DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS LT1611/LT1613 1.4MHz Switching Regulators in SOT-23 U DESCRIPTIO The LT®1611 and LT1613 are 5-lead SOT-23, current mode DC/DC converters. Intended for small, low power applications, both operate from inputs as low as 1V and switch at 1.4MHz, allowing the use of tiny, low cost capacitors and inductors. DC233 contains three switching regulator circuits. Two of these demonstrate the use of the LT1613CS5 in a simple boost regulator circuit and in an uncoupled SEPIC circuit. Both circuits produce 3.3V or 5V (jumper selected). The boost circuit produces 200mA in a typical application and occupies less than 0.2 square inches of circuit board area. The SEPIC circuit allows operation from input voltages either higher or lower than the output, making this circuit suitable for single Li-Ion cell to 3.3V conversion or four alkaline cells to 5V conversion. Typical output current for the SEPIC circuit is 120mA. The third circuit demonstrates the LT1611CS5 in a low noise inverting circuit. This circuit can convert 5V to – 5V at 160mA. , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation. W U WW PERFOR A CE SU ARY PARAMETER CONDITIONS VALUE Boost Input Voltage (Note 1) VOUT = 3.3V VOUT = 5V 1V to 3.6V 1V to 5.3V Maximum Load Current (Min) VOUT = 3.3V, VIN = 1.5V VOUT = 5V, VIN = 3V 115mA 190mA Shutdown Current (Typ) VIN = 1.5V, SHDN = 0V 10µA W U U TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO 5VOUT Efficiency (Boost) 90 90 80 80 VIN = 2.4V 70 EFFICIENCY (%) EFFICIENCY (%) 3.3VOUT Efficiency (Boost) VIN = 1.5V 60 50 40 30 VIN = 3.3V VIN = 2.4V 70 60 50 40 0 50 100 150 200 LOAD CURRENT (mA) 250 300 DC233 TA01 30 0 50 100 150 200 LOAD CURRENT (mA) 250 300 DC233 TA02 1 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U DESCRIPTIO The LT1611 and LT1613 will find applications in batterypowered products, such as pagers, digital cameras, cellular phones, cordless phones and palmtop computers. The small circuit size and low component count make these parts suitable for use in PC cards, miniature disk drives and flash memory products, and for generating local logic supplies—for example, converting 3.3V to 5V. The LT1611 produces a very low noise negative output and is suitable for generating negative rails for op amp circuits and disk drives. WW U W PERFOR A CE SU ARY PARAMETER CONDITIONS VALUE SEPIC Input Voltage (Note 1) 1V to 6V Maximum Load Current (Min) VOUT = 3.3V, VIN = 3V VOUT = 5V, VIN = 5V 130mA 120mA Shutdown Current (Typ) VIN = 3V, SHDN = 0V 0.5µA Inverter Input Voltage (Note 1) 1V to 6V Maximum Load Current (Min) VOUT = – 5V, VIN = 5V 165mA Shutdown Current (Typ) VIN = 5V, SHDN = 0V 0.5µA Note 1: This limit is based on the DC233 circuits. The LT1611 and LT1613 can operate from higher supply voltages. U W TYPICAL PERFOR A CE CHARACTERISTICS 5VOUT Efficiency (SEPIC) 90 90 80 80 VIN = 2.7V 60 50 40 30 80 70 VIN = 4V 60 50 40 0 100 150 50 LOAD CURRENT (mA) 200 DC233 TA03 2 90 EFFICIENCY (%) VIN = 4.2V 70 – 5VOUT Efficiency (Inverter) VIN = 6V EFFICIENCY (%) EFFICIENCY (%) 3.3VOUT Efficiency (SEPIC) 30 VIN = 5V 70 VIN = 3.3V 60 50 40 0 100 150 50 LOAD CURRENT (mA) 200 DC233 TA04 30 0 100 150 50 LOAD CURRENT (mA) 200 DC233 TA05 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U W TYPICAL PERFOR A CE CHARACTERISTICS Max Load Current vs VIN (Boost) MAXIMUM OUTPUT CURRENT, MIN (mA) 500 BOOST 400 300 VOUT = 3.3V VOUT = 5V 200 100 0 3 2 1 4 5 INPUT VOLTAGE (V) DC233 TA06 Max Load Current vs VIN (SEPIC) MAXIMUM OUTPUT CURRENT, MIN (mA) 250 SEPIC 200 VOUT = 3.3V 150 VOUT = 5V 100 50 0 2 6 4 INPUT VOLTAGE (V) 8 DC233 TA07 Max Load Current vs VIN (Inverter) MAXIMUM OUTPUT CURRENT, MIN (mA) 250 INVERTER 200 VOUT = 5V 150 100 50 0 2 6 4 INPUT VOLTAGE (V) 8 DC233 TA08 3 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U UU U W W SCHE ATIC A D CO ECTIO DIAGRA S Boost L4 4.7µH VIN (3.3VOUT) 1V TO 3.6V (5VOUT) 1V TO 5.3V D2 VOUT C7 0.1µF + VIN C8 15µF U2 LT1613 + R5 71.5k C10 1µF C9 15µF C11 0.1µF FB SHDN SHDN R4 100k SW GND R6 59k JP2 GND DC233 F01a SEPIC C3 1µF L1 10µH VIN 1V TO 6V D1 VOUT C1 0.1µF + C2 15µF SW VIN U1 LT1613 SHDN SHDN L2 10µH R1 100k + R2 71.5k C4 15µF C5 1µF C6 0.1µF FB GND R3 59k JP1 GND DC233 F01b Inverting C14 1µF L5 22µH VIN 1V TO 6V L6 22µH VOUT + C13 15µF VIN SW R7 30.1k FB SHDN SHDN D3 U3 LT1611 + C12 0.1µF GND C16 1µF C15 15µF C17 0.1µF R8 10k GND