3mm × 3mm Monolithic DC/DC Boost/Inverting Converters with 65V Power Switches Joshua Moore The vast array of power supply rails required by modern electronics has popularized the use of compact, easy-to-use monolithic DC/DC converters, such as the LT3580 boost/ inverting converter. The LT8580, LT8570, and LT8570-1 build on the success of the LT3580, increasing the switch voltage to 65V and the input voltage to 40V, while retaining features and pin compatibility. The LT8580 includes a 65V, 1A power switch, whereas the LT8570 and LT8570-1 step the switch current limit down to 0.5A and 0.25A, respectively. Various current options enable application optimization—a monolithic converter sized for specific demands can be smaller and more efficient than one designed for greater load currents. Optimized sizing for current limit helps limit input and output current in the event of a short or failure. In addition to the new options, all devices in the family—LT®3580, LT8580, LT8570, and LT8570-1—are pin compatible. With a few simple component changes, the same PCB layout can be used for a range of applications, allowing fast turnaround design changes and reuse. The LT8580, LT8570, and LT8570-1 retain the LT3580’s features such as single resistor feedback, for both positive and negative output voltages, overtemperature protection, frequency foldback, and an external clock input pin. And, like the LT3580, many features are user adjustable, including oscillator frequency, soft-start, UVLO and output voltage. All are available in thermally enhanced 8-pin 3mm × 3mm DFN or 8-pin MSE packages. 65V POWER SWITCH The LT8580/LT8570/LT8570-1 incorporate an internal 65V power switch, for applications with high input and output voltages. Furthermore, VIN is capable of handling up to 40V. This can greatly simplify applications. For example, Figure 1 shows the necessary circuitry to create a 48V output with the LT3580; the LT8570’s 65V switch simplifies the circuit in Figure 2. Table 1. Feature comparison of monolithic pin-compatible boost/inverting DC/DC converters LT3580 LT8570 LT8570-1 LT8580 Input Range 2.5V to 32V 2.55V to 40V 2.55V to 40V 2.55V to 40V Max Switch Voltage 42V 65V 65V 65V Max Switch Current 2A 1A 0.5A 0.25A Integrated Power Switch L L L L Frequency Foldback L L L L External Clock Input L L L L Overtemperature Protection L L L L Positive And Negative Output Voltages L L L L Single Resistor Feedback L L L L Packages 8-Pin MSE 8-Pin MSE 8-Pin MSE 8-Pin MSE 14 | July 2015 : LT Journal of Analog Innovation design features All devices in the family—LT3580, LT8580, LT8570, and LT8570-1—are pin compatible. With a few simple component changes, the same PCB layout can be used for a wide variety of applications, allowing fast turnaround design changes and reuse. D3 D2 R1 L1 VIN 12V VOUT 48V COUT2 D1 C1 L1 VIN 12V RSHDN VOUT 48V RSHDN VIN SW SHDN FB VIN RFB COUT1 LT3580 CIN SYNC GND SYNC RC RT CF CC CSS The LT8580, LT8570 and LT8570-1 include a number of configuration options. The oscillator frequency can be adjusted from 200kHz to 1.5MHz. While lower switching frequencies tend to be more efficient, higher switching frequencies offer smaller solution sizes. Also, choice of oscillator frequency may be useful for avoiding interference with sensitive RF circuitry. 15µH FBX VIN 56.2k SYNC 6.04k 47pF 0.22µF FBX 3.3nF Figure 3. LT8580 configured as 5V input to 12V output boost converter 130k 1µF LT8570-1 0.47µF SS VOUT 12V 50mA SW SHDN 4.7µF VC GND A final configuration option is softstart. By varying the soft-start capacitor, the user can adjust the rate of increase of the inductor current. The faster the inductor current increases, the faster the output rises during start-up. However, allowing the inductor current to increase slowly reduces output voltage overshoot and avoids large input transient currents during start-up. 10k 130k LT8580 SYNC CF CC CSS 47µH VIN 5V SW SHDN 2.2µF Another configuration option is undervoltage lockout, which, for most applications, is configurable with just one resistor from VIN to SHDN. This allows the parts to be used in situations where source impedance may be high, the source may ramp slowly or where it is desirable that the part not discharge the source below some threshold. VOUT 12V 200mA 10k VIN RC SS GND