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MIC2571
Micrel
MIC2571
Single-Cell Switching Regulator Preliminary Information
General Description
Micrel's MIC2571 is a micropower boost switching regulator that operates from one alkaline, nickel-metal-hydride cell, or lithium cell. The MIC2571 accepts a positive input voltage between 0.9V and 15V. Its typical no-load supply current is 120A. The MIC2571 is available in selectable fixed output or adjustable output versions. The MIC2571-1 can be configured for 2.85V, 3.3V, or 5V by connecting one of three separate feedback pins to the output. The MIC2571-2 can be configured for an output voltage ranging between its input voltage and 36V, using an external resistor network. The MIC2571 has a fixed switching frequency of 20kHz. An external SYNC connection allows the switching frequency to be synchronized to an external signal. The MIC2571 requires only four components (diode, inductor, input capacitor and output capacitor) to implement a boost regulator. A complete regulator can be constructed in a 0.3 in2 area. All versions are available in an 8-lead MSOP with an operating range from -40C to +85.
Features
* Operates from a single-cell supply 0.9V to 15V operation * 120A typical quiescent current * Complete regulator fits 0.3 in2 area * 2.85V/3.3V/5V selectable output voltage (MIC2571-1) * Adjustable output up to 36V (MIC2571-2) * 1A current limited pass element * Frequency synchronization input * 8-lead MSOP package
Applications
* * * * * * * Pagers LCD bias generator Battery-powered, hand-held instruments Palmtop computers Remote controls Detectors Battery Backup Supplies
Typical Applications
L1 150H
8
D1 MBR0530 5V/5mA
8
L1 150H
D1 MBR0530 3.3V/8mA
IN SW 1V to1.5V 1 Cell C1* 47F 16V 2.85V MIC2571-1 SYNC
7
1 6 5 4
IN SW 1V to1.5V 1 Cell C2 47F 16V C1* 47F 16V 2.85V MIC2571-1 SYNC
7
1 6 5 4
3.3V 5V GND
2
3.3V 5V
GND
2
C2 47F 16V
* Needed if battery is 4" from MIC2571 Circuit size < 0.3 in2 excluding C1
* Needed if battery is 4" from MIC2571 Circuit size < 0.3 in2 excluding C1
Single-Cell to 5V DC-to-DC Converter
Single-Cell to 3.3V DC-to-DC Converter
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Ordering Information
Part Number MIC2571-1BMM MIC2571-2BMM Temperature Range -40C to +85C -40C to +85C Voltage Selectable* Adjustable Frequency 20kHz 20kHz Package 8-lead MSOP 8-lead MSOP
* Externally selectable for 2.85V, 3.3V, or 5V
Pin Configuration
MIC2571-1
SW GND NC 5V 1 2 3 4 8 7 6 5 IN SYNC 2.85V 3.3V SW GND NC NC 1 2 3 4
MIC2571-2
8 7 6 5 IN SYNC FB NC
Selectable Voltage 20kHz Frequency
Adjustable Voltage 20kHz Frequency
8-Lead MSOP (MM)
4
Pin Description
Pin No. (Version) 1 2 3 4 (-1) 4 (-2) 5 (-1) 5 (-2) 6 (-1) 6 (-2) 7 8
Pin Name SW GND NC 5V NC 3.3V NC 2.85V FB SYNC IN
Pin Function Switch: NPN output switch transistor collector. Power Ground: NPN output switch transistor emitter. Not internally connected. 5V Feedback (Input): Fixed 5V feedback to internal resistive divider. Not internally connected. 3.3V Feedback (Input): Fixed 3.3V feedback to internal resistive divider. Not internally connected. 2.85V Feedback (Input): Fixed 2.85V feedback to internal resistive divider. Feedback (Input): 0.22V feedback from external voltage divider network. Synchronization (Input): Oscillator start timing. Oscillator synchronizes to falling edge of sync signal. Supply (Input): Positive supply voltage input.
Example: (-1) indicates the pin description is applicable to the MIC2571-1 only.
