. The output voltage
is a function of the feedback voltage, derived from
RTOP and RBOT resistors, as shown in Figure 13. The
resistors’ values can be higher than indicated in
Figure 13. A potential issue with higher value resistors
is environmental contamination, which can create a
leakage current path on the PCB. This will affect the
feedback voltage and the output voltage regulation.
Designers should use resistors that are larger than
1 M with precaution. In normal humidity conditions,
the FB input leakage is very low and the resistors’
values will not affect the stability of the system. The
internal Error Amplifier is a trans-conductance type;
gain is not related to the resistors’ values. To calculate
the resistor values, the following equation can be used:
5.0V
VOUT
RTOP
976k
1.21V
VFB
RBOT
309k
FIGURE 13:
Feedback Resistors Divider
Values for 5.0V Output Voltage.
EQUATION 1:
V OUT
R TOP = R BOT   ------------- – 1
V FB
where VFB voltage for MCP1640 is 1.21V.
VOUT
4u7
P1
324k
RTOP2
L1
536k
GND
RTOP1
VIN
GND
P1
2.0V
6
2 GND VOUT 5
4
3
FB
EN
1 SW
EN Switch
C1
10 µF
P1
VIN
U1
MCP1640/B/C/D
S1
P1
VOUT SEL
3.3V
C2
10 µF
RBOT
309k
S1
P1
FIGURE 14:
MCP1640/B/C/D - SOT23, Two Output Voltages Options (2.0V and 3.3V) Using a
Switch to Connect RTOP Resistors in Parallel.
 2010 Microchip Technology Inc.
DS01337A-page 7
AN1337
RTOP = 536 k and RBOT = 309 k
or
RTOP = 6.8 M and RBOT = 3.9 M.
Manually-selected multiple output voltages can be
designed using jumpers or miniature switches. For
boost converters, the removal of the feedback resistors
when using jumpers, must be avoided. If the feedback
loop is opened, the output voltage will increase above
the absolute maximum output limits of the MCP1640
and damages the device. To solve this problem,
connect resistors in parallel with the switches, as
shown in Figure 14 (2.0V and 3.3V output application).
When switch VOUT SEL is open, the output is 3.3V,
because only the RTOP1 is connected. If the switch is
closed, the output is 2.0V, while RTOP1 and RTOP2 are
connected in parallel (the equivalent resistance is
approximately 202 k).
RTOP2 is calculated by using the resistance value for
VOUT1 = 3.3V and the equivalent resistance (REQ), for
VOUT2 = 2.0V.
EQUATION 2:
V OUT1 
R TOP1 = R BOT   --------------–1
 V FB

where: RBOT is user’s choice.
EQUATION 3:
VOUT2
R EQ = R BOT   --------------- – 1
V FB
where: REQ = RTOP1 II RTOP2
With RBOT selected and REQ, we can calculate RTOP2:
EQUATION 4:
 RTOP1  R EQ 
R TOP2 = ----------------------------------- R TOP1 – R EQ 
DS01337A-page 8
TIPS ON HOW TO INCREASE
BATTERY LIFE
MCP1640 was developed to increase battery lifetime.
Low input voltage operation, PFM/PWM mode, up to
96% efficiency, low quiescent current, True Output
Disconnect and Input-to-Output Bypass shutdown
options are only a few of the features that help extend
the battery life.
How to Estimate the Battery Service Time
The primary battery capacity (expressed in terms of
mAh) is an indication of the battery life for a specific
drain rate, at a specific cutoff voltage. For an alkaline
battery, the discharge curve (Battery Voltage vs.
Service Time) is given for a constant discharge current
and a specified cutoff voltage. Using this curve, the
available capacity can be obtained by multiplying the
drain current (mA) with time (hours) at the cutoff
voltage required. Figure 15 shows a typical 100 mA
constant current discharge curve at room temperature
for an AA/LR6 alkaline battery, that can be found in the
manufacturer's battery data sheet. For example, this
battery
would
have
a
capacity
of
100 mA x 25h = 2500 mAh under 100 mA drain, with a
0.8V cutoff. The same battery, at 1.2V cutoff and with
the same 100 mA drain current, would be
100 mA x 15h = 1500 mAh.
