advertisement Tiny Versatile Buck Regulators Operate from 3.6V to 36V Input Design Note 367 Hua (Walker) Bai Introduction Linear Technology offers two new buck regulators that operate from a wide input voltage range (3.6V to 36V) and take so little space that they easily solve many difficult power supply problems. The LT®1936 and LT1933 are perfect for applications with disparate power inputs or wide range input power supplies such as automotive batteries, 24V industrial supplies, 5V logic supplies and various wall adapters. Both parts are monolithic current mode PWM regulators which provide excellent line and load regulation and dynamic response. They operate at a 500kHz switching frequency, enabling the use of small, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. Small Size and Versatility The LT1936 regulator includes a 1.9A power switch in a tiny, thermally enhanced 8-lead MSOP. The LT1933 regulator includes an internal 0.75A power switch in a tiny 6-lead ThinSOT™ package, which occupies less than 0.15in2 board space. The LT1936 offers the option of external compensation for design flexibility or internal compensation for compact solution size. Both parts offer soft-start via the SHDN pin, thus reducing maximum inrush currents during start-up. Both parts also have a very low, 2μA shutdown current which significantly extends battery life in applications that spend long periods of time in sleep or shutdown mode. During D2 CMDSH-4E 1 VIN 4.5V TO 36V C6 22μF OPT 2 + C1 2.2μF 50V 5 8 C5 0.22μF BOOST LT1936 VIN SW 3 SHDN COMP VC C1: TDK C3225X5R1H225M C2: TDK C3225X5R0J476M 7 C6: SANYO 50CV22BS D1: DIODES, INC DFLS140L L1: TOKO D63CB P/N: A916CY-100M FB GND 4 L1 10μH R2 17.4k 1% 6 D1 R1 10k 1% Figure 1. Typical Application of LT1936 Accepts 4.5V to 36V and Produces 3.3V/1.2A 07/05/367_conv VOUT 3.3V 1.2A C2 47μF 6.3V DN367 F01 short circuit, both parts offer frequency foldback, where the switching frequency decreases by about a factor of ten. The lower frequency allows the inductor current to safely discharge, thereby preventing current runaway. LT1936 Produces 3.3V at 1.2A from 4.5V to 36V Figure 1 shows a typical application for the LT1936. This circuit generates 3.3V at 1.2A from an input of 4.5V to 36V. With the same input voltage range, the LT1933 circuit can supply 500mA. The typical output voltage ripple of the Figure 1 circuit is less than 16mV while efficiency is as high as 89%. Excellent transient response is possible with either external compensation or the internal compensation; this circuit uses internal compensation to minimize component count. A high ESR electrolytic capacitor, C6 in Figure 1, is recommended to damp overshoot voltage in applications where the circuit is plugged into a live input source through long leads. For more information, refer to the LT1933 or LT1936 data sheet. Producing a Lower Output Voltage from the LT1936 In order to fully saturate the internal NPN power transistor of the LT1936, the BOOST pin voltage must be at least 2.3V above the SW pin voltage. A charge pump comprising D2 and C5 creates this headroom in Figure 1. Nevertheless, when the output voltage is less than 2.5V, different approaches are needed. Figure 2 shows one example. It allows VIN to go up to 36V and generates 1.4A at 1.8V. In this circuit, Q2 serves as an inexpensive Zener. The emitter-base breakdown voltage of Q2 gives a stable 6V reference. The charging current for the BOOST capacitor, C5, passes through the follower, Q1. R4, Q1 and Q2 limit the BOOST pin voltage below its maximum rating of 43V. If the maximum VIN in an application is less than 20V, simply tie VIN to D2 to allow a lower minimum input voltage. