LMZ23605TZX

LMZ23605 5A SIMPLE SWITCHER® Power Module with 36V Maximum Input Voltage April 5, 2011 LMZ23605 5A SIMPLE SWITCHER® Power Module with 36V Maximum Input Voltage Easy to use 7 pin package Performance Benefits ■ ■ ■ ■ ■ High efficiency reduces system heat generation Complies with EN55022 Class B (Note 5) Low component count, only 5 external components Low output voltage ripple Uses PCB as heat sink, no airflow required System Performance 30116986 TO-PMOD 7 Pin Package 10.16 x 13.77 x 4.57 mm (0.4 x 0.542 x 0.18 in) θJA = 12°C/W, θJC = 1.9°C/W (Note 6) RoHS Compliant Efficiency VIN = 12V VOUT = 5.0V 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 0 1 2 3 4 DISSIPATION (W) 5 Electrical Specifications ■ ■ ■ ■ ■ 30W maximum total output power Up to 5A output current Input voltage range 6V to 36V Output voltage range 0.8V to 6V Efficiency up to 92% Key Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Integrated shielded inductor Simple PCB layout Frequency synchronization input (650 kHz to 950 kHz) Flexible startup sequencing using external soft-start, tracking and precision enable Protection against inrush currents and faults such as input UVLO and output short circuit – 40°C to 125°C junction temperature range Single exposed pad and standard pinout for easy mounting and manufacturing Fast transient response for powering FPGAs and ASICs Fully enabled for Webench® Power Designer Pin compatible with LMZ22005/LMZ23603/LMZ22003 Thermal derating curve VIN = 12V VOUT = 5.0V 6 MAXIMUM OUTPUT CURRENT (A) 5 4 3 2 1 0 θJA=12°C/W 30116903 30 40 50 60 70 80 90 100 110 120 130 AMBIENT TEMPERATURE (°C) 30116989 Applications ■ ■ ■ ■ Point of load conversions from 12V and 24V input rail Time critical projects Space constrained / high thermal requirement applications Negative output voltage applications See AN-2027 Radiated EMI (EN 55022) of Demo Board (See AN–2125) 50 45 AMPLITUDE (dBuV/m) 40 35 30 25 20 15 10 5 0 0 Class A Limit Class B Limit Horiz Peak Horiz Quasi-peak 200 400 600 800 FREQUENCY (MHz) 1000 30116995 © 2011 National Semiconductor Corporation 301169 www.national.com LMZ23605 Simplified Application Schematic 30116901 Connection Diagram 30116902 Top View 7-Lead TO-PMOD Ordering Information Order Number LMZ23605TZ LMZ23605TZE LMZ23605TZX Package Type TO-PMOD-7 TO-PMOD-7 TO-PMOD-7 NSC Package Drawing TZA07A TZA07A TZA07A Supplied As 45 Units in a Rail 250 Units on Tape and Reel 500 Units on Tape and Reel Pin Descriptions Pin 1 2 Name Description VIN Supply input — Nominal operating range is 6V to 36V . A small amount of internal capacitance is contained within the package assembly. Additional external input capacitance is required between this pin and exposed pad (PGND). SYNC Sync Input — Apply a CMOS logic level square wave whose frequency is between 650 kHz and 950 kHz to synchronize the PWM operating frequency to an external frequency source. When not using synchronization connect to ground. The module free running PWM frequency is 812 kHz (Typ). EN Enable — Input to the precision enable comparator. Rising threshold is 1.279V typical. Once the module is enabled, a 21 uA source current is internally activated to facilitate programmable hysteresis. Feedback — Internally connected to the regulation amplifier, over-voltage comparators. The regulation reference point is 0.796V at this input pin. Connect the feedback resistor divider between the output and AGND to set the output voltage. 3 4 5 AGND Analog Ground — Reference point for all stated voltages. Must be externally connected to PGND (EP). FB www.national.com 2 LMZ23605 Pin 6 7 EP Name Description SS/ TRK Soft-Start/Track — To extend the 1.6 mSec internal soft-start connect an external soft start capacitor. For tracking connect to an external resistive divider connected to a higher priority supply rail. See applications section VOUT Output Voltage — Output from the internal inductor. Connect the output capacitor between this pin and exposed pad. PGND Exposed Pad / Power Ground Electrical path for the power circuits within the module. — NOT Internally connected to AGND / pin 4. Used to dissipate heat from the package during operation. Must be electrically connected to pin 4 external to the package. 