1.6
Battery Voltage (V)
As an example, for VOUT = 3.3V, the boost application
resistor values are:
100 mA Constant Current Discharge @ 21°C
1.4
1.2
1
0.8
0
5
10
15
20
25
Discharge Time (h)
FIGURE 15:
Typical AA/LR6 Alkaline
Battery Discharge Curve @ 100 mA to 0.8V
cutoff.
 2010 Microchip Technology Inc.
AN1337
In conclusion, the service time of the alkaline batteries
is dependent upon the discharge current and the cutoff
voltage. The primary battery is more efficient at lower
discharge currents, as shown in Figure 17. The cutoff
voltage will impact the battery run time. Generally, if the
battery is discharged to 0.8V, approximately 95% of the
battery capacity is used.
For rechargeable cells, a good start to approximate the
service time is Peukert's Law, elaborated by the
German scientist W. Peukert in 1897, which expressed
the capacity of a lead-acid battery in terms of the rate
at which the battery is discharged.
EQUATION 5:
C
t = ---k
I
1000
Where:
100
cutoff 1.0V
cutoff 1.1V
10
1
1
10
100
1000
Discharge Current (mA)
FIGURE 16:
Typical Constant Current
Discharge Characteristics @ 21oC to Different
Cutoff Voltages, for an AA/LR6 Alkaline Battery.
Battery Capacity (mAh)
3000
AA/LR6 Alkaline Battery
o
at 21 C and 0.8V cutoff
2500
2000
1500
t
=
time of discharge (h)
C
=
capacity of cell (Ah)
I
=
discharge current (A)
k
=
Peukert constant
For a lead-acid battery, the value of k is typically
between 1.1 and 1.3. However, for an ideal battery, the
constant k equals 1. In this case, the actual capacity is
independent of the drain current.
1.6
Battery Voltage (V)
Service Time (h)
cutoff 0.8V
Charge to 180mA x 16h @ 21oC; Discharge with:
900 mA (0.5C), 360 mA (0.2C) and 180 mA (0.1C)
1.5
1.4
1.3
1.2
1.1
1
0.5C
0.9
0.2C
0.1C
0.8
1000
0
500
2
4
6
8
10
12
Discharge Time (h)
0
25
100
250
500
Discharge Rate (mA)
FIGURE 18:
Typical Discharge Time vs.
Battery Voltage Graphs at Different Discharge
Rate, for 1800 mAh NiMh Battery.
FIGURE 17:
Battery Capacity vs. Drain
Current Chart @ 21oC to 0.8V Cutoff.
If the load does not require permanent constant
current, and the application is pulsed on and off, the
operating on-time can impact battery service time. The
amount of additional service time depends on the load
current and the on/off time of the load. In this case,
there is no simple equation to calculate the battery life.
For a boost convertor working at a constant output
current, the output power is also constant, therefore the
efficiency of the system must be considered (which is
high for MCP1640), to calculate the input current.
Because the current consumption increases as the
battery voltage drops, the input power can be
considered quasi-constant in low power applications, if
the efficiency is high. For such applications, the lifetime
estimation can be within an acceptable tolerance on
the curves presented in Figure 16, considering the
average power consumed.
 2010 Microchip Technology Inc.
DS01337A-page 9
AN1337
This is a simple way to estimate the lifetime of a
rechargeable battery. Figure 18 shows a typical
discharge curve for a 1800 mAh NiMh cell. Battery
lifetime depends on the charge current, discharge
current and cutoff voltage. If the 0.9V cutoff is used, the
estimated service time will be approximately:
EQUATION 6:
1800mAh
t = ------------------------ = 2h
900mA
when discharging with 0.5 C, or:
EQUATION 7:
1800mAh
t = ------------------------ = 5h
360mA
when discharging at 0.2 C.
