National Semiconductor Application Note 2027 Alan Martin March 24, 2010 Introduction is referred to ground while disabled and to –VOUT once enabled. This adds a large and possibly undesirable amount of hysteresis to the simplest form of enable. To alleviate this situation several level shift methods are discussed in the following text. It should also be noted that the maximum output current obtained from the module is decreased from that obtained in the conventional buck configuration. (See graphs) Further, since the ground terminal of the module is connected to –VOUT the maximum positive input voltage into the inverting application is decreased by the amplitude of the output voltage.Thus for a –5V output application the maximum input voltage will be 37V. This configuration can be applied to the whole family of LMZ1420x and LMZ1200x modules so long as the input voltage and output current limits are observed. Be aware that efficiency is lower in the inverting configuration resulting in higher dissipation for a given output power and that thermal derating may need to be observed when operating at maximum output current. This application note illustrates how to apply the LMZ14203 integrated buck module into the buck-boost configuration such that a positive input voltage can be used to create a regulated negative output voltage. It is also shown how the conventional non-inverting demonstration and evaluation boards for the device can be connected into the inverting configuration without the need to acquire a new PCB to evaluate the application. Refer to AN-2024, AN-2031, and AN-2032 for detailed descriptions on the PCB assemblies in the conventional buck topology. Figure 1 illustrates the method of reassigning the terminals of the evaluation board (or demo board) for the inverting application. Careful labeling of leads is suggested to avoid confusion since the terminals formerly at ground potential are now connected to –VOUT, and the connection formerly assigned to +Vout is now connected to ground. The 3 connections to the power path are straightforward. But there may be additional circuitry required for the enable signal to function as desired in the target system as the precision enable reference voltage Inverting Application for the LMZ14203 SIMPLE SWITCHER Power Module Inverting Application for the LMZ14203 SIMPLE SWITCHER® Power Module 30113401 FIGURE 1. Evaluation board connections for inverting application AN-2027 © 2010 National Semiconductor Corporation 301134 www.national.com AN-2027 30113406 FIGURE 2. Inverting application schematic for simple enable and –5VOUT. For BOM refer to AN-2024. the enable input of the LMZ14203 which in this application is referenced to –VOUT. The level shift is quite straight forward and can be accomplished with a single transistor. The transistor type can either be small signal PNP or low level Pchannel mosfet. The transistor terminal connections are essentially identical. These circuits are shown in Figures 3 and 4. For applications where precision UVLO is needed with a small and controllable amount of hysteresis then an adjustable shunt reference can be configured as a precision comparator to meet the requirements. Suggested circuitry is detailed in figure 5. The first is based on the common LMV431 type device with a PNP inverter output section. The other circuit uses the similar LM4041 that differs in that it has high side feedback reference and the inversion is not required. Either circuit is both low cost and compact. Enable Options Essentially there are three methods for enabling the module in the inverting application. The first is the precision threshold shown in figure 2. UVLO (Under Voltage Lock Out) is determined by the values of RENT and RENB in the same manner as described in the data sheet. Once the module is enabled –VOUT goes from its initial ground potential to the regulated negative VOUT level at a rate determined by the soft-start capacitor. Since RENB is also tied to –VOUT a reinforcing action occurs that increases the 90 mV hysteresis level substantially such that the total hysteresis is essentially the magnitude of Vout. As previously suggested, a hysteresis level this large may be undesirable in certain system situations so two other methods are described as alternatives. Many systems have ground referred control or supervisory logic signals that need to be level shifted for compatibility with www.national.com 2 AN-2027 External Enable Logic Level Shifters 30113402 FIGURE 3. PNP Level Shifter 30113403 FIGURE 4. P-ch MOSFET Level Shifter 30113407 FIGURE 5. Adjustable Shunt Reference Based Precision UVLO Circuits 3 www.national.com AN-2027 Typical Performance Characteristics Max IOUT vs Input Voltage Max IOUT vs Input Voltage 30113416 30113413 Efficiency at Max IOUT Efficiency at Max IOUT 30113414 30113417 Dissipation at Max IOUT Dissipation at Max IOUT 30113418 30113415 www.national.com 4 AN-2027 Output Ripple Output Ripple 30113420 30113419 Startup Startup 30113421 30113422 5 www.national.com Inverting Application for the LMZ14203 SIMPLE SWITCHER Power Module Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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