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 countries. 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, dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN, ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mTouch, Omniscient Code Generation, PICC, PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, REAL ICE, rfLAB, Select Mode, Total Endurance, TSHARC, UniWinDriver, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2010, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. ISBN: 978-1-60932-566-4 Microchip received ISO/TS-16949:2002 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified. 2010 Microchip Technology Inc. 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