Figure 2. LT8570 configured for 48V output USER CONFIGURABILITY VIN 5V COUT VC RT Figure 1. LT3580 configured for 48V output RFBX FBX LT8570 CIN SS RT SW SHDN VC RT RT D1 RT VC GND 56.2k 6.04k SS 47pF 0.22µF 3.3nF Figure 4. LT8570-1 configured as a 5V input to 12V output boost converter July 2015 : LT Journal of Analog Innovation | 15 VIN 9V TO 16V UP TO 40V TRANSIENT • C1 1µF L1 22µH D1 L2 22µH 487k SW SHDN COUT 4.7µF VC GND 84.5k CIN 4.7µF 16.2k SS 0.22µF BOOST CONVERTER COUT 4.7µF VC GND 13.7k SS 47pF 0.22µF 10nF Figure 6. LT8580 configured as 5V–40V In to −15V out dual inductor inverting converter LT8580 required 0805 size capacitors, the LT8570-1 can use 0603 size capacitors. SEPIC CONVERTERS The SEPIC topology creates a positive voltage where the input voltage may be less than or greater than the output voltage. Due to a lack of DC path from input to output, it also offers output disconnect, so that there is no output voltage if the converter is shut down. Output disconnect makes the converter resistant to damage in case of output shorts. The application in Figure 5 shows the LT8580 configured to produce 12V from an input range of 9V to 16V, and able to survive 40V transients on VIN . 90 640 80 560 70 480 60 400 50 320 40 240 160 30 EFFICIENCY POWER LOSS 20 10 0 50 100 150 LOAD CURRENT (mA) 200 POWER LOSS (mW) The boost topology creates an output voltage greater than the input voltage. Since the boost converter is the simplest topology for LT8580, LT8570, and LT8570-1, it can clearly illustrate how the different current limits affect solution size. Figure 3 shows the LT8580 in a 12V out boost converter and Figure 4 shows the LT8570-1 in the same converter. Note that the only significant circuit changes required between the two are the inductor, the input capacitor, and the output capacitor. Both applications use similar inductors in the Würth WE-LQS family, but the LT8580 requires an inductor that is 5mm × 5mm, while LT8570-1 can use an inductor that is only 3mm × 3mm. This reduces the inductor footprint from 25mm2 to 9mm2. At the same time, the height drops from 4mm to 1.5mm. Also, where 182k L1, L2: COILCRAFT 22µH MSD7342-223 D1: CENTRAL SEMI CMMSH1-60 CIN: 4.7µF, 50V, 1206, X5R COUT : 4.7µF, 25V, 1206, X7R C1: 1µF, 100V, 0805, X7S EFFICIENCY (%) The FBX pin on LT8580/LT8570/LT8570-1 makes setting output voltage easy for both inverting and noninverting topologies. In both cases, only a single resistor from VOUT to FBX is needed to set the output voltage—the converter topology determines whether the output is positive or negative. 16 | July 2015 : LT Journal of Analog Innovation SYNC 113k Figure 5. LT8580 configured as 9V–16V In to 12V output SEPIC converter SIMPLE AND EASY OUTPUT VOLTAGE CONFIGURATION FBX LT8580 1nF L1, L2: WÜRTH 22µH WE-DD 744877220 D1: DIODES INC. DFLS1100 CIN: 4.7µF, 50V, 1206, X7R COUT : 4.7µF, 25V, 1206, X7R C1: 1µF, 100V, 0805, X7S VOUT –15V 90mA (VIN = 5V) 210mA (VIN = 12V) 420mA (VIN = 40V) • SW SHDN RT 22pF L2 22µH D1 VIN LT8580 RT C1 1µF L1 22µH 10k 130k FBX SYNC • VIN 5V TO 40V • VIN CIN 4.7µF VOUT 12V 240mA 80 0 Figure 7. Efficiency and power loss for Figure 6 with VIN = 12V DUAL INDUCTOR INVERTING CONVERTER The dual inductor inverting topology creates a negative voltage from a positive input voltage, which may be greater than or less than the magnitude of the output voltage. This topology, like the SEPIC, has output disconnect. In addition, this topology tends to have a quieter output than the boost or SEPIC, since L2 is in series with the output. The converter in Figure 6 shows the LT8580 configured as a dual inductor inverting converter with a −15V output, and Figure 7 shows the efficiency and power loss versus load. CONCLUSION The popular LT3580 monolithic boost/ inverting converter has been joined by the pin-compatible LT8580, LT8570, and LT8570-1 converters, which add current options and higher voltages, while retaining the features of the LT3580. These new options provide an additional means of optimizing a power supply for a given application. Depending on the intended load, solution size and part counts can be reduced. By retaining pin compatibility, transition within the LT3580, LT8580, LT8570 and LT8570-1 family of parts is easy, allowing simple design changes and PCB reuse. n