3 www.national.com LMZ23605 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to PGND EN, SYNC to AGND SS/TRK, FB to AGND AGND to PGND Junction Temperature Storage Temperature Range -0.3V to 40V -0.3V to 5.5V -0.3V to 2.5V -0.3V to 0.3V 150°C -65°C to 150°C ESD Susceptibility (Note 2) For soldering specifications: see product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf ± 2 kV Operating Ratings (Note 1) 6V to 36V 0V to 5.0V −40°C to 125°C VIN EN, SYNC Operation Junction Temperature Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of -40°C to +125°C. Minimum and Maximum limits are guaranteed through test, design or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated the following conditions apply: VIN = 12V, Vout = 3.3V Symbol Parameter Conditions Min (Note 3) Typ (Note 4) Max (Note 3) Units SYSTEM PARAMETERS Enable Control VEN VEN-HYS Soft-Start ISS tSS Current Limit ICL fosc fsync VIL-sync VIH-sync Sync d.c. Dmax VFB VFB-OV IFB IQ ISD TSD TSD-HYST θJA Current limit threshold Free-running oscillator frequency Synchronization range Synchronization logic zero amplitude Synchronization logic one amplitude Synchronization duty cycle range Maximum Duty Factor In-regulation feedback voltage Feedback over-voltage protection threshold Feedback input bias current Non Switching Input Current Shut Down Quiescent Current Thermal Shutdown Thermal shutdown hysteresis Junction to Ambient(Note 6) VFB= 0.86V VEN= 0V Rising Falling 4 layer Evaluation Printed Circuit Board, 60 vias, No air flow 2 layer JEDEC Printed Circuit Board, No air flow θJC www.national.com EN threshold trip point EN input hysteresis current SS source current Internal soft-start interval VEN rising VEN > 1.279V VSS = 0V 1.10 1.279 –21 1.458 V µA 40 50 1.6 60 µA msec A d.c. average Sync input connected to ground. 5.4 711 650 812 914 950 0.4 1.5 15 50 83 85 Internal Switching Oscillator kHz kHz V V % % 0.816 V V nA mA μA °C °C °C/W °C/W °C/W Relative to AGND Relative to AGND. Regulation and Over-Voltage Comparator VSS >+ 0.8V IO = 5A 0.776 0.796 0.86 5 2.6 70 165 15 12.0 21.5 1.9 Thermal Characteristics Junction to Case No air flow 4 LMZ23605 Symbol Parameter Conditions Min (Note 3) Typ (Note 4) 9 ±0.02 1 86 81.5 80 76 Max (Note 3) Units PERFORMANCE PARAMETERS(Note 7) Δ VO ΔVO/ΔVIN ΔVO/ΔIOUT η η η η Output voltage ripple Line regulation Load regulation Peak efficiency Full load efficiency Peak efficiency Full load efficiency Cout = 220uF w/ 7 milliohm ESR + 100uF X7R + 2 x 0.047uF BW@ 20 MHz VIN = 12V to 36V, IO= 0.001A VIN = 12V, IO= 0.001A to 5A VIN = 12V VO = 3.3V IO = 1A VIN = 12V VO = 3.3V IO = 5A VIN = 24V VO = 3.3V IO = 2A VIN = 24V VO = 3.3V IO = 5A mV PP % mV/A % % % % Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test method is per JESD-22-114. Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL). Note 4: Typical numbers are at 25°C and represent the most likely parametric norm. Note 5: EN 55022:2006, +A1:2007, FCC Part 15 Subpart B: 2007. See AN-2125 and layout for information on device under test. Vin = 24V Vo = 3.3V Io = 5A Note 6: Theta JA measured on a 3.5” x 3.5” four layer board, with three ounce copper on outer layers and two ounce copper on inner layers, sixty 10 mil thermal vias, no air flow, and 1W power dissipation. Refer to application note layout diagrams. Note 7: Refer to BOM in Table 1. 