Depending on the battery state – number of charging/
discharging cycles or charging algorithms, ambient
temperature – the lifetime decreases, in contrast with
the calculated value.
Regardless of the selected battery type, when
powering a boost DC-DC application, a boost device
with a lower input shutdown voltage and lower start-up
voltage, such as MCP1640, becomes important (down
to 0.35V).
Increasing Battery Service Time
Using MCP1640 – Tips and Tricks
The key features of the MCP1640 that help increase
the life of the battery are:
• Up to 96% efficiency
• PFM mode for lighter load (see Figures 3 and 7)
• Low input start-up voltage, typically 0.65V at 1 mA
load
• Low shutdown voltage (MCP1640/B/C/D devices
continuously operating down to 0.35V)
• True output disconnect EN option, preventing
leakage current from input to output by removing
the P-Channel MOS bulk diode (less than 1 µA is
consumed from the battery in this mode)
• 19 µA quiescent current
For applications powered by non-rechargeable
batteries, such as alkaline, that consume a few mA, the
MCP1640 device can operate to the minimum input
voltage necessary to completely remove all the energy
from the battery. As shown in Figure 19, the MCP1640
will start with 1 mA load from a minimum input of 0.65V,
and will continuously regulate the output voltage as the
input voltage drops to 0.35V. It is important to know the
minimum operating voltage of the MCP1640 device, to
estimate the life of the battery below the cutoff value
(0.8V).
1.00
1.20
VOUT = 3.3V
VOUT = 5.0V
0.85
1.00
Startup
VIN (V)
VIN (V)
Startup
0.70
0.55
0.80
0.60
Shutdown
Shutdown
0.40
0.40
0.25
0.20
0
20
40
60
IOUT (mA)
FIGURE 19:
DS01337A-page 10
80
100
0
10
20
30
40
50
60
IOUT (mA)
Minimum Start-up and Shutdown VIN into Resistive Load vs. IOUT.
 2010 Microchip Technology Inc.
AN1337
Depending on the design considerations (size, cost,
etc.) and load requirements, here are a few tips to
improve battery life:
USING THE INPUT-TO-OUTPUT BYPASS
OPTION (MCP1640C/D) FOR LONGER SLEEP
MODE LOADS
• Choose an inductor with lower DC series
resistance (see Figure 11)
• Choose input and output ceramic capacitors (with
lower DC series resistance)
• Increase output capacitor up to 100 µF (see
Figure 12)
• Increase the input capacitor to reduce the input
voltage ripple and lower the source impedance
• Increase feedback resistors (in terms of M)
• Pulse EN pin to turn on and off the device,
accepting a larger output ripple voltage to reduce
the average input current. In microcontroller
applications, this method can reduce no load
standby current.
When the EN pin is low, the MCP1640C and
MCP1640D enter in an Input-to-Output Bypass
Shutdown mode. During Shutdown, the internal
P-Channel MOS transistor is turned on and input
voltage is bypassed through the P-Channel to the
output. This option reduces the quiescent current, in
applications that operate in Sleep mode directly from
the source, but require a higher voltage for the normal
operating mode. In Shutdown mode, MCP1640C/D
consumes less than 1 µA from the battery. A part of the
current is also consumed by the feedback resistors.
INCREASING THE VALUE OF FEEDBACK
RESISTORS
The feedback resistor network (connected between
VOUT and GND) that biases the FB pin (RTOP and RBOT
in Figures 13 and 14) can be increased. Larger value
resistors will not affect MCP1640’s stability. If the
environmental conditions permit (no excessive
humidity), the megohm resistors can be used, without
affecting stability.
3.3V
RTOP
6M8
(536k)
VOUT
DISABLING FEEDBACK RESISTORS DURING
SHUTDOWN FOR MCP1640C/D
Depending on the values of the RTOP and RBOT , and
on the range of VOUT, the current consumed by the
feedback network can be several µA, which is more
than the MCP1640 consumes in Shutdown mode.