DC233 F01c Figure 1. DC233 Schematics TOP VIEW SW 1 TOP VIEW 5 VIN GND 2 4 SHDN NFB 3 4 SW 1 5 VIN GND 2 4 SHDN FB 3 S5 PACKAGE 5-LEAD PLASTIC SOT-23 S5 PACKAGE 5-LEAD PLASTIC SOT-23 LT1611CS5 LT1613CS5 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS PARTS LIST REFERENCE DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE Boost C7, C11 2 0805YC104MAT1A 0.1µF 16V X7R 0805 Capacitor AVX C8, C9 2 TAJA156M010R 15µF 10V 20% Tantalum Capacitor AVX (843) 946-0362 (207) 282-5111 C10 1 0805ZC105MAT1A 1µF 10V X7R 0805 Capacitor AVX (843) 946-0362 D2 1 MBR0520LT1 0.5A 20V SOD123 Schottky Diode ON Semiconductor (602) 244-6600 JP2 1 2802S-2-G1 2-Pin Header, 0.079 Center Comm Con (626) 301-4200 L4 1 LQH3C4R7M24 4.7µH Inductor Murata (770) 436-1300 R4 1 CR16-1003FM 100k 1/10W 1% 0603 Resistor TAD (800) 508-1521 R5 1 CR16-7152FM 71.5k 1/10W 1% 0603 Resistor TAD (800) 508-1521 R6 1 CR16-5902FM 59k 1/10W 1% 0603 Resistor TAD (800) 508-1521 U2 1 LT1613CS5 SOT-23 DC/DC Converter LTC (408) 432-1900 1 CCIJ2MM-138G Shunt, 0.079 Center Comm Con (626) 301-4200 C1, C6 2 0805YC104MAT1A 0.1µF 16V X7R 0805 Capacitor AVX (843) 946-0362 C2, C4 2 TAJA156M010R 15µF 10V 20% Tantalum Capacitor AVX (207) 282-5111 C3, C5 2 0805ZC105MAT1A 1µF 10V X7R 0805 Capacitor AVX (843) 946-0362 SEPIC D1 1 MBR0520LT1 0.5A 20V SOD123 Schottky Diode ON Semiconductor (602) 244-6600 JP1 1 2802S-2-G1 2-Pin Header, 0.079 Center Comm Con (626) 301-4200 L1, L2 2 LQH3C100K24 10µH Inductor Murata (770) 436-1300 R1 1 CR16-1003FM 100k 1/10W 1% 0603 Resistor TAD (800) 508-1521 R2 1 CR16-7152FM 71.5k 1/10W 1% 0603 Resistor TAD (800) 508-1521 R3 1 CR16-5902FM 59k 1/10W 1% 0603 Resistor TAD (800) 508-1521 U1 1 LT1613CS5 SOT-23 DC/DC Converter LTC (408) 432-1900 1 CCIJ2MM-138G Shunt, 0.079 Center Comm Con (626) 301-4200 C12, C17 2 0805YC104MAT1A 0.1µF 16V X7R 0805 Capacitor AVX (843) 946-0362 C13, C15 2 TAJA156M010R 15µF 10V 20% Tantalum Capacitor AVX (207) 282-5111 C14, C16 2 0805ZC105MAT1A 1µF 10V X7R 0805 Capacitor AVX (843) 946-0362 D3 1 MBR0520LT1 0.5A 20V SOD123 Schottky Diode ON Semiconductor (602) 244-6600 L5, L6 2 LQH3C220K34 22µH Inductor Murata (770) 436-1300 R7 1 CR16-3012FM 30.1k 1/10W 1% 0603 Resistor TAD (800) 508-1521 R8 1 CR16-1002FM 10k 1/10W 1% 0603 Resistor TAD (800) 508-1521 U3 1 LT1611CS5 SOT-23 DC/DC Converter LTC (408) 432-1900 Inverting 5 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS QUICK START GUIDE DC233 contains three switching regulator circuits. Two of these demonstrate the use of the LT1613CS5 in a simple boost regulator circuit and in an uncoupled SEPIC circuit. Both circuits produce 3.3V or 5V (jumper selected). The third circuit demonstrates the LT1611CS5 in a low noise inverting circuit, producing a – 5V output. The three circuits are electrically isolated from each other, and have their own grounds. Each circuit has a similar set of inputs and outputs—this quick-start guide applies to all. 1. The output of the boost and SEPIC circuits can be set to either 3.3V or 5V. The board is shipped with a jumper in place that programs the output for 5V. Remove the jumper to program the circuit for 3.3V out. 2. Apply a voltage source to the input of the circuit between the VIN and GND terminals. A benchtop supply with a 1A current limit is a good choice for this source. The circuit will operate from an input voltage between 1V and 6V. Do not apply more than 6V to the circuit. Note that the boost circuit will regulate the output only when the input voltage is less than the desired output voltage. 3. Attach a voltmeter or oscilloscope probe between the VOUT and GND terminals of the circuit in order to monitor the output. To start the circuit, tie the SHDN terminal to the VIN terminal. The LT1611/LT1613 will begin regulating the output voltage. 4. Attach a load to the output. The power capability of these circuits depends on the input voltage. A 100Ω one-half watt resistor soldered between the VOUT and GND pins of the circuit is a good starting point, and will allow you to observe the operation of the circuit. 5. The circuit can be placed in shutdown mode by either floating the SHDN terminal or tying it to ground. 