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Absolute Maximum Ratings
Supply Voltage (VIN) ..................................................... 18V Switch Voltage (VSW) .................................................... 36V Switch Current (ISW) ....................................................... 1A Sync Voltage (VSYNC) .................................... -0.3V to 15V Storage Temperature (TA) ....................... -65C to +150C MSOP Power Dissipation (PD) ................................ 250mW
Operating Ratings
Supply Voltage (VIN) .................................... +0.9V to +15V Ambient Operating Temperature (TA) ........ -40C to +85C Junction Temperature (TJ) ....................... -40C to +125C MSOP Thermal Resistance (JA) .......................... 240C/W
Electrical Characteristics
VIN = 1.5V; TA = 25C, bold indicates -40C TA 85C; unless noted Parameter Input Voltage Quiescent Current Fixed Feedback Voltage Condition Startup guaranteed, ISW = 100mA Output switch off MIC2571-1; V2.85V pin = VOUT, ISW = 100mA MIC2571-1; V3.3V pin = VOUT, ISW = 100mA MIC2571-1; V5V pin = VOUT, ISW = 100mA MIC2571-2, [adj. voltage versions], ISW = 100mA, Note 1 MIC2571-2, [adj. voltage versions] MIC2571-1; V2.85V pin = VOUT, ISW = 100mA MIC2571-1; V3.3V pin = VOUT, ISW = 100mA MIC2571-1; V5V pin = VOUT, ISW = 100mA MIC2571-1; V2.85V pin = VOUT MIC2571-1; V3.3V pin = VOUT MIC2571-1; V5V pin = VOUT MIC2571-2, [adj. voltage versions]; VFB = 0V 1.0V VIN 12V VIN = 1.0V, ISW = 200mA VIN = 1.2V, ISW = 600mA VIN = 1.5V, ISW = 800mA Output switch off, VSW = 36V MIC2571-1, -2; ISW = 100mA Min 0.9 120 2.85 3.30 5.00 220 220 6 65 75 120 4.5 4.5 4.5 25 0.35 200 400 500 1 20 36 0.7 35 1.1 VFB < VREF, ISW = 100mA 67 Typ Max 15 Units V V A V V V mV mV mV mV mV mV A A A nA %/V mV mV mV A kHz V V s A %
Reference Voltage Comparator Hysteresis Output Hysteresis
Feedback Current
Reference Line Regulation Switch Saturation Voltage
Switch Leakage Current Oscillator Frequency Maximum Output Voltage Sync Threshold Voltage Switch On Time Currrent Limit Duty Cycle
General Note: Devices are ESD protected; however, handling precautions are recommended. Note 1: Measured using comparator trip point.
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Typical Characteristics
Switch Saturation Voltage
1.0 TA = -40C SWITCH CURRENT (A) SWITCH CURRENT (A) 0.8 0.6 0.4 0.2 VIN = 1.0V 0 0 0.2 0.4 0.6 0.8 SWITCH VOLTAGE (V) 1.0 0 1.4V 1.3V 1.2V 1.1V 0.8 1.0
Switch Saturation Voltage
1.0 1.4V TA = 25C 0.6 0.4 0.2 1.1V 1.0V VIN = 0.9V 0 0.2 0.4 0.6 0.8 SWITCH VOLTAGE (V) 1.0 1.3V SWITCH CURRENT (A) 1.2V 0.8 0.6
Switch Saturation Voltage
1.4V TA = 85C 1.3V 1.2V 1.1V 1.0V
0.4 0.2 0
VIN = 0.9V
0
0.2 0.4 0.6 0.8 SWITCH VOLTAGE (V)
1.0
Oscillator Frequency vs. Temperature
30 OSC. FREQUENCY (kHz) VIN = 1.5V ISW = 100mA 25 75
Oscillator Duty Cycle vs. Temperature
200 QUIESCENT CURRENT (A) VIN = 1.5V ISW = 100mA 175 150 125 100 75
Quiescent Current vs. Temperature
VIN = 1.5V
DUTY CYCLE (%)
70 65 60 55
20
4
15 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
50 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
50 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
Feedback Current vs. Temperature
10 FEEDBACK CURRENT (A) FEEDBACK CURRENT (nA) 8 6 4 2 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C) VIN = 1.5V MIC2571-1 50 40 30 20 10
Feedback Current vs. Temperature
200 VIN = 2.5V MIC2571-2 QUIESCENT CURRENT (A) 175 150 125 100 75 50 25 0 0
Quiescent Current vs. Supply Voltage
-40C +25C +85C
0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
2 4 6 8 SUPPLY VOLTAGE (V)
10
SWITCH LEAKAGE CURRENT (nA)
Output Current Limit vs. Temperature
1.75 1.50 CURRENT LIMIT (A) 1.25 1.00 0.75 0.50 0.25 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
Switch Leakage Current vs. Temperature
1000 OUTPUT HYSTERESIS (mV) 100 10 1 0.1 0.01 -60 -30 0 30 60 90 120 150 TEMPERATURE (C) 150 125 100
Output Hysteresis vs. Temperature
5V
3.3V 75 50 25 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C) VOUT = 2.85V