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. 400 Q1 D2 C6 22μF OPT 1 2 8 + 380 Q2 C1 2.2μF 50V 5 C5 0.22μF BOOST LT1936 3 SW VIN 360 L1 10μH R2 10k 1% COMP SHDN 6 FB VC GND C1: TDK C3225X5R1H225M 4 7 C2, C8: TDK C3216X5R0J476M C6: SANYO 50CV22BS R3 D1: DIODES, INC DFLS140L 68.1k D2: CENTRAL SEMI CMDSH-4E L1: TOKO D63CB P/N: A916CY-100M C3 Q1, Q2: DIODES, INC MMBT3904 150pF VOUT 1.8V 1.4A C2 47μF 6.3V 240 + C4 10μF 50V OPT C1 2.2μF 50V GND BOOST SHDN SW GND FB 2 3 C2: TAIYO YUDEN JMK316BJ226KL-T C3: TDK C3225X7R1H225KT C4: SANYO 50CV10BJ D1: ON SEMI MBRM140T3 L1: TOKO D63CB P/N: A916CY-100M 12 20 16 VIN (V) 24 28 DN367 F04 1 LT1933 4 8 4 DN367 F01 D2 VIN 260 C4 18pF Negative Output from a Buck Regulator The circuit shown in Figure 3 can generate a negative voltage of –3.3V from a buck regulator such as the LT1933. This circuit effectively sets the ground reference of the LT1933 to –3.3V. The average inductor current of this circuit is the summation of the input and output current. The available output current is a function of the input voltage as shown in Figure 4. 5 320 280 C8 47μF 6.3V Figure 2. This Circuit Generates Lower Output Voltage While Allowing Maximum Input Up to 36V VIN 4.5V TO 30V 340 300 R1 20k 1% C7 100pF D1 IOUT (mA) R4 10k VIN 5V TO 25V OPERATING 36V TRANSIENT 6 C1 0.1μF L1 10μH D1 R2 16.5k 1% C2 22μF 6.3V –VOUT DN367 F03 3.3V R3 10k 1% Figure 3. This Circuit Produces –3.3V from 4.5V to 30V Figure 4. Maximum Output Current of the Circuit in Figure 3 as a Function of the Input Voltage Tiny Circuit Generates 3.3V and 5V from a Minimum 4.5V Supply The circuit in Figure 5 is capable of generating two output voltages from a minimum 4.5V supply. One output is 3.3V at 300mA, the other 5V at 50mA. The circuit is especially useful in automotive cold crank conditions when the battery voltage drops below 5V but both the 3.3V and 5V outputs need to be alive. If more current is needed, the circuit can also be implemented using the LT1936. Even though the input of the LT1761-5 is unregulated, the 5V output is regulated by the LT1761-5 LDO. To maintain regulation, the 3.3V output current should be always well above the 5V output current, especially when VIN is low. Conclusion The LT1933 and LT1936 step-down switching regulators accept a wide variety of input sources as well as offer compact, efficient and versatile solutions to many otherwise hard-to-solve problems. 1 C5 4.7μF 3 VIN OUT LT1761-5 SHDN BYP 5 4 C6 0.01μF VOUT2 5V C7 50mA 10μF GND 5 VIN 4.5V TO 36V VIN BOOST LT1933 + GND C4 10μF 50V OPT 4 C3 2.2μF 50V SHDN SW FB GND 2 2 1 6 C1 0.1μF L1 t t D1 VOUT1 3.3V 300mA D3 D2 3 R3 10k 1% R2 16.5k 1% C2 22μF 6.3V C2: TAIYO YUDEN JMK316BJ226KL-T C3: TDK C3225X7R1H225KT C4: SANYO 50CV10BJ C5: TAIYO YUDEN TMK325BJ475MN C7: TAIYO YUDEN JMK316BJ106ML D1: DIODES, INC B160 D2: ON SEMI MBRM140T3 L1: COILTRONICS CTX25-1P DN367 F05 Figure 5. From a Minimum 4.5V, This Circuit Produces Two Outputs at 3.3V/300mA and 5V/50mA Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn367f_conv LT/TP 0705 305K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005