DESIGN FEATURES Tiny SOT-23 Step-Down Regulator Switches at 1MHz for Space-Critical by Damon Lee Applications Introduction As portable devices continue to shrink, the need for progressively smaller components increases. To use smaller capacitors and inductors, switching regulators need to run at ever higher frequencies in ever smaller packages. To help meet this growing demand, Linear Technology introduces the LTC1701 5-lead SOT-23, step-down, current mode, DC/DC converter. Intended for low- to medium-power applications, it operates from a 2.5V to 5.5V input voltage and switches at 1MHz. The high switching frequency allows the use of tiny, low cost capacitors and inductors, which can be 2mm in height or less. Combined with the tiny SOT-23, the area consumed by the complete DC/DC converter can be less than 0.3in2, as shown in Figure␣ 1. The output voltage is adjustable from 1.25V to 5V. The LTC1701 can also be used as a zeta converter for battery-powered applications. A builtin 0.28Ω switch allows up to 500mA of output current at high efficiency. OPTI-LOOP compensation allows the transient response to be optimized over a wide range of loads and output capacitors. VIN 2.5V–5.5V C1 + 10µF 6.3V L1 4.7µH VIN R4 1M LTC1701 ITH/RUN R3 5.1k VOUT 2.5V/0.5A SW D1 VFB R2 121k GND C2 47µF 6V R1 121k C3 330pF L1: SUMIDA CD43-R47 C1: TAIYO YUDEN JMK316BJ106ML C2: SANYO POSCAP 6TPA47M D1: ON MBRM120L + (847) 956-0667 (408) 573-4150 (619) 661-6835 (800) 282-9855 Figure 2. High efficiency 2.5V/500mA step-down regulator High Efficiency 2.5V The LTC1701 incorporates a Step-Down DC/DC Converter current mode, constant-off-time architecture and includes automatic, power saving Burst Mode operation to reduce gate charge losses at low load currents. With no load, the converter draws only 135µA; in shutdown, it draws less than 1µA, making it ideal for battery-powered applications. In dropout, the internal P-channel MOSFET switch is turned on continuously, maximizing the usable battery life. A typical application for the LTC1701 is a 2.5V step-down converter, as shown in Figure 2. This circuit converts a 2.5V to 5.5V input supply to a regulated 2.5V output supply at up to 500mA. The efficiency peaks at 94% with a 3.3V input supply, as shown in Figure 3. The graphs show an improvement in efficiency above 100mA, where Burst Mode operation is disabled. Burst Mode operation provides better efficiency at lower currents by producing a single pulse or a group of pulses that are repeated 100 95 EFFICIENCY (%) VIN = 3.3V 90 VIN = 5.0V 85 80 75 VOUT = 2.5V 70 1 Figure 1. LTC1701 evaluation circuit Linear Technology Magazine • February 2000 10 100 LOAD CURRENT (mA) 1000 Figure 3. Efficiency of Figure 2’s circuit 5 DESIGN FEATURES VOUT 50mV/DIV VOUT 1V/DIV ITH/RUN 2V/DIV IL1 200mA/DIV IL1 500mA/DIV Figure 5. Start-up with 3.3V input into a 6Ω load Figure 4. Example of Burst Mode operation periodically, as shown in Figure 4. By switching intermittently, the switching losses, which are dominated by the gate-charge losses of the power MOSFET, are minimized. Start-up waveforms from a 3.3V input into a 6Ω load are pictured in Figure 5. The converter reaches regulation in approximately 200µs, depending on the load. Soft-start can be implemented by ramping the voltage on the I TH /RUN pin, which requires only an RC delay with a small Schottky diode, as shown in Figure 6. Single-Cell Li-Ion to 3.3V Zeta Converter Some designs need the ability to maintain a regulated output voltage while the input voltage may be either above or below the desired output. When the input is above the output, the circuit must behave like a buck regulator; when the input is below the output, it must behave like a boost regulator. The circuit configuration C6 4.7µF VIN C4 1µF R4 1M + C1 22µF 6.3V L1 4.7µH VIN D1 CC RUN C1 RC Figure 6. Soft-start hookup can be attributed to the dominance of switching losses across most of the current range. Since Li-Ion batteries spend most of their lives with a cell voltage in the 3.6V–4.0V range, the typical efficiency is about 81%. 