Analyzing Figure 13, when two batteries are in series
(VIN = 2.4 V typical), the current consumed by the
feedback resistors with the EN pin low can be
approximated using Equation 8:
EQUATION 8:
 2.4V 
I = ----------------------------------- = 1.87  A
 976k + 309k 
Note:
1.21V
VFB
RBOT
3M9
(309k)
FIGURE 20:
Increase of the Feedback
Resistors Value for a 3.3V Output.
Voltage on inductor or P-Channel is
not considered.
By increasing the RTOP and RBOT at 6.8 M and
3.9 M, the consumed current will be lower, as
demonstrated in the following equation:
EQUATION 9:
 2.4V 
I = ----------------------------------------------- = 0.23  A
 6.8M  + 3.9M  
Through Hole Technology (THT) resistors can be used
to avoid potential issues with environmental
contamination. Smaller package-sized resistors 0805
and 0603, with megohm values, can create a leakage
current path on PCB that will change the VFB voltage.
Tests with THT resistors have favorable results, for
example with RTOP = 6.8 M and RBOT = 3.9 M.
 2010 Microchip Technology Inc.
DS01337A-page 11
AN1337
VIN
VOUT
VOUT
EN
FB
RTOP
976k
RBOT
309k
VIN
VOUT
uC PIC
I/O
VDD
RTOP
976k
FB
VOUT
VFB
RBOT
309k
EN
FDN337N
DS01337A-page 12
VIN
VFB
Switch
FIGURE 21:
Bypass Option.
pin is low, the transistor is off, removing the feedback
current path. It is recommended to use an N-Channel
with a low VGSth. A good choice would be FDN337N,
with a gate threshold below 2V. Using the FDN337N for
the feedback divider, the input current for the
MCP1640C is reduced to 0.75 µA in Standby mode by
using the Input-to-Output bypass option.
MCP1640C
VIN
MCP1640C
One solution could be the removal of the feedback
resistors during shutdown by using an N-Channel
MOSFET to eliminate the FB divider current path, as
shown in Figure 21. The transistor’s gate is controlled
by the EN pin. When EN is high and MCP1640C/D is
operating in Boost mode, the N-Channel FET is turned
on, and the feedback network is closed. When the EN
Drive EN pin
from I/O PIC® MCU pin
FDN337N
Removing Feedback Resistors when EN is Low for MCP1640C, with Input-to-Output
 2010 Microchip Technology Inc.
AN1337
REDUCING STANDBY – NO LOAD
INPUT CURRENT IN
MICROCONTROLLER APPLICATIONS
microcontroller requires a minimum of 2V to operate. In
Sleep mode, the microcontroller consumes very few
µA. The input current measured for a typical application
similar to Figure 7 is 40 µA to 100 µA. The MCP1640
operating in PFM mode can be used in True Output
Disconnect mode, to lower the input current consumed
from the battery by using the microcontroller in Sleep
mode.
When an application is powered by a single alkaline or
NiCd/NiMh battery (VIN = 1.2V), and the application
operates for a long period in Standby mode (remote
controls, electronic torch, etc.), the block diagram
described in Figure 21 is not applicable, because the
Load Switch
VIN
CIN
10 µF
VOUT
MCP1640
FB
R1
1M
EN
RT
1M3
VOUT
COUT
10 µF
RM
240k
GP1 3
COUT
1.2V
ON/OFF
NDS7002
4
VDD 5
6 GP3
1
CIN+
PIC10F206
SOT23-6
RB
510k
VIN
CIN
10 µF
R1
1M
VOUT
MCP1640
FB
EN
RT
1M3
RM
240k
GP1 3
COUT
1.2V
ON/OFF
4
COUT
10 µF R
LOAD
Load
Switch
NDS7002
VDD 5
6 GP3
1
CIN+
PIC10F206
SOT23-6
RB
510k
FIGURE 22:
Typical Applications Using MCP1640C with PIC® Microcontroller Attached – Reducing
Standby No Load Current.
