DESIGN IDEAS 4A, 4MHz Monolithic Synchronous Regulator with Tracking offers a Compact Solution for Power Supply Sequencing by Joey M. Esteves Introduction The LTC3416 offers a compact and efficient voltage regulator solution for systems that require power supply sequencing between different supply voltages. Many microprocessors and DSP chips need a core power supply and an I/O power supply that must be sequenced during start-up. Without proper power supply sequencing, latch-up or excessive current draw may occur that could lead to damage to the microprocessor’s I/O ports or the I/O ports of a supporting device such as memory, logic, FPGAs, or data converters. The LTC3416 operates from an input voltage range of 2.25V to 5.5V and can generate an output voltage between 0.8V to 5V. The internal power MOSFET switches have a low 67mΩ on-resistance, thus allowing the LTC3416 to deliver up to 4A of output current while achieving efficiencies as high as 91%. The LTC3416 employs a constant frequency, current-mode architecture with a frequency range of 300KHz to 4MHz. Forced continuous operation allows the LTC3416 to maintain a constant frequency throughout the entire load range, making it easier to filter the switching noise and reduce 100 90 EFFICIENCY (%) 80 70 60 VIN = 3.3V 50 40 30 20 10 0 0.01 VOUT = 1.8V f = 2MHz 0.10 1 LOAD CURRENT (A) 10 Figure 2. Efficiency vs load current Linear Technology Magazine • May 2004 I/O SUPPLY 7 VIN 3.3V CIN1 100µF ×2 R4 255k PGOOD RPG 100k 14 16 4 17 3 2 ROSC 127k R3 200k 5 1 10 PVIN SW PVIN SW SVIN SW RUN SW LTC3416 PGOOD VFB TRACK NC RT NC SGND ITH PGND PGND PGND PGND 2.5V 8 L1* 0.2µH 9 R1 255k 12 13 COUT 100µF ×2 C2 22pF X7R VOUT1 1.8V 4A 19 6 R2 200k 15 L1: TOKO FDV0620-R20M CIN1, COUT: TDK C4532X5R0J107M 18 20 11 CITH 820pF X7R RITH 7.5k C1 47pF X7R Figure 1. A 1.8V/4A step-down regulator with tracking the RF interference—important for EMI-sensitive applications. The switching frequency can be set externally with a resistor or synchronized to an external clock, where each switching cycle begins at the falling edge of the external clock signal. Since the output voltage ripple is inversely proportional to switching frequency and inductor value, a designer can take advantage of the LTC3416’s high switching frequency to use smaller inductors without compromising the output voltage ripple. Lower inductor values translate directly to smaller case sizes, reducing the overall size of the system. OPTI-LOOP® compensation allows the transient response to be optimized over a wide range of loads and output capacitors, including ceramics. For increased thermal handling, the LTC3416 is offered in a 20-Lead TSSOP package with an exposed pad to facilitate heat sinking. Authors can be contacted at (408) 432-1900 Voltage tracking is enabled by applying a ramp voltage to the TRACK pin. When the voltage on the TRACK pin is below 0.8V, the feedback voltage regulates to this tracking voltage. When the tracking voltage exceeds 0.8V, tracking is disabled and the feedback voltage regulates to the internal reference voltage. Tracking is implemented by connecting an extra resistor divider to the I/O supply voltage. The ratio of this divider should be selected to be the same as that of the LTC3416’s feedback resistor divider. continued on page 33 500mV/ DIV 5ms/DIV VIN = 3.3V, VOUT = 1.8V TRACKING 2.5V Figure 3. Start-up and shut-down tracking 31 DESIGN IDEAS Boost Converter Drives 1A White LEDs by Keith Szolusha White LEDs are brighter and more powerful than ever. High-power white LEDs, because of their extreme luminous density and ultra-compact size, are replacing conventional bulbs in flashlights, headlamps, streetlights, and many automotive applications— anywhere a conventional bulb might be found. Some new white LEDs, such as Lumileds’ Luxeon™ series, improve on conventional bulbs in several characteristics, including greater luminescence, improved response time, and increased durability with decreased size and cost. The challenge in using white LEDs in portable applications is powering them with the wide input voltage range that batteries present, such as 3.3V to 4.2V from a lithium-ion. LEDs require constant current to maintain constant luminosity. The battery-LED DC/DC converter must both step up and step down the source voltage to a 3.0V to 3.6V LED forward voltage range at a constant LED current such as 1A. The LT3436EFE 800kHz boost converter in Figure 1 provides 1A driving D2 1A CONSTANT CURRENT 0.050Ω 1% VOUT = VIN + VLED L1 49.9k 1% D1 UPS120 VIN 3.3V TO 4.2V SINGLE LI-ION VIN LED ON 4.7µF X5R 6.3V CERAMIC LT1783 LT3436 SHDN SYNC + VSW GND – 78.7k VC FB Q2 Q1 8.2k 0.1µF 4.99k 1.21k 1% 22µF X5R 10V CERAMIC D2: LUMILEDS LXHL-PW09 3.6V 1A EMITTER L1: CDRH6D28-3R0 Q1: MMBT2222A (FOR OVERVOLTAGE PROTECT) Q2: FMMT3906 Figure 1. LT3436EFE boost converter drives Luxeon III 1A 3.6V white LED with 70% efficiency. current for the Luxeon III series white LED LXHL-PW09 from a lithium-ion battery. The Luxeon III white LED has a forward voltage range from 3.0V to 3.6V. By tying the LED from the output of the boost converter back to the input, as opposed to ground, the boost converter is capable of both stepping-up and stepping-down its input voltage to the LED. The effective output voltage of the converter is a boosted voltage of VIN plus VLED as shown in the schematic. The LT1783 1.25MHz SOT -23 rail-to-rail op amp provides the current-sense capability and regulates the diode current to 1A when the LED ON switch is closed. When the switch is open, the LT3436 consumes only 6μA in shutdown. Figure 3 shows the relationship between the output voltage waveform of the LTC3416 and the I/O supply voltage during start-up. Ceramic capacitors offer low cost and low ESR, but many switching regulators have difficulty operating with them because the extremely low ESR can lead to loop instability. The phase margin of the control loop can drop to inadequate levels without the aid of the zero that is normally generated from the higher ESR of tantalum capacitors. The LTC3416, however, includes OPTI-LOOP compensation, which allows it to operate properly with ceramic input and output capacitors. The LTC3416 allows loop stability to be achieved over a wide range of loads and output capacitors with proper selec- tion of the compensation components on the ITH pin. Luxeon is a trademark of Lumileds Lighting. LTC3416, continued from page 31 1.8V/4A Converter Tracks a 2.5V I/O Supply Figure 1 shows a 1.8V step-down DC/ DC converter tracking an I/O supply voltage of 2.5V. This circuit operates from an input voltage of 3.3V and provides a regulated 1.8V output at up to 4A of load current. Efficiency is as high as 90% and is shown in Figure 2. The switching frequency for this circuit is set at 2MHz by a single external resistor, ROSC. Operating at a frequency this high allows the use of a lower valued and physically smaller inductor. During start-up, the output of the LTC3416 coincidentally tracks the I/O supply voltage. Once the I/O supply voltage exceeds 1.8V, tracking is disabled and the LTC3416 regulates its output voltage to 1.8V. Linear Technology Magazine • May 2004 Conclusion The LTC3416 with its tracking ability is well suited to applications involving microcontroller-based circuits with dual supply architectures. It’s high switching frequency and internal low RDS(ON) power switches allow the LTC3416 to provide a small solution size with high efficiency for systems with power supply sequencing requirements. for the latest information on LTC products, visit www.linear.com 33