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Block Diagrams
VBATT IN SYNC MIC2571-1 Oscillator 0.22V Reference VOUT
Driver
SW
5V
3.3V
2.85V
GND
Selectable Voltage Version with External Components
VBATT IN SYNC MIC2571-2 Oscillator 0.22V Reference
VOUT
Driver
SW
FB
GND
Adjustable Voltage Version with External Components
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Functional Description
The MIC2571 switch-mode power supply (SMPS) is a gated oscillator architecture designed to operate from an input voltage as low as 0.9V and provide a high-efficiency fixed or adjustable regulated output voltage. One advantage of this architecture is that the output switch is disabled whenever the output voltage is above the feedback comparator threshold thereby greatly reducing quiescent current and improving efficiency, especially at low output currents. Refer to the Block Diagrams for the following discription of typical gated oscillator boost regulator function. The bandgap reference provides a constant 0.22V over a wide range of input voltage and junction temperature. The comparator senses the output voltage through an internal or external resistor divider and compares it to the bandgap reference voltage. When the voltage at the inverting input of the comparator is below 0.22V, the comparator output is high and the output of the oscillator is allowed to pass through the AND gate to the output driver and output switch. The output switch then turns on and off storing energy in the inductor. When the output switch is on (low) energy is stored in the inductor; when the switch is off (high) the stored energy is dumped into the output capacitor which causes the output voltage to rise. When the output voltage is high enough to cause the comparator output to be low (inverting input voltage is above 0.22V) the AND gate is disabled and the output switch remains off (high). The output switch remains disabled until the output voltage falls low enough to cause the comparator output to go high. There is about 6mV of hysteresis built into the comparator to prevent jitter about the switch point. Due to the gain of the feedback resistor divider the voltage at VOUT experiences about 120mV of hysteresis for a 5V output.
Supply Voltage
5V
VIN
0V IPEAK 0mA
Output Voltage
Inductor Current
5V Time
Figure 1. Typical Boost Regulator Waveforms Synchronization The SYNC pin is used to synchronize the MIC2571 to an external oscillator or clock signal. This can reduce system noise by correlating switching noise with a known system frequency. When not in use, the SYNC pin should be grounded to prevent spurious circuit operation. A falling edge at the SYNC input triggers a one-shot pulse which resets the oscillator. It is possible to use the SYNC pin to generate oscillator duty cycles from approximately 20% up to the nominal duty cycle. Current Limit Current limit for the MIC2571 is internally set with a resistor. It functions by modifying the oscillator duty cycle and frequency. When current exceeds 1.2A, the duty cycle is reduced (switch on-time is reduced, off-time is unaffected) and the corresponding frequency is increased. In this way less time is available for the inductor current to build up while maintaining the same discharge time. The onset of current limit is soft rather than abrupt but sufficient to protect the inductor and output switch from damage. Certain combinations of input voltage, output voltage and load current can cause the inductor to go into a continuous mode of operation. This is what happens when the inductor current can not fall to zero and occurs when:
4
Appications Information
Oscillator Duty Cycle and Frequency The oscillator duty cycle is set to 67% which is optimized to provide maximum load current for output voltages approximately 3x larger than the input voltage. Other output voltages are also easily generated but at a small cost in efficiency. The fixed oscillator frequency (options -1 and -2) is set to 20kHz.