 2010 Microchip Technology Inc.
DS01337A-page 13
AN1337
For Sleep mode or light load applications, the
MCP1640’s enable input is pulsed at a slow rate to
reduce the average input current. The EN pin drive
frequency depends on the MCP1640 output capacitor
value and microcontroller sleep current. The
microcontroller will wake-up only to turn on the
MCP1640 for a short period of time to pump-up the
output voltage. The typical time to charge the output
capacitor voltage to 3.3V is 750 ns, with a load less
than 10 µA.
FIGURE 23:
Experimental Results – Output Voltage and Drive Signal (Left) and Short Pulse Input
Current (Right) Using Switching Method for EN Pin (also see Figures 22 and 25).
For example, Figure 22 shows two low-cost and lowcomponent applications that use a PIC10F206 to
perform the main goal: reduce the input current in
Standby when load is disconnected. When MCP1640 is
in Shutdown, it typically consumes 0.75 µA, but with
the analog comparator enabled, the PIC10F206
consumes more than 100 µA. To reduce this current,
the microcontroller operates in Sleep mode most of the
time. The comparator is periodically enabled (using the
internal timer of the microcontroller) to verify the output
voltage of the MCP1640. On the schematic in
Figure 22, the PIC10F206 consumes approximately
10 µA for a short period of time (when EN signal is
high), and about 2 µA when in Sleep mode.
To avoid losing power on the passive components, the
application also uses MCP1640’s feedback network as
an input to the PIC MCU comparator (CIN+ comparator
input). The inverter input, CIN-, is connected to a 0.6V
internal PIC MCU reference. For this application, the
threshold of the comparator is around 2.3V. The
positive duty cycle is less than 1%, and the frequency
of the EN signal is ~0.5 Hz (see Figure 23). The
microcontroller periodically enables the MCP1640 to
keep its bias at a minimum of 2.0V. Figure 24
demonstrates that the input current with no load is
reduced by approximately 87%, from 90 µA to 11 µA.
Using the push button as a wake-up feature, the EN
signal goes high permanently, powering the
microcontroller with a regulated 3.3V.
DS01337A-page 14
1000
No Load Current (uA)
There are different hardware and software methods to
determine the output voltage level of the MCP1640/C
device and/or the frequency of the EN signal used to
enable and disable the MCP1640.
VOUT = 3.3V, EN=1
100
10
VOUT = 3.4...2.3V, EN Switched
1
0.8
1
1.2
Input Voltage (V)
1.4
1.6
FIGURE 24:
No Load Current Reduced
with 87% Using EN Switched Method.
Because PIC10F206 is powered from the MCP1640’s
output, the application starts with EN high for a short
period. An N-Channel MOS transistor is used to drive
the EN pin.
 2010 Microchip Technology Inc.
AN1337
VOUT
R1
1M
R2
1M3
L1
1.5V
C2
10 µF
4µ7
NDS7002A
Q1
C1
10 µF
1 SW
6
VIN
2
5
GND VOUT
3 EN
FB 4
U1
MCP1640
R3
240k
LED
Load
VOUT
R0
120
R5
220
VOUT
PIC10F206
3
4 GP2/TOCKI/
GP1/ICSPCLK
COUT/FOSC4
/CIN5
VSS 2
VDD
6 GP3/MCLR/VP
VPP
Q2
FDN337
PGC
GP1/ICSPDAT 1
/CIN+
PGD
R4
510k
S1
J3
1
2
3
4
5
VPP
C5
1µ
VOUT
PGD
PGC
1
2
3
4
5
ICD2 Interface
FIGURE 25:
Application Example – MCP1640 and PIC10F206, to Reduce Standby Current.
The source code for MPLAB® IDE with HI-TECH C
compiler, used in the application illustrated in
Figure 25, is listed in Appendix A: “Source Code
Example”. The code can be easily modified to use with
any PIC microcontroller with compatible peripherals.
The Watchdog Timer enables the PIC MCU
periodically. Its internal comparator is enabled for a
short period of time to verify MCP1640’s output voltage
level. If VOUT is lower than the 2.3 V threshold voltage,
fixed by R2, R3 and R4 resistors, a short low-level
signal will drive the gate of NDS7002 transistor low,
enabling the MCP1640. The output capacitor holds the
output above 2.3V for more than two seconds.
