advertisement A Simple Ultra-Low Dropout Regulator – Design Note 32 Jim Williams Linear voltage regulators with low dropout characteristics are a frequent requirement, particularly in battery powered applications. It is desirable to maintain regulation until the battery is almost entirely depleted. Regulator dropout limits significantly impact useful battery life, and as such should be minimized. Figure 1 shows dropout characteristics for a monolithic regulator, the LT®1085. The <1.5V dropout performance is about twice as good as standard monolithic regulators. In many cases this device will serve nicely, but applications requiring lower dropout mandate a different approach. Figure 2’s simple regulator has only 85mV dropout at 2.5A – a 13x improvement. At lower currents dropout decreases to vanishingly small values. This circuit is particularly applicable in battery driven lap top computers, where multi-output power supplies are used. In operation, the LT1431 shunt regulator adjusts its output (“collector”) to whatever value is required to force circuit output to 5V. The LT1431’s internal trimming eliminates the usual feedback resistors and trimpots. Q1, 6VIN (TYP. 5.04V – 7.2V) the pass element, runs as a voltage overdriven source follower. This configuration offers the lowest possible dropout voltage,* although it does require a +12V bias source for Q1’s gate. This +12V source is commonly present in lap top computers and similar devices because of disc drive and peripheral power requirements. Power drain on the +12V supply is a few milliamperes. Providing short circuit protection without introducing significant loss requires care. A1 achieves this by sensing across a 0.002Ω shunt (1.5" of #23 wire). This introduces only 6mV of drop at the circuits 3A current limit threshold. A 6mV current limit trip point is derived by grounding A1’s offset pin 5. The 6mV input offset generated at A1 by doing this is stable over time, tem*A detailed discussion of various methods for achieving low dropout appears as Appendix A (“Achieving Low Dropout”) in LTC Application Note 32, “High Efficiency Linear Regulators.” L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Q1 MTP50N05EL D 0.002Ω (1.5" #23 WIRE) + S 5VOUT 1.5k – 1N4148 EOS = 6mV SEE TEXT A1 LT1006 12V 5 + MINIMUM INPUT/OUTPUT DIFFERENTIAL (V) 47μF 12V 2 LT1085 1 COLLECTOR V+ FIGURE 2 GNDF 0 0 3 2 1 OUTPUT CURRENT (A) REF RMID GNDS DN032 F02 4 DN032 F01 Figure 1. Dropout Performance for a Low Dropout Monolithic Regulator vs Figure 2 03/90/32_conv LT1431 RTOP CONNECT ALL LABELED RETURNS TO A SINGLE POINT AT THE 6VIN SOURCE Figure 2. Ultra-Low Dropout Regulator perature and unit-unit variation, and substitutions for A1 are not advisable. Currents beyond 3A cause A1 to pull low, stealing Q1’s gate drive and shutting off the regulators output. Under overload conditions A1 and Q1 from a well controlled linear current control loop with smooth limiting. Figure 3 details dropout characteristics. Results for the MTP50N05EL MOSFET specified for Q1 show only 85mV dropout, decreasing to just 8mV at 0.25A. For comparison, data for some higher resistance transistors also appears. Q1’s source follower connection makes regulator dynamics quite good compared to common source/ emitter approaches. Figure 4 shows no load (Trace A low) to full load (Trace A high) response. Regulator output (Trace B) dips only 200mV and recovers quickly with clean damping. The positive slew recovery time is due to the 1.5kΩ bias resistor acting against Q1’s input capacitance (Trace C is Q1’s gate). Quicker response is 300 275 Q1 = MTP15N08L (0.135Ω) DROPOUT VOLTAGE (mV) 250 225 Q1 = MTP25N05L (0.08Ω) 200 2.5A = 338mV DROPOUT 175 Q1 = MTP40N06EL (0.06Ω) 150 125 100 possible by a reduction in this value, although current drain from the +12V supply will increase. The value used represents a good compromise. Transient recovery for load removal is also well controlled. This regulator offers a simple solution to applications requiring extremely low dropout over a range of output currents. The performance, low parts count and lack of trimming make it an attractive alternative to other approaches. For reference, pertinent information on construction of wire shunts appears in Figures 5 and 6. WIRE GAUGE μΩ/INCH 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 83 100 130 160 210 265 335 421 530 670 890 1000 1300 1700 2100 2700 75 50 0 Figure 5. Resistance vs Size for Various Copper Wire Types Q1 = MTP50N05EL (0.032Ω) 25 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 OUTPUT CURRENT (A) DN032 F03 Figure 3. Dropout Characteristics for Figure 2. Q1’s Saturation Directly Influences Performance. LENGTH DETERMINES SHUNT VALUE MAJOR CURRENT FLOW MAJOR CURRENT FLOW LOW RESISTANCE WIRE A = 5V/DIV SENSE POINT WIRE B = 100mV/DIV C = 0.5V (AC COUPLED) PRINTED CIRCUIT VERSION HORIZ = 1μs/DIV DN032 F04 Figure 4. Transient Response for a Full Load Step. Follower Connection Provides Clean Dynamics. Data Sheet Download www.linear.com Linear Technology Corporation SENSE POINT WIRE SENSE POINT TRACE MAJOR CURRENT FLOW SENSE POINT TRACE DN032 F06 Figure 6. Detail of a Low Resistance Current Shunt For applications help, call (408) 432-1900 dn32f_conv IM/GP 03890 165K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1990