duty cycle
VOUT + VDIODE - VIN VOUT + VDIODE - VSAT
Output Waveforms The voltage waveform seen at the collector of the output switch (SW pin) is either a continuous value equal to VIN or a switching waveform running at a frequency and duty cycle set by the oscillator. The continuous voltage equal to VIN happens when the voltage at the output (VOUT) is high enough to cause the comparator to disable the AND gate. In this state the output switch is off and no switching of the inductor occurs. When VOUT drops low enough to cause the comparator output to change to the high state the output switch is driven by the oscillator. See Figure 1 for typical voltage waveforms in a boost application.
Current "ratchet" without current limit
Inductor Current
Current Limit Threshold
Continuous Current
Discontinuous Current Time
Figure 2. Current Limit Behavior 1997 4-81
MIC2571
Figure 2 shows an example of inductor current in the continuous mode with its associated change in oscillator frequency and duty cycle. This situation is most likely to occur with relatively small inductor values, large input voltage variations and output voltages which are less than ~3x the input voltage. Selection of an inductor with a saturation threshold above 1.2A will insure that the system can withstand these conditions. Inductors, Capacitors and Diodes The importance of choosing correct inductors, capacitors and diodes can not be ignored. Poor choices for these components can cause problems as severe as circuit failure or as subtle as poorer than expected efficiency.
Micrel Capacitors It is important to select high-quality, low ESR, filter capacitors for the output of the regulator circuit. High ESR in the output capacitor causes excessive ripple due to the voltage drop across the ESR. A triangular current pulse with a peak of 500mA into a 200m ESR can cause 100mV of ripple at the output due the capacitor only. Acceptable values of ESR are typically in the 50m range. Inexpensive aluminum electrolytic capacitors usually are the worst choice while tantalum capacitors are typically better. Figure 4 demonstrates the effect of capacitor ESR on output ripple voltage.
5.25 OUTPUT VOLTAGE (V)
a. Inductor Current
5.00
b.
c.
4.75
Time
0
500 1000 TIME (s)
1500
Figure 3. Inductor Current: a. Normal, b. Saturating and c. Excessive ESR
Figure 4. Output Ripple
Inductors Inductors must be selected such that they do not saturate under maximum current conditions. When an inductor saturates, its effective inductance drops rapidly and the current can suddenly jump to very high and destructive values. Figure 3 compares inductors with currents that are correct and unacceptable due to core saturation. The inductors have the same nominal inductance but Figure 3b has a lower saturation threshold. Another consideration in the selection of inductors is the radiated energy. In general, toroids have the best radiation characteristics while bobbins have the worst. Some bobbins have caps or enclosures which significantly reduce stray radiation. The last electrical characteristic of the inductor that must be considered is ESR (equivalent series resistance). Figure 3c shows the current waveform when ESR is excessive. The normal symptom of excessive ESR is reduced power transfer efficiency. Note that inductor ESR can be used to the designers advantage as reverse battery protection (current limit) for the case of relatively low output power one-cell designs. The potential for very large and destructive currents exits if a battery in a one-cell application is inserted backwards into the circuit. In some applications it is possible to limit the current to a nondestructive (but still battery draining) level by choosing a relatively high inductor ESR value which does not affect normal circuit performance.