 2010 Microchip Technology Inc.
This solution demonstrates a method that can be used
for any PIC MCU application that runs in Sleep mode
for extended periods of time. By implementing this
method, battery life can be extended up to 10 times.
DS01337A-page 15
AN1337
CONCLUSIONS
REFERENCES
In the low voltage boost applications that are powered
by batteries, the MCP1640 offers flexible options to
help increase the battery lifetime. The MCP1640
device can easily be attached to a microcontroller and
used in applications that work for extended periods of
time in Standby mode, because they consume less µA
of current than one-cell battery applications. Battery life
is extended by using the MCP1640 family, due to its
low operating voltage capability.
[1]
MCP1640/B/C/D Data Sheet, “0.65V Start-up
Synchronous Boost Regulator with True Output
Disconnect or Input/Output Bypass Option”,
(DS22234)
[2]
AN1311, “Single Cell Input Boost Converter
Design”, (DS01311)
[3]
“Alkaline Manganese Dioxide – Handbook and
Application Manual”, Energizer Battery Manufacturing Inc.
[4]
Energizer E91 Product Data Sheet, Energizer
Holdings, Inc.
[5]
“Alkaline
Manganese
Dioxide
Battery,
MN1500_US_CT, AA (LR6), Zn/MnO2 battery
Product Data Sheet”, Duracell®
[6]
GP180AAH Product Data Sheet, GP Batteries
DS01337A-page 16
 2010 Microchip Technology Inc.
AN1337
Software License Agreement
The software supplied herewith by Microchip Technology Incorporated (the “Company”) is intended and supplied to you, the
Company’s customer, for use solely and exclusively with products manufactured by the Company.
The software is owned by the Company and/or its supplier, and is protected under applicable copyright laws. All rights are reserved.
Any use in violation of the foregoing restrictions may subject the user to criminal sanctions under applicable laws, as well as to civil
liability for the breach of the terms and conditions of this license.
THIS SOFTWARE IS PROVIDED IN AN “AS IS” CONDITION. NO WARRANTIES, WHETHER EXPRESS, IMPLIED OR STATUTORY, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE APPLY TO THIS SOFTWARE. THE COMPANY SHALL NOT, IN ANY CIRCUMSTANCES, BE LIABLE FOR
SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES, FOR ANY REASON WHATSOEVER.
APPENDIX A:
SOURCE CODE EXAMPLE
//--------------------------------------------------------------------------// This software example is the property of Microchip Technology Incorporated
// Program:
MCP1640EV-LSBC.c
// Author:
Sergiu Oprea & Valentin C. Constantin (Microchip)
//
// PIC Processor: PIC10F206
// Description:
//
Demonstrated reducing no load current for MCP1640
//
in PIC attach application using HI-TECH C compiler
//
// Modifications: //
//---------------------------------------------------------------------------
#include <htc.h>
__CONFIG (WDTEN & MCLRDIS & UNPROTECT); // start with GP3 as input pin & WDT enabled
//--------------------------------------------------------------------------// Section:
Processor I/O Definitions
//--------------------------------------------------------------------------#define EN
GP2
#define LED GP1
//DRIVE MCP1640 EN PIN FROM GP2 WITH INVERTED SIGNAL
//DRIVE LED LOAD FROM GP1
#define button GP3
//INPUT BUTTON ON PORT GP3
//--------------------------------------------------------------------------// Section:
COMPARATOR SETUP
//--------------------------------------------------------------------------#define COMP_SETUP_1 0b01111011; //comparator enabled
#define COMP_SETUP_2 0b01110011; //comparator disabled
//--------------------------------------------------------------------------// Section:
WORKING VARIABILE
 2010 Microchip Technology Inc.
DS01337A-page 17
AN1337
//--------------------------------------------------------------------------unsigned char temp = 0x00;
bit
button_state; // STAE OF PUSH-BUTTON
bit
LED_STATE;
// STAE OF LOAD-LED
//--------------------------------------------------------------------------// Code Segment
//--------------------------------------------------------------------------/****************************************************************************
Function:
void main (void)
Summary:
Main program entry point.