6. Proper hook-up is essential for accurate and meaningful evaluation of efficiency and regulation. Figure 2 shows the appropriate arrangement of the supply, load, ammeters and voltmeters. IIN DC233 A IOUT VIN A BENCH SUPPLY 1V TO 6V/1A + LOAD VIN – + VOUT – VOUT GND SHDN Figure 2. Proper Hook-Up for Evaluating the DC233 6 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO INTRODUCTION DC233 contains three switching regulator circuits. Two of these demonstrate the use of the LT1613CS5 in a simple boost regulator circuit and in an uncoupled SEPIC1 circuit. Both circuits produce 3.3V or 5V (jumper selected). The SEPIC circuit allows operation from input voltages either higher or lower than the output, making this circuit suitable for single Li-Ion cell to 3.3V conversion or four alkaline cells to 5V conversion. The third circuit demonstrates the LT1611CS5 in a low noise inverting circuit. This circuit can convert 5V to – 5V at 160mA. The three circuits on the DC233 are electrically isolated from each other and have their own grounds. Because the three circuits are functionally similar and have the same input and output connections (VIN, VOUT, GND and SHDN), many of the comments that follow will apply to all three. Each circuit is described in more detail in its individual section. This manual describes the operation of these demonstration circuits, their performance, and variations on the basic circuits. For a thorough discussion of the LT1611 and LT1613 and their applications, please consult the parts’ data sheets. LT1611/LT1613 will default to its shutdown mode. Tie the SHDN terminal of the DC233 to the VIN to start the regulator. Apply a load between the VOUT and GND terminals, using either a fixed resistor, a decade resistor box (provided it is rated for the power) or an active load. A simple initial load might be a one-half watt, 100Ω resistor. Warning: because the boost circuit contains a DC path between the input and output (through inductor L4 and diode D2), the circuit is not protected against a shorted output. It is recommended that preliminary testing of the circuit be performed using a current-limited supply on the input. Figure 3 shows some of the boost circuit’s operating waveforms. The scope photo shows the output voltage, the current through the internal power switch (the current into the SW pin) and the voltage on the SW pin of the LT1613. The SEPIC and inverting circuits display similar waveforms. VOUT 50mV/DIV ISW 200mA/DIV Hook-Up and Initial Tests DC233 contains fairly simple, low power switching regulators. However, some precautions are necessary in order to test the circuits safely. Proper hook-up and accurate measurements are necessary for meaningful evaluation of efficiency and line and load regulation. Refer to Figure 2 for proper connections. The outputs of the boost and SEPIC circuits can be set to either 3.3V or 5V. The board is shipped with a jumper in place that programs the output for 5V. Remove the jumper to program the circuit for 3.3V out. The input can safely accept a voltage as high as 6V. A good starting point is to apply 2.5V between the VIN and GND terminals of the DC233, using a benchtop supply with a 1A current limit. Because the SHDN pin has been left floating, the VSW 5V/DIV 0.2µs/DIV DC231 F03 Figure 3. Operating Waveforms of the DC233 Boost Circuit (VIN = 2V, VOUT = 3.3V, IOUT = 80mA) PERFORMANCE Efficiency The efficiency of the DC233 circuits is plotted in the Typical Performance section of this manual. Efficiency measurements should be made with care, as there are plenty of opportunities for errors to creep in. 1SEPIC is an acronym for "single-ended primary-inductance converter.” 7 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO The efficiency is defined as the power delivered to the load divided by the power drawn from the input supply. Normally, the average input voltage, input current, output voltage and output current are measured under steady-state conditions and the efficiency is calculated from these values. Each should be measured with the highest accuracy and precision possible. BOOST Figure 2 shows connections for the proper measurement of efficiency and output regulation. The input and output voltages are measured at the DC233 terminals in order to avoid including voltage drops across ammeters and terminal connections. It is best to take all of these measurements at one time. Be aware that most digital multimeters drop significant voltage when they are used as ammeters, so you must measure the input voltage while the ammeter is in the circuit—the input voltage will be lower than the voltage at