Output Diode Finally, the output diode must be selected to have adequate reverse breakdown voltage and low forward voltage at the application current. Schottky diodes typically meet these requirements. Standard silicon diodes have forward voltages which are too large except in extremely low power applications. They can also be very slow, especially those suited to power rectification such as the 1N400x series, which affects efficiency. Inductor Behavior The inductor is an energy storage and transfer device. Its behavior (neglecting series resistance) is described by the following equation:
I= V xt L
where: V = inductor voltage (V) L = inductor value (H) t = time (s) I = inductor current (A) If a voltage is applied across an inductor (initial current is zero) for a known time, the current flowing through the inductor is a linear ramp starting at zero, reaching a maximum value at the end of the period. When the output switch is on, the voltage across the inductor is:
V1 = VIN - VSAT
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When the output switch turns off, the voltage across the inductor changes sign and flies high in an attempt to maintain a constant current. The inductor voltage will eventually be clamped to a diode drop above VOUT. Therefore, when the output switch is off, the voltage across the inductor is:
Micrel
Referring to Figure 1, it can be seen the peak input current will be twice the average input current. Rearranging the inductor equation to solve for L: L= V x t1 I
VIN(min) 2 x Average IIN(max) x t1
V2 = VOUT + VDIODE - VIN
For normal operation the inductor current is a triangular waveform which returns to zero current (discontinuous mode) at each cycle. At the threshold between continuous and discontinuous operation we can use the fact that I1 = I2 to get:
L=
V1 x t1 = V2 x t 2
t V1 =2 t1 V2
where t1 =
duty cycle fOSC
To illustrate the use of these equations a design example will be given: Assume: MIC2571-1 (fixed oscillator) VOUT = 5V IOUT(max) =5mA VIN(min) = 1.0V efficiency = 75%. Average IIN(max) = L= 5V x 5mA = 33.3mA 1.0V x 0.75
This relationship is useful for finding the desired oscillator duty cycle based on input and output voltages. Since input voltages typically vary widely over the life of the battery, care must be taken to consider the worst case voltage for each parameter. For example, the worst case for t1 is when VIN is at its minimum value and the worst case for t2 is when VIN is at its maximum value (assuming that VOUT, VDIODE and VSAT do not change much). To select an inductor for a particular application, the worst case input and output conditions must be determined. Based on the worst case output current we can estimate efficiency and therefore the required input current. Remember that this is power conversion, so the worst case average input current will occur at maximum output current and minimum input voltage.
Average IIN(max) = VOUT x IOUT(max) VIN(min) x Efficiency
1.0V x 0.7 2 x 33.3mA x 20kHz L = 525H Use the next lowest standard value of inductor and verify that it does not saturate at a current below about 75mA (< 2 x 33.3mA).
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Application Examples
L1 150H
8
D1 MBR0530
1
VOUT 5V/5mA
IN 1V to 1.5V 1 Cell C1* 47F 16V MIC2571
SW 5V
4
SYNC GND
7 2
C2 47F 16V
* Needed if battery is more than 4" away from MIC2571 U1 C1 C2 D1 L1 Micrel Sprague Sprague Motorola Coilcraft MIC2571-1BMM 594D476X0016C2T Tantalum ESR = 0.11 594D476X0016C2T Tantalum ESR = 0.11 MBR0530T1 DO1608C-154 DCR = 1.7