Description:
Main program entry point.
The system will initialize the PIC processor
and peripherals and then loop forever while monitoring the MCP1640 state.
Returns:
None
**************************************************************************/
void main(void)
{
OPTION = 0b11011011;
//GP2 is set as output
TRIS = 0b11111001;
//GP2, GP1 direction is output
CMCON0 = COMP_SETUP_2;
//Comparator is disabled
if((STATUS & 0xF8) == 0x18) //Power On Reset?
{
EN = 0;
//On Power On Reset starts MCP1640 switching
}
LED = 0;
//Turn-off the load - LED
button_state = 0;//if push-button hold-on set the button state to low,
temp = 0;
LED_STATE = 0;
//and LED state to low
/* main forever loop */
while(1)
{
if(!button)
//if push-button is pressed,
{
CLRWDT();
temp++;
//wait
if(temp == 20)
{
DS01337A-page 18
 2010 Microchip Technology Inc.
AN1337
temp = 0;
if(!button_state)
//and button state is low,
{
if(LED==0)
//and LED off
{
EN = 0;
//turn ON the MCP1640 output,
for(temp=0;temp<100;temp++) NOP();
LED_STATE = 1;
LED = 1;
//and turn ON the LED
}
else { LED_STATE = 0; LED = 0; }
button_state = 1;
//else keep the LED OFF
}
}
}
else button_state = 0;
//if no push button pushed detected and LED is OFF:
if ((LED_STATE==0)&&(button))
{
CLRWDT();
CMCON0 = COMP_SETUP_1; //Enable comparator; 0.6V internal reference
for(temp=0;temp<10;temp++) NOP();
//delay for stable comp output
if(CMCON0&0x80)
//check comparator output and
{
CMCON0 = COMP_SETUP_2;
//disable it,
EN = 1;
//stop switching MCP1640
temp = GPIO;
//read the output latch to avoid false interrupt on PIN Change
SLEEP();
//and go to SLEEP Mode
}
else
{
CMCON0 = COMP_SETUP_2;
//else keep disable it,
EN = 0;
//start MCP1640
for(temp=0;temp<150;temp++) NOP(); //for short period
EN = 1;
//and stop it.
temp = GPIO; //read the output latch to avoid false interrupt on PIN Change
SLEEP();
//and go to SLEEP Mode
}
}
else
{
CLRWDT();
//reset the internal timer
}
}
}
 2010 Microchip Technology Inc.
DS01337A-page 19
AN1337
NOTES:
DS01337A-page 20
 2010 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
devices in life support and/or safety applications is entirely at
the buyer’s risk, and the buyer agrees to defend, indemnify and
hold harmless Microchip from any and all damages, claims,
suits, or expenses resulting from such use. No licenses are
conveyed, implicitly or otherwise, under any Microchip
intellectual property rights.
Trademarks
The Microchip name and logo, the Microchip logo, dsPIC,
KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART,
PIC32 logo, rfPIC and UNI/O are registered trademarks of
Microchip Technology Incorporated in the U.S.A. and other
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FilterLab, Hampshire, HI-TECH C, Linear Active Thermistor,
MXDEV, MXLAB, SEEVAL and The Embedded Control
Solutions Company are registered trademarks of Microchip
Technology Incorporated in the U.S.A.
Analog-for-the-Digital Age, Application Maestro, CodeGuard,
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ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial
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SQTP is a service mark of Microchip Technology Incorporated
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All other trademarks mentioned herein are property of their
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© 2010, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
ISBN: 978-1-60932-566-4
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and India. The Company’s quality system processes and procedures
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devices, Serial EEPROMs, microperipherals, nonvolatile memory and
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and manufacture of development systems is ISO 9001:2000 certified.
 2010 Microchip Technology Inc.
DS01337A-page 21
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DS01337A-page 22
 2010 Microchip Technology Inc.
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