the output of your benchtop supply. Input Range and Power Capability Testing in Your System The power capability of the DC233 boost circuit is determined primarily by the input voltage and by the current limit of the LT1613’s internal power switch and, to a lesser extent, by the value of the inductor L4. Therefore, the maximum load current that this circuit can supply depends on the input voltage. A graph of maximum load appears in the Typical Performance section of this manual. This curve is based on the minimum current limit specification in the LT1613 data sheet. A typical LT1613 will deliver more current. As load current is increased beyond this level, the output voltage will sag as the LT1613 reaches its current limit. You may want to paste this circuit into your system to test compatibility. This should be done with care, since long hook-up wires and ground loops can introduce noise sources and regulation problems that would not be present if the DC/DC converter were properly designed into your PCB. Treat the DC233 as a 3-terminal device, with VIN, VOUT and GND terminals. Wire the DC233 to your circuit board with wires as short as practical, to points on the circuit board that are close to each other. Also, add high frequency bypass capacitors (0.1µF ceramics) from VIN and VOUT to ground on your circuit board. If you are bringing power directly to the DC233, use two wires from the input source to the VIN and GND terminals of the DC233. The output power should be applied to your system as described above and either the input supply or your circuit should be floating in order to avoid ground loops. 8 The boost circuit is the simplest LT1613 circuit. It can be used to convert a low voltage to a higher output voltage, for example, converting 1- or 2-cell alkaline batteries to 3.3V or 5V or generating a local 5V logic supply from a 3.3V rail. The LT1613 will typically run from inputs down to 0.9V and is guaranteed to operate from inputs above 1V. The maximum allowable input voltage to this circuit is 6V, which is based on the voltage ratings of the input and output capacitors, C8 and C9. The boost circuit will allow the LT1613 to regulate the output only when the input voltage is less than the desired output voltage plus one diode drop. This means that the practical input range is 0.9V to 3.6V for a 3.3V output and 0.9V to 5.3V for a 5V output. Be aware that L4 and D2 provide a direct path between the input and output, and that this circuit does not limit the output current. As an increasing load drags the output voltage below the input, a larger current will flow, limited only by the impedance of the power source, inductor L4 and diode D2. DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO The SHDN pin of the LT1613 is tied directly to the SHDN terminal of the DC233 and has been left floating. In this condition, or with this pin grounded, the LT1613 is in its shutdown mode. In this state, the LT1613 will draw less than 1µA from the input. However, the inductor L4 and catch diode D2 provide a path from the input to the output and the feedback divider (R8, R10 and R11) may draw a few µA, depending on the input voltage. In addition, the load can draw power from the input while the LT1613 is shut down. SEPIC The SEPIC circuit can be implemented with either a pair of inductors or a 1:1 transformer. Figure 4 shows the transformer arrangement. The DC233 layout includes pads for installation of two types of 1:1 surface mount transformers from Sumida. The Sumida CLS62-100 is a 10µH inductor with two windings that can be used as a transformer. This coupled inductor reduces the ripple current in the LT1613, raising the output power capability of the circuit by 20%. There are also pads that accept the Sumida CLQ61B-8R2. Use this part to implement a low profile design. It can be mounted within a routed hole (not present on the DC233 circuit board), reducing the inductor height to less than 1.5mm above the top surface of the printed circuit board. The LT1613 SEPIC circuit is slightly more complicated than the boost circuit, but it can regulate the output over a wider input voltage range. It might be used, for example, to convert a Li-Ion cell input (2.7V to 4.2V) to a 3.3V output. The LT1613 will typically run from inputs down to 0.9V, and is guaranteed to operate from inputs above 1V. The maximum allowable input voltage to this circuit