5 www.national.com LMZ23605 Typical Performance Characteristics Unless otherwise specified, the following conditions apply: VIN = 12V; Cin = 2 x 10μF + 1μF X7R Ceramic; CO = 220μF Specialty Polymer + 10 uF Ceramic; Tambient = 25° C for waveforms. Efficiency and dissipation plots marked with * have cycle skipping at light loads resulting in slightly higher output ripple – See applications section. Efficiency 6.0V output @ 25°C ambient 100 90 80 70 60 50 40 0 10 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116987 Dissipation 6.0V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116988 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 10 Vin Efficiency 5.0V output @ 25°C ambient 100 90 80 70 60 50 40 0 9 VIn 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116952 Dissipation 5.0V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116953 www.national.com EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin 6 LMZ23605 Efficiency 3.3V output @ 25°C ambient 100 90 80 70 60 50 40 0 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116954 Dissipation 3.3V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116955 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin Efficiency 2.5V output @ 25°C ambient 90 80 70 60 50 40 30 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116956 Dissipation 2.5V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116957 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin Efficiency 1.8V output @ 25°C ambient 90 80 70 60 50 40 30 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116958 Dissipation 1.8V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116959 EFFICIENCY (%) 36 Vin* 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin 7 www.national.com LMZ23605 Efficiency 1.5V output @ 25°C ambient 85 75 EFFICIENCY (%) 65 55 45 35 25 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin* 36 Vin* 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116960 Dissipation 1.5V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116961 36 Vin* 30 Vin* 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin Efficiency 1.2V output @ 25°C ambient 80 70 60 50 40 30 20 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin* 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116962 Dissipation 1.2V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116963 EFFICIENCY (%) 36 Vin* 30 Vin* 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin Efficiency 1.0V output @ 25°C ambient 80 70 60 50 40 30 20 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin* 30 Vin* 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116964 Dissipation 1.0V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116965 www.national.com EFFICIENCY (%) 36 Vin* 30 Vin* 24 Vin* 20 Vin 12 Vin 9 VIn 6 Vin 8 LMZ23605 Efficiency 0.8V output @ 25°C ambient 70 60 50 40 30 20 10 0 6 Vin 9 Vin 12 Vin 20 Vin* 24 Vin* 30 Vin* 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116990 Dissipation 0.8V output @ 25°C ambient 7 6 DISSIPATION (W) 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116991 EFFICIENCY (%) 36 Vin* 30 Vin* 24 Vin* 20 Vin* 12 Vin 9 Vin 6 Vin Efficiency 6.0V output @ 85°C ambient 100 90 80 70 60 50 40 0 10 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116926 Dissipation 6.0V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116927 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 10 Vin Efficiency 5.0V output @ 85°C ambient 100 90 80 70 60 50 40 0 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116928 Dissipation 5.0V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116929 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin 9 www.national.com LMZ23605 Efficiency 3.3V output @ 85°C ambient 90 80 70 60 50 40 30 0 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 1 2 3 4 OUTPUT CURRENT (A) 5 30116930 Dissipation 3.3V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116931 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin Efficiency 2.5V output @ 85°C ambient 90 80 70 60 50 40 30 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116932 Dissipation 2.5V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116933 EFFICIENCY (%) 36 Vin 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin Efficiency 1.8V output @ 85°C ambient 90 80 70 60 50 40 