Example 1. 5V/5mA Regulator
L1 150H
8
D1 MBR0530
1
VOUT 3.3V/8mA
IN 1V to 1.5V 1 Cell C1* 47F 16V MIC2571
SW
5
3.3V SYNC GND
7 2
C2 47F 16V
* Needed if battery is more than 4" away from MIC2571 U1 C1 C2 D1 L1 Micrel Sprague Sprague Motorola Coilcraft MIC2571-1BMM 594D476X0016C2T Tantalum ESR = 0.11 594D476X0016C2T Tantalum ESR = 0.11 MBR0530T1 DO1608C-154 DCR = 1.7
Example 2. 3.3V/8mA Regulator
L1 150H
8
D1 MBR0530
1
VOUT 12V/2mA
IN 1.0V to 1.5V 1 Cell C1* 47F 16V MIC2571
SW FB
R2 1M 1%
6
C2 15F 25V
SYNC GND
7 2
R1 20k 1%
* Needed if battery is more than 4" away from MIC2571 VOUT = 0.22V U1 C1 C2 D1 L1 Micrel Sprague Sprague Motorola Coilcraft (1 + R2/R1) MIC2570-2BMM 594D476X0016C2T Tantalum ESR = 0.11 594D156X0025C2T Tantalum ESR = 0.22 MBRA0530T1 DO1608C-154 DCR = 1.7
Example 3. 12V/40mA Regulator
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MIC2571
L1 150H
8
Micrel
D1 C3 47F 16V MBR0530 VOUT/+IOUT 5V/2mA
IN 1V to 1.5V 1 Cell C1* 47F 16V MIC2571
SW
1
5V SYNC
7
4
GND
2
C2 47F 16V
D2 MBR0530 D3 MBR0530 R1 220k C4 47F 16V -VOUT/-IOUT -5V/2mA -IOUT +IOUT
* Needed if battery is more than 4" away from MIC2571 U1 C1 C2 C3 C4 D1 D2 D3 L1 Micrel Sprague Sprague Sprague Sprague Motorola Motorola Motorola Coilcraft
MIC2571-1BMM 594D476X0016C2T Tantalum ESR = 0.11 594D476X0016C2T Tantalum ESR = 0.11 594D476X0016C2T Tantalum ESR = 0.11 594D476X0016C2T Tantalum ESR = 0.11 MBR0530T1 MBR0530T1 MBR0530T1 DO1608C-154 DCR = 1.7
Example 4. 5V/2mA Regulator
L1 1V to 1.5V 1 Cell 47H Q1 2N3906 D1 MBRA140 VOUT 5V/15mA
C1 100F 10V
8
R1 51k
IN MIC2571
1
SW 5V
4
4
C2 100F 10V
SYNC GND
7 2
Minimum Start-Up Supply Voltage VIN = 1V, ILOAD = 0A VIN = 1.2V, ILOAD = 15mA U1 Micrel MIC2571-1BMM C1 AVX TPSD107M010R0100 Tantalum ESR = 0.1 C2 AVX TPSD107M010R0100 Tantalum ESR = 0.1 D1 Motorola MBRA140T3 L1 Coilcraft DO3308P-473 DCR = 0.32
Example 5. 5V/15mA Regulator
L1 1V to 1.5V 1 Cell
8
D3 1N4148
1
150H IN
SW FB
C1 15F 25V
R2 1.1MEG 1.1% C2 0.1F R1 20k 1%
C1 47F 16V
MIC2571
6
SYNC GND
7 2
D1 MBR0530
-VOUT = - 0.22V *(1+R2/R1) + 0.6V U1 C1 C2 C3 D1 D2 L1 Micrel Sprague Sprague Sprague Motorola Motorola Coilcraft
D2 MBR0530
R3 220k
MIC2571-2BM 594D476X0016C2T Tantalum ESR = 0.11 594D156X0025C2T Tantalum ESR = 0.22 594D156X0025C2T Tantalum ESR = 0.22 MBR0530T1 MBR0530T1 DO1608C-154 DCR = 1.7
C2 15F 25V -VOUT -12V/2mA
Example 6. -12V/2mA Regulator
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Suggested Manufacturers List
Inductors Capacitors Diodes
Coilcraft 1102 Silver Lake Rd. Cary, IL 60013 PH (708) 639-2361
FX (708) 639-1469
AVX Corp. 801 17th Ave. South Myrtle Beach, SC 29577 PH (803) 448-9411
FX (803) 448-1943
General Instruments (GI) 10 Melville Park Rd. Melville, NY 11747 PH (516) 847-3222
FX (516) 847-3150
Coiltronics 6000 Park of Commerce Blvd. Boca Raton, FL 33487 PH (407) 241-7876
FX (407) 241-9339
Sanyo Video Components Corp. 2001 Sanyo Ave. San Diego, CA 92173 PH (619) 661-6835
FX (619) 661-1055
International Rectifier Corp. 233 Kansas St. El Segundo, CA 90245 PH (310) 322-3331
FX (310) 322-3332
Sumida 637 E. Golf Road, Suite 209 Arlington Heights, IL PH (708) 956-0666 FX (708) 956-0702
Sprague Electric
Lower Main Street 60005Sanford, ME 04073 PH (207) 324-4140
Motorola Inc. 3102 North 56th St. MS 56-126 Phoenix, AZ 85018 PH (602) 244-3576
FX (602) 244-4015
Evaluation Board Layout
Component Side and Silk Screen (Not Actual Size)
Solder Side and Silk Screen (Not Actual Size)
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