DESIGN IDEAS L Miniscule Step-Up Controller Yields Wide Input and Output Ranges by Theo Phillips and Hong Ren Introduction Space-Saving Design 17.4k 1% ITH 47pF LTC3872 GND 11k 1% 34.8k 1% VIN VFB CIN2 470µF VIN 3.3V SW M1 CIN 1nF controllers that use a constant on-time switching scheme, which can result in a lower operating frequency and physically larger filter components. No RSENSE™ technology eliminates the need for a separate current- D1 COUT 100µF x2 VOUT 5V 2A 100 10 90 80 1 70 60 50 0.1 40 30 0.01 20 10 RUN/SS VIN CIN 10µF 10µH LTC3872 12.1k 1% + Figure 1. A 3.3V input, 5V output boost regulator delivering up to 2A. The large input capacitor CIN2 protects against inductive ringing in long traces to the main input supply, which could happen during load transients and startup. IPRG GND CIN1 10µF L1: TOKOFDV0630-2R2 D1: MBR B320 M1: VISHAY SILICONIX Si3460DV CIN2: SANYO 6TPE470M 0.022µF ITH L1 1.0µH 5.11k VFB RUN/SS NGATE 1M 4.7nF 93.1k 1% VIN IPRG POWER LOSS (W) Matching the LTC3872’s small size, its 550kHz constant frequency operation allows the use of a small-footprint surface mount inductor and ceramic capacitors. This saves space over boost 1.8nF EFFICIENCY (%) Small in stature, but big in power, the LTC3872 boost controller can produce an expansive range of output voltages with the bare minimum in components. Its design forgoes a few features and pins (TRACK, SYNC, and MODE, for instance) to squeeze into a slender 8-pin 3mm × 2mm DFN or leaded TSOT-23 package. Nevertheless, it gives up little in versatility, providing up to 60V in regulated output from a 2.75V–9.8V input. This input range accommodates one or two lithium-ion batteries, in addition to the common 3.3V and 5V input rails. 0 VIN 5V M1 COUT1 22µF COUT2 68µF COUT3: NTS60X5R1H685MT M1: VISHAY SILICONIX Si4850 D1: MBRB320 Figure 3. A 5V input, 48V output boost regulator delivering up to 0.5A ILOAD 200mA/DIV 10mA TO 375mA STEP 2ms/DIV 400µs/DIV Figure 4. Startup and load transient for the circuit of Figure 3 Linear Technology Magazine • September 2006 0.001 VOUT 48V 0.5A sensing resistor. Just a few additional resistors and capacitors are needed to program the output voltage and close the feedback loop; the user can adjust OPTI-LOOP® compensation to accommodate whatever output voltage and filter components are chosen. Light Load Efficiency VIN = 5V VOUT = 48V 500mV/DIV IL 2A/DIV 10k D1 464k 1% VOUT 20V/DIV 10 100 1k LOAD CURRENT (mA) Figure 2. Efficiency and Power Loss for the application circuit of Figure 1. SW NGATE 1 At light loads, pulse skip mode maintains constant frequency operation. This has the dual benefit of minimizing ripple and keeping switching noise within a predictable, easily filtered band. Output voltage remains stable in the presence of transients, due to the LTC3872’s current mode architecture. 25 L DESIGN IDEAS A 3.3V Input, 5V/2A Output Boost Converter Figure 1 shows a typical LTC3872 application—a 3.3V input to 5V output boost regulator which can deliver up to 2A load current. Figure 2 shows the efficiency/power loss curve. In spite of the converter’s small size, efficiency peaks at 90% and stays above 80% down to 20mA. In shutdown mode it draws only 8µA. The LTC3872 uses the drain to source voltage of the external N-channel MOSFET to sense the inductor current. Eliminating a separate sense resistor can increase efficiency by 1%–2% at heavy loads. Absent a short circuit at the output, the maximum current that the converter can draw from VIN is determined by the RDS(ON) of the MOSFET (a function of the gate drive voltage VIN ). This maximum current can be adjusted by using the three-state current limit programming pin IPRG. A 5V Input, 48V/0.5A Output Boost Converter Figure 3 shows the LTC3872’s ability to deliver high output voltage. In this topology, the limitation on VOUT is the 60V maximum rating of the SW pin. Where even higher output voltages LTC2952, continued from page 18 ON/OFF signal is caused by a valid pushbutton OFF. From the start of the shutdown sequence, the system power turns off in 500ms, unless an edge (a high-tolow or low-to-high transition) at the WDE pin is detected within the 500ms period to extend the wait period for another 500ms. This KILL Wait time (500ms/cycle) is designed to allow the system to finish performing its house keeping tasks before shut down. Once the µP finishes performing its power down operations, it can either let the KILL Wait time expire on its own or set the KILL pin low to immediately terminate the KILL Wait time. When the KILL Wait time expires, the LTC2952 sets EN low. This turns off the DC/DC converter connected to the EN pin. In the sequence shown in 26 VSW 20V/DIV VSW 20V/DIV IL 2A/DIV IL 2A/DIV LOAD = 1mA 1µs/DIV LOAD = 1mA 1µs/DIV Figure 5. At light loads, the circuit of Figure 3 uses pulse skip mode. In this mode operation does not exceed the (80%) maximum duty cycle of the converter at 550kHz. At heavy loads, the maximum duty cycle is extended by allowing the switching frequency to fall. against overshoot and undershoot during startup and load transients (Figure 4). At high duty cycle under heavy loads, the commutation cycle (here, 1/550kHz) is too brief to allow the average inductor current to equal the converter’s required input current. In this case, the on-time of MOSFET M1 is extended, and inductor current ramps up to the level required to maintain output regulation (Figure 5). Figure 6. A typical LTC3872 application occupies just 2.25cm2. are required, a sense resistor can be inserted between the source of the MOSFET and ground, with the SW pin tied to the high side of the sense resistor. The output is well-controlled Figure 10, the KILL Wait time is reset twice with edges on the WDE pin (t5 and t6) before finally expiring (t7). When the DC/DC converter is turned off (EN goes low), it can take a significant amount of time for its output level to decay to ground. In order to guarantee that the µP has always powered down properly before it is re-started, another 500ms (Enable Lock Out period) timer is started to allow for the DC/DC converter output power level to power down completely to ground. During this Enable Lock Out period, the EN pin remains in its low state regardless of any transition at the internal ON/OFF signal. At the end of the 500ms Enable Lock Out time (t8) the LTC2952 goes into its reset state, ready for the next turn on sequence. Note that at this reset Conclusion The LTC3872 is a tiny current-mode, non-synchronous boost controller that requires no sense resistor—a typical design occupies 2.25cm2 (Figure 6). The small solution size and wide input voltage range make it an easy fit for a variety of applications. L state the EN pin remains strongly pulling down. Conclusion The LTC2952 is a versatile, full featured Power Path Management IC that provides robust pushbutton ON/OFF control with a simple and graceful communication interface to the system microprocessor. Its wide voltage range, gate drive capability and low power fit an extensive number of applications requiring efficient management of two or more power paths. To further complement the requirements of highly reliable systems, the LTC2952 also offers voltage and watchdog monitoring capabilities. L Authors can be contacted at (408) 432-1900 Linear Technology Magazine • September 2006