30 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116934 Dissipation 1.8V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116935 www.national.com EFFICIENCY (%) 36 Vin* 30 Vin 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin 10 LMZ23605 Efficiency 1.5V output @ 85°C ambient 80 70 60 50 40 30 20 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin* 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116936 Dissipation 1.5V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116937 EFFICIENCY (%) 36 Vin* 30 Vin* 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin Efficiency 1.2V output @ 85°C ambient 80 70 60 50 40 30 20 0 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin* 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 5 30116938 Dissipation 1.2V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116939 EFFICIENCY (%) 36 Vin* 30 Vin* 24 Vin 20 Vin 12 Vin 9 Vin 6 Vin Efficiency 1.0V output @ 85°C ambient 75 65 EFFICIENCY (%) 55 45 35 25 15 6 Vin 9 Vin 12 Vin 20 Vin 24 Vin* 30 Vin* 36 Vin* 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116940 Dissipation 1.0V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116941 36 Vin* 30 Vin* 24 Vin* 20 Vin 12 Vin 9 Vin 6 Vin 11 www.national.com LMZ23605 Efficiency 0.8V output @ 85°C ambient 70 60 Dissipation 0.8V output @ 85°C ambient 8 7 DISSIPATION (W) 6 5 4 3 2 1 0 5 30116992 EFFICIENCY (%) 50 40 30 20 0 36 Vin* 30 Vin* 24 Vin* 20 Vin* 12 Vin 9 Vin 6 Vin 6 Vin 9 Vin 12 Vin 20 Vin* 24 Vin* 30 Vin* 36 Vin* 1 2 3 4 OUTPUT CURRENT (A) 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116993 Thermal derating VIN = 12V, VOUT = 5.0V 6 MAXIMUM OUTPUT CURRENT (A) 5 4 3 2 1 0 θJA=12°C/W 30 40 50 60 70 80 90 100 110 120 130 AMBIENT TEMPERATURE (°C) 30116989 Thermal derating VIN= 12V, VOUT = 3.3V 6 MAXIMUM OUTPUT CURRENT (A) 5 4 3 2 1 0 θJA = 12 °C/W 30 40 50 60 70 80 90 100 110 120 130 TEMPERATURE (°C) 30116994 Thermal derating VIN = 24V, VOUT = 5.0V 6 MAXIMUM OUTPUT CURRENT (A) 5 4 3 2 1 θJA=12°C/W 0 30 40 50 60 70 80 90 100 110 120 130 AMBIENT TEMPERATURE (°C) 30116996 Thermal derating VIN = 24V, VOUT = 3.3V MAXCIMUM OUTPUT CURRENT (A) 6 5 4 3 2 1 0 θJA=12°C/W 30 40 50 60 70 80 90 100 110 120 130 AMBIENT TEMPERATURE (°C) 30116997 www.national.com 12 LMZ23605 Normalized Line and load regulation VOUT = 3.3V NORMALLIZED OUTPUT VOLTAGE (V/V) 1.002 Output ripple 12VIN 3.3VO @ 5A, BW = 20 MHz 1.001 1.000 9 Vin 12 Vin 20 Vin 24 Vin 30 Vin 36 Vin 0 1 2 3 4 OUTPUT CURRENT (A) 5 30116942 30116905 0.999 0.998 Output ripple 12VIN 3.3VO@ 5A BW = 250 MHz Transient response 12VIN 3.3VO0.5 to 5A Step 30116906 30116908 Short circuit current vs input voltage 9 8 7 CURRENT (A) 6 5 4 3 2 1 0 0 4 8 12 16 20 24 28 32 36 INPUT VOLTAGE (V) 30116918 Output Current Input Current 13 www.national.com LMZ23605 Block Diagram 30116910 General Description The LMZ23605 SIMPLE SWITCHER© power module is an easy-to-use step-down DC-DC solution capable of driving up to 5A load. The LMZ23605 is available in an innovative package that enhances thermal performance and allows for hand or machine soldering. The LMZ23605 can accept an input voltage rail between 6V and 36V and deliver an adjustable and highly accurate output voltage as low as 0.8V. The LMZ23605 only requires two external resistors and three external capacitors to complete the power solution. The LMZ23605 is a reliable and robust design with the following protection features: thermal shutdown, programmable input under-voltage lockout, output over-voltage protection, short-circuit protection, output current limit, and allows startup into a pre-biased output. The sync input allows synchronization over the 650 to 950 kHz switching frequency range. Design Steps for the LMZ23605 Application The LMZ23605 is fully supported by Webench® which offers: component selection, electrical and thermal simulations. Additionally there are both evaluation and demonstration boards that may be used as a starting