is 6V, which is based on the voltage ratings of the input capacitor, C2. Unlike the boost circuit, the SEPIC can regulate the output voltage when the input voltage is higher. As in the boost circuit, the power capability of the DC233 SEPIC circuit is determined primarily by the input voltage and by the current limit of the LT1613’s internal power switch and, to a lesser extent, by the value of the inductors L1 and L2. Therefore, the maximum load current that this circuit can supply depends on the input voltage. A graph of maximum load appears in the Typical Performance section of this manual. This curve is based on the minimum current-limit specification in the LT1613 data sheet. A typical LT1613 will deliver more current. As load current is increased beyond this level, the output voltage will sag as the LT1613 reaches its current limit. • VIN VOUT + Input Range and Power Capability L3 • Shutdown Mode + SW VIN + LT1613 SHDN SHDN FB GND DC233 F04 Figure 4. Transformer Arrangement for the SEPIC Shutdown Mode Float the SHDN terminal of the DC233 or tie it to ground to shut down the LT1613. The coupling capacitor C3 provides a DC block between the input and output of the SEPIC circuit. This provides an automatic disconnect function; when the LT1613 is placed in shutdown mode, the load cannot draw current from the input source. The shutdown current consumption is less than 1µA. 9 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO INVERTER VOUT + VIN • • + SW VIN + The LT1611 inverter uses a very low noise circuit topology. Both the input and output of this circuit are connected to inductors and AC current into the input and output capacitors is very low. This results in low voltage ripple at the input and output. This circuit provides lower noise and better regulation than switched capacitor inverters of equivalent power. L7 LT1611 SHDN SHDN FB GND Input Range and Power Capability DC233 F05 The LT1611 will typically run from inputs down to 0.9V and is guaranteed to operate from inputs above 1V. The maximum allowable input voltage to this circuit is 6V, which is based on the voltage ratings of the input capacitor C13 and coupling capacitor C14. This inverting circuit can regulate a negative output voltage whose magnitude is either greater or less than the input voltage. The power capability of the DC233 inverter is determined primarily by the input voltage and by the current limit of the LT1611’s internal power switch and, to a lesser extent, by the value of the inductors L5 and L6. Therefore, the maximum load current that this circuit can supply depends on the input voltage. A graph of maximum load appears in the Typical Performance section of this manual. This curve is based on the minimum current limit specification in the LT1611 data sheet. A typical LT1611 will deliver more current. As load current is increased beyond this level, the output voltage will sag as the LT1611 reaches its current limit. The inverter can be implemented with either a pair of inductors or with a 1:1 transformer. Figure 5 shows the transformer arrangement. The DC233 layout includes pads for installation of two types of 1:1 surface mount transformers from Sumida. The Sumida CLS62-100 is a 10µH inductor with two windings that can be used as a transformer. The Sumida CLS62-220 22µH inductor will increase power capability by ≈10% and decrease output ripple at the expense of slightly lower efficiency. There 10 Figure 5. Transformer Arrangement for the Inverter are also pads that accept the Sumida CLQ61B-8R2. Use this part to implement a low profile design. It can be mounted within a routed hole (not present on the DC233 circuit board), reducing the inductor height to less than 1.5mm above the top surface of the printed circuit board. Shutdown Mode Float the SHDN terminal of the DC233 or tie it to ground to shut down the LT1611. The coupling capacitor C14 provides a DC block between the input and output of the inverter. This provides an automatic disconnect function: when the LT1611 is placed in shutdown mode, the load cannot draw current from the input source. The shutdown current consumption is less than 1µA. DESIGN ALTERNATIVES Component Selection The components used for the DC233 emphasize low cost and small size. Other component choices can provide improved performance. As described above, for example, replacing the inductors in the SEPIC and