advertisement A Simple Ultra-Low Dropout Regulator – Design Note 44 Jim Williams Single Devices Now Provide This Function Switching regulator post regulators, battery powered apparatus, and other applications frequently require low drop-out linear regulators. Often, battery life is significantly affected by the regulator’s dropout performance. Figure 1’s simple circuit offers lower dropout voltage than any monolithic regulator. Dropout is below 50mV at 1A, increasing to only 450mV at 5A. Line and load regulation are within 5mV, and initial output accuracy is inside 1%. Additionally, the regulator is fully short circuit protected, and has a no load quiescent current of 600μA. Circuit operation is straightforward. The 3-pin LT®1123 regulator (TO-92 package) servo controls Q1’s base to maintain its feedback pin (FB) at 5V. The 10μF output capacitor provides frequency compensation. If the circuit is located more than six inches from the input source the optional 10μF capacitor should bypass the + 100 +5VOUT 10μF 20Ω DRIVE LT1123 FB GND * = OPTIONAL (SEE TEXT) MJE1123 = MOTOROLA 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. 110 + 10μF* 600Ω Normally, configurations of this type offer unpredictable short circuit protection. Here, the MJE1123 transistor shown has been specially designed for use with the LT1123. Because of this, beta based current limiting is practical. Excessive output current causes the LT1123 to pull down harder on Q1 until beta limiting occurs. Under these conditions the controlled pull down current combines with Q1’s beta and safe operating area characteristics to provide reliable short circuit limiting. Figure 3 details current limit characteristics for 30 randomly selected transistors. DN044 F01 POWER LIMITING RESISTOR INPUT Q1 MJE1123 input. The optional 20Ω resistor limits LT1123 power dissipation and is selected based upon the maximum expected input voltage (see Figure 2). 90 80 70 60 50 ASSUME: VBE = 0.9V VSAT, LT1123 = 2V IDRIVE MAX = 150mA 40 30 20 10 5 Figure 1. The Ultra-Low Dropout Regulator. LT1123 Combines with Specially Designed Transistor for Lowest Dropout and Short Circuit Protection 02/91/44_conv 15 10 INPUT VOLTAGE (V) 20 DN044 F02 Figure 2. LT1123 Power Dissipation Limiting Resistor Value vs Input Voltage 0.75 0.60 DROPOUT VOLTAGE (V) NUMBER OF UNITS 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00 OUTPUT CURRENT (A) 0.45 0.30 0.15 0 0 1 3 4 2 OUTPUT CURRENT (A) DN044 F03 Figure 4 shows dropout characteristics. Even at 5A, dropout is about 450mV, decreasing to only 50mV at 1A. Monolithic regulators cannot approach these figures, primarily because monolithic power transistors do not offer Q1’s combination of high beta and excellent saturation. For comparison, Figure 5 compares the circuits performance against some popular monolithic regulators. Dropout is ten times better than 138 types, and significantly better than the other types shown. Because of Q1’s high beta, base drive loss is only 1%-2% of output current, even at full 5A output. This maintains high efficiency under the low VIN – VOUT conditions the circuit will typically operate at. As an exercise, the MJE1123 was replaced with a 2N4276, a Germanium device. This combination provided even lower dropout performance, although current limit characteristics cannot be guaranteed. Data Sheet Download www.linear.com Linear Technology Corporation DN044 F04 Figure 4. Dropout Voltage vs Output Current 3.0 2.5 DROPOUT VOLTAGE (V) Figure 3. Short Circuit Current for 30 Randomly Selected MJE1123 Transistors at VIN = 7V 5 LT138 2.0 1.5 LT1084 1.0 LT1185 LT1123/2N4276 0.5 LT1123/MJE1123 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 DN044 F05 Figure 5. Dropout Voltage vs Output Current for Various Regulators For applications help, call (408) 432-1900 dn44f_conv IM/GP 0291 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1991