2mm High, 1.5V Converter In many applications, the height constraint can be more of a concern than the area constraint. Small, low profile inductors and capacitors can be used with the LTC1701, due to the high switching frequency of 1MHz. In Figure 9, a circuit is shown that uses low profile components to produce a 2mm 85 VOUT 3.3V SW ITH/RUN R1 VIN = 4.0V 80 VIN = 3.5V LTC1701 ITH/RUN R3 5.1k VFB D1 L2 4.7µH + GND C3 330pF R1 20.5k R2 34k C2 22µF 6.3V EFFICIENCY (%) VIN 2.5V–4.2V Li-Ion known as a zeta converter is a very simple design that can meet this requirement. A single lithium-ion battery is a popular choice for many portable applications due to its light weight and high energy density, but it has a cell voltage that ranges from 4.2V to 2.5V. Thus, a simple buck or boost topology cannot be used to provide a 3.3V output voltage. In Figure 7, the LTC1701 is used in a zeta configuration to supply a constant 3.3V with over 200mA of load current. The circuit uses a single, dual-winding inductor (a 1:1 transformer) for better performance, although two separate inductors can also be used with somewhat lower efficiency. The components shown in the schematic result in a 3mm high converter, suitable for portable applications. As can be seen in Figure 8, the overall efficiency does not vary much with supply voltage variations, except at high currents (over 100mA). This VIN = 2.5V 75 VIN = 3.0V 70 65 VOUT = 3.3V C1,C2: C6: L1, L2: D1: AVX TAJA226M006R TAIYO YUDEN JMK212BJ475MG SUMIDA CLQ72 SERIES ON MBR0520L (207) 282-5111 (408) 573-4150 (847) 956-0667 (800) 282-9855 Figure 7. Single-cell Li-Ion to 3.3V zeta converter 6 60 1 10 100 LOAD CURRENT (mA) 1000 Figure 8. Efficiency of Figure 7’s circuit Linear Technology Magazine • February 2000 DESIGN FEATURES SW VIN R4 1M + LTC1701 ITH/RUN + C2 22µF 6.3V 85 VIN = 3.3V 80 C5 4.7µF R2 20k GND R3 5.1k C4 1µF D1 VFB VIN = 2.5V VOUT 1.5V/0.5A C3 330pF EFFICIENCY (%) C1 15µF 10V 90 L1 4.7µH VIN 2.5V–5.5V R1 100k 75 VIN = 5.0V 70 65 60 55 C1: C2: C4: C5: L1: D1: VOUT = 1.5V 50 AVX TAJA156M010R AVX TAJA226M006R TAIYO YUDEN LMK212BJ105MG TAIYO YUDEN JMK212BJ475MG MURATA LQH3C4R7M24 ON MBRM120L 1 (803) 946-0524 (408) 573-4150 (814) 237-1431 (800) 282-9855 2.5V Converter with All Ceramic Capacitors The low cost and low ESR of ceramic capacitors make them a very attractive choice for use in switching regulators. Unfortunately, the ESR is so low that loop stability problems may result. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumental in providing acceptable loop phase margin. Ceramic capacitors remain capaci- 1000 Figure 10. Efficiency of Figure 9’s circuit Figure 9. 2mm high 1.5V converter high (nominal), 1.5V step-down converter that occupies less than 0.3in2. The photograph in Figure 1 shows an example of a layout with these components. The efficiency, shown in Figure 10, peaks at 88%. As can be seen, the overall efficiency tends to degrade with a larger VIN-to-VOUT ratio, which is typical for step-down regulators. 10 100 LOAD CURRENT (mA) Conclusion tive to beyond 300kHz and usually resonate with their ESL before ESR damping becomes effective. Also, ceramic caps are prone to temperature effects, which require the designer to check loop stability over the full operating temperature range. For these reasons, great care must be taken when using only ceramic input and output capacitors. The OPTI-LOOP compensation components can be adjusted when ceramic capacitors are used. For a detailed explanation of optimizing the compensation components, refer to LTC Application Note 76. Figure 11 shows one example of an all-ceramic-capacitor circuit; its efficiency graph is shown in Figure 12. The efficiency in this case has a very flat peak at 93% due to the relatively low output capacitance and the low ESR of the ceramic capacitors. The LTC1701 is a small, monolithic, step-down regulator that switches at high frequencies, allowing the use of tiny, low cost capacitors and inductors for a cost- and space-saving DC/ DC converter. Although the LTC1701 was designed for basic buck applications, the architecture is versatile enough to produce an effective zeta converter, due in part to its power saving Burst Mode operation and its optimized OPTI-LOOP compensation. By combining a high switching frequency and an onboard P-channel MOSFET in a tiny SOT-23 package, the LTC1701 is ideal for space-critical portable applications. 100 VIN = 3.0V 95 90 R4 1M LTC1701 ITH/RUN R3 5.1k C4 1µF 10V VFB R2 121k GND C5 1µF 10V VIN = 5.0V 80 75 70 65 60 55 C3 180pF TAIYO YUDEN JMK316BJ106ML TAIYO YUDEN LMK212BJ105MG MURATA LQH3C4R7M24 ON MBRM120L C2 10µF 6.3V D1 VOUT 2.5V/0.5A EFFICIENCY (%) SW VIN C1 10µF 6.3V C1, C2: C4, C5: L1: D1: 85 L1 4.7µH VIN 2.5V–5.5V R1 121k C6 33pF (408) 573-4150 VOUT = 2.5V 50 1 10 100 LOAD CURRENT (mA) 1000 Figure 12. Efficiency of Figure 11’s circuit (814) 237-1431 (800) 282-9855 Figure 11. All-ceramic-capacitor converter delivers 2.5V at 500mA. Linear Technology Magazine • February 2000 7