inverting circuits results in greater output current capability. This section will describe some other alternatives. DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO Diodes D1, D2 and D3 (Motorola MBR0520LT1) are onehalf amp, 20V Schottky diodes. This is a good choice for nearly any LT1611/LT1613 application, unless the output voltage or the circuit topology requires a diode rated for higher reverse voltages. Motorola also offers 30V and 40V versions. Most one-half amp and one amp Schottky diodes are suitable; these are available from many manufacturers. If you use a silicon diode, it must be an ultrafast recovery type. Efficiency will be lower due to the silicon diode’s higher forward voltage drop. Inductors used with the LT1611 and LT1613 should be rated for approximately 0.5A. The value of the inductor should be matched to the power requirements and operating voltages of the application. In most cases a value of 4.7µH or 10µH is suitable. The Murata inductors used on the DC233 are small and inexpensive and are a good fit for the LT1611 and LT1613. Alternatives are the CD43 series from Sumida and the DO1608 series from Coilcraft. These inductors are slightly larger but will result in slightly higher circuit efficiency. The voltage rating of the input capacitor limits the input voltage range of the circuits. The input range to the SEPIC and inverting circuit can be raised to 10V by replacing the input capacitor (C2 or C13) with a 16V capacitor and (in the case of the inverter) the coupling capacitor (C14) with a 16V part. Note that, in power supply applications, most tantalum capacitor manufacturers recommend using a capacitor with a voltage rating higher than the operating voltage. The coupling capacitor in the SEPIC and inverting circuits (C3 or C14) should have a low ESR to ensure good efficiency and must have an adequate ripple current rating. It also must have a suitable voltage rating. In the case of the SEPIC circuit, it should be rated for the maximum input voltage or higher; in the inverter, its voltage rating must be higher than the sum of the magnitudes of the input and output voltages. If a coupled inductor is used, the value of this ceramic capacitor can be reduced to 0.22µF from the 1µF used here. Lower Ripple The quality of the output capacitor is the greatest determinant of the output voltage ripple. The output capacitor performs two major functions: it must have enough capacitance to satisfy the load under transient conditions and it must shunt the AC component of the current coming through the diode from the inductor. The ripple on the output results when this AC current passes through the finite impedance of the output capacitor. The capacitor should have low impedance at the 1.4MHz switching frequency of the LT1611/LT1613. At this frequency, the impedance is usually dominated by the capacitor’s equivalent series resistance (ESR). Choosing a capacitor with lower ESR will result in lower output ripple. The DC233 uses a combination of two capacitors to achieve these ends. The 15µF tantalum output capacitor (C4, C9 or C15) provides the bulk capacitance for good transient response. A 1µF ceramic capacitor (C5, C10 or C16) in parallel with the tantalum capacitor provides a low impedance bypass at the switching frequency. This results in low output ripple and helps to maintain good efficiency at high loads by eliminating AC losses in the main output capacitor. This combination output capacitor provides good performance at low cost. Both capacitors are quite small. However, low ESR and the required bulk output capacitance can be obtained using a single larger output capacitor. Larger tantalum capacitors, newer capacitor technologies (for example the POSCAP from Sanyo and SPCAP from Panasonic) or large value ceramic capacitors will reduce the output ripple. Note, however, that the stability of the circuit depends on both the value of the output capacitor and its ESR. When using low value capacitors or capacitors with very low ESR, circuit stability should be evaluated carefully, as described below. 