advertisement Monolithic Step-Down Regulator Withstands The Rigors of Automotive Environments and Consumes Only 100µA of Quiescent Current – Design Note 378 Rich Philpott Introduction Automobile electronic systems place high demands on today’s DC/DC converters. They must be able to precisely regulate an output voltage in the face of wide temperature and input voltage ranges—including load dump transients in excess of 60V and cold crank voltage drops to 4V. The converter must also be able to minimize battery drain in always-on systems by maintaining high efficiency over a broad load current range. Similar demands are made by many 48V nonisolated telecom applications, 40V FireWire peripherals and battery-powered applications with auto plug adaptors. The LT3437’s best in class performance meets all of these requirements in a small thermally enhanced 3mm × 3mm DFN package. Features of the LT3437 The LT® 3437 is a 200kHz fixed frequency, 500mA monolithic buck switching regulator. Its 3.3V to 80V input voltage range makes the LT3437 ideal for harsh automotive environments. Micropower bias current and Burst Mode® operation help to maintain high efficiency over the entire load range and result in a no load quiescent current of only 100µA for the circuit in Figure 1. The LT3437 has an undervoltage lockout and a shutdown pin with an accurate threshold for a <1µA quiescent current shutdown mode. VIN 3.75V TO 80V* 2.2µF 100V CER VIN VC 1500pF 0.1µF SW LT3437 330pF 0.1µF The fuzziness seen on the output voltage is due to the ESR of the output capacitor and the change in inductor current ripple as the input voltage transitions between , LTC, LT and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 100µH BAS21 LOAD DUMP 10MQ100N VIN 20V/DIV CSS VBIAS 27pF 24k SYNC GND 165k + FB 100k * FOR INPUT VOLTAGES ABOVE 60V SOME RESTRICTIONS MAY APPLY. SEE ABSOLUTE MAXIMUM RATINGS IN DATA SHEET Figure 1. 14V to 3.3V Step-Down Converter with 100µA No Load Quiescent Current 12/05/378 Brutal Input Transients Figure 2 shows the LT3437’s reaction to the severe input transients that are possible in an automotive environment. Here, the input voltage rises from a nominal 12V to 80V in a 100ms load dump pulse, then drops to 4V in a 150ms cold crank pulse. The 200kHz fixed frequency and current mode topology of the LT3437 allow it to take it all in its stride—response to the input transients are less than 1% of the regulated voltage. VOUT 3.3V 250mA BOOST SHDN External synchronization can be implemented by driving the SYNC pin with a logic-level input. The SYNC pin also doubles as Burst Mode defeat for applications where lower output ripple is desired over light load efficiency. A single capacitor provides soft-start capability which limits inrush current and output voltage overshoot during startup and recovery from brown-out situations. The LT3437 is available in either a low profile 3mm × 3mm 10-pin DFN or 16-pin TSSOP package both with an exposed pad leadframe for low thermal resistance. 100µF 6.3V TANT COLD CRANK 0V VOUT 20mV/DIV AC COUPLED DN378 F01 50ms/DIV DN378 F02 Figure 2. Output Voltage Response to Load Dump and Cold Crank Input Transients levels. This ripple can be eliminated by changing the output capacitor type from tantalum to ceramic. Conclusion The LT3437’s wide input range, low quiescent current, robust design and small thermally enhanced packages make it an ideal solution for all automotive and wide input voltage, low quiescent current applications. 100 500 180 90 450 160 80 140 70 EFFICIENCY (%) 200 120 100 80 400 EFFICIENCY 350 60 300 50 250 40 200 POWER LOSS 60 30 150 40 20 100 20 10 50 0 0 0 10 20 30 40 50 60 INPUT VOLTAGE (V) 70 80 0 0.1 1 DN378 F03 Figure 3. Supply Current vs Input Voltage for Circuit in Figure 1 POWER LOSS (mW) SUPPLY CURRENT (µA) Low Quiescent Currents Many of today’s automotive applications are migrating to always-on systems which require low average quiescent current to prolong battery life. Loads are switched off or reduced during low demand periods, then activated for short periods. Quiescent current for the application circuit in Figure 1 is less than 1µA in shutdown mode, and a mere 100µA (Figure 3) for an input voltage of 12V under a no load condition. The LT3437 provides excellent step response from a no-load to load situation as shown in Figure 4. Automatic Burst Mode operation ensures efficiency over the entire load range as seen in Figure 5. Burst Mode operation can be defeated if lower ripple is desired over light load efficiency by pulling the SYNC pin high or driving it with an external clock. Soft-Start Capability The rising slope of the output voltage is determined by the output voltage and a single capacitor. Initially, when the output voltage is close to zero, the slope of the output is determined by the soft-start capacitor. As the output voltage increases, the output slope is increased to full bandwidth near the regulated voltage. Since the circuit is always active, inrush current and voltage overshoot are minimized for startup and recovery from overload (brown-out) conditions. Figure 6 illustrates the effect of several soft-start capacitor values. 10 100 LOAD CURRENT (mA) 1k DN378 F05 Figure 5. Efficiency and Power Loss vs Load Current for the Circuit in Figure 1 CSS = GND VOUT 50mV/DIV VOUT 1V/DIV CSS = 0.01µF CSS = 0.1µF 200mA IOUT 100mA/DIV 0mA 1ms/DIV DN378 F04 Figure 4. Output Voltage Response for 0mA to 200mA Load Step COUT = 100µF ILOAD = 200mA VIN = 12V 1ms/DIV DN378 F06 Figure 6. Output Voltage Soft-Start Data Sheet Download http://www.linear.com For applications help, call (408) 432-1900, Ext. 2759 Linear Technology Corporation dn378f LT/TP 1205 409K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●