11 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO Loop Compensation The LT1611 and LT1613 are current mode, PWM switching regulators. Each uses a linear control loop to regulate its output. This control loop is compensated internally, eliminating several external components. However, the stability of the control loop depends on the value of the output capacitor and its ESR. A tantalum capacitor’s combination of capacitance and ESR will result in stable operation. As the amount of capacitance or ESR is decreased, the phase margin of the circuit will decrease and the transient response of the circuit may ring or the circuit may become unstable. After the power components (including the output capacitor) have been chosen, the circuit should be tested under transient loads for stable response. Linear Technology’s Application Note 19 provides details of this method. All-Ceramic Design Large value ceramic capacitors that are suitable for use as the main output capacitor of an LT1611/LT1613 regulator are now available. These capacitors have very low ESR and therefore offer very low output ripple in a small package. However, you should approach their use with some caution. Ceramic capacitors are manufactured using a number of dielectrics, each with different behavior across temperature and applied voltage. Y5V is a common dielectric type used for high value capacitors, but it can lose more than 80% of the original capacitance with applied voltage and extreme temperatures. The transient behavior and loop stability of the switching regulator depend on the value of the output capacitor, so you may not be able to afford this loss. Other dielectrics (X7R and X5R) result in more stable characteristics and are suitable for use as the output capacitor. The X7R type has better stability across temperature, whereas the X5R is less expensive and is available in higher values. 12 The second concern in using ceramic capacitors is that many switching regulators benefit from the ESR of the output capacitor because it introduces a zero in the regulator’s loop gain. This zero may not be effective because the ceramic capacitor’s ESR is very low. Most current mode switching regulators can be easily compensated without this zero. Any design should be tested for stability at the extremes of operating temperatures; this is particularly so of circuits that use ceramic output capacitors. Figure 6 shows a boost design that uses ceramic capacitors at both the input and output, resulting in small circuit size and very low noise. A capacitor has been added in the feedback path for phase lead, compensating for the output capacitor’s low ESR. Figure 7 compares the transient response and output ripple of the DC233 boost circuit with those for the all-ceramic design. The lower trace in each scope photo shows the load current stepping from 50mA to150mA. The upper trace shows the output as it responds to this load step. The output ripple for the DC233 boost circuit appears when the load current is high and is approximately 30mVP-P. The low ESR, 10µF ceramic capacitor results in output ripple under 5mVP-P. 4.7µH VOUT 5V VIN C21 2.2µF SW VIN 100k 220pF LT1613 SHDN SHDN C22 10µF FB GND 32.4k DC233 F06 C21: TAIYO YUDEN LMK212BJ225MF (0805 CASE SIZE) C22: TAIYO YUDEN JMK316BJ106ML (1206 CASE SIZE) Figure 6. This Boost Design Uses Ceramic Input and Output Capacitors for Small Circuit Size and Low Noise DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U OPERATIO VOUT 100mV/DIV ILOAD 100mA/DIV 0.1ms/DIV DC231 F07a VOUT 100mV/DIV ILOAD 100mA/DIV 0.1ms/DIV DC231 F07b Figure 7. Transient Response of the DC233 Boost Circuit (Top Photo) and All Ceramic Design in Figure 6. (VIN = 3V, VOUT = 5V) 13 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS W U PCB LAYOUT A D FIL 14 Component Side Silkscreen Component Side Component Side Solder Mask Component Side Paste Mask DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS W U PCB LAYOUT A D FIL Solder Side Solder Side Solder Mask Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 DEMO MANUAL DC233 SOT-23 SWITCHING REGULATORS U PC FAB DRAWI G 2.125 NOTES: UNLESS OTHERWISE SPECIFIED 1. MATERIAL: FR4 OR EQUIVALENT EPOXY, 2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006 TOTAL OF 2 LAYERS 2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC) 3. SOLDER MASK: BOTH SIDES USING SR1020 OR EQUIVALENT 4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK 5. ALL DIMENSIONS IN INCHES D A B A A C A C 2.975 A A B A SYMBOL DIAMETER NUMBER OF HOLES A 0.020 24 B 0.035 4 C 0.065 12 D A 0.072 2 TOTAL HOLES 42 233 FAB A 16 C D Linear Technology Corporation dc233f LT/TP 0400 500 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 2000