point for design. The following list of steps can be used to manually design the LMZ23605 application. All references to values refer to the typical applications schematic figure 4. • Select minimum operating VIN with enable divider resistors • Program VO with resistor divider selection • Select CO • Select CIN • Determine module power dissipation • Layout PCB for required thermal performance ENABLE DIVIDER, RENT, RENB AND RENHSELECTION Internal to the module is a 2 mega ohm pull-up resistor connected from VIN to Enable. For applications not requiring precision under voltage lock out (UVLO), the Enable input may be left open circuit and the internal resistor will always enable the module. In such case, the internal UVLO occurs typically at 4.3V (VINrising). In applications with separate supervisory circuits Enable can be directly interfaced to a logic source. In the case of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than the LMZ23605 output rail. Enable provides a precise 1.279V threshold to allow direct logic drive or connection to a voltage divider from a higher enable voltage such as VIN. Additionally there is 21 μA(typ) of switched offset current allowing programmable hysteresis. See Figure 1. The function of the enable divider is to allow the designer to choose an input voltage below which the circuit will be disabled. This implements the feature of programmable UVLO. The two resistors should be chosen based on the following ratio: RENT / RENB = (VIN UVLO / 1.279V) – 1 (1) The LMZ23605 typical application shows 12.7kΩ for RENB and 42.2kΩ for RENT resulting in a rising UVLO of 5.46V. Note that this divider presents 8.33V to the input when the divider is raised to 36V which would exceed the recommended 5.5V limit for Enable. A midpoint 5.1V Zener clamp is applied to allow the application to cover the full 6V to 36V range of operation. The zener clamp is not required if the target application prohibits the maximum Enable input voltage from being exceeded. Additional enable voltage hysteresis can be added with the inclusion of RENH. It is possible to select values for RENT and RENB such that RENH is a value of zero allowing it to be omitted from the design. Rising threshold can be calculated as follows: VEN(rising) = 1.279 ( 1 + (RENT|| 2 meg)/ RENB) www.national.com 14 LMZ23605 Whereas the falling threshold level can be calculated using: VEN(falling) = VEN(rising) – 21 µA ( RENT|| 2 meg || RENTB + RENH ) Enable input detail 30116909 FIGURE 1. OUTPUT VOLTAGE SELECTION Output voltage is determined by a divider of two resistors connected between VO and ground. The midpoint of the divider is connected to the FB input. The regulated output voltage determined by the external divider resistors RFBT and RFBB is: VO = 0.796V * (1 + RFBT / RFBB) (2) Rearranging terms; the ratio of the feedback resistors for a desired output voltage is: RFBT / RFBB = (VO / 0.796V) - 1 (3) These resistors should generally be chosen from values in the range of 1.0 kΩ to 10.0 kΩ. For VO = 0.8V the FB pin can be connected to the output directly and RFBB can be set to 8.06kΩ to provide minimum output load. A table of values for RFBT , and RFBB, is included in the simplified applications schematic on page 2. SOFT-START CAPACITOR SELECTION Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time. Upon turn-on, after all UVLO conditions have been passed, an internal 1.6mSec circuit slowly ramps the SS/TRK input to implement internal soft start. If 2 mSec is an adequate turn– on time then the Css capacitor can be left unpopulated. Longer soft-start periods are achieved by adding an external capacitor to this input. Soft start duration is given by the formula: tSS = VREF * CSS / Iss = 0.796V * CSS / 50uA (4) This equation can be rearranged as follows: CSS = tSS * 50μA / 0.796V (5) Using a 0.22μF capacitor results in 3.5 msec typical soft-start duration; and 0.47μF results in 7.5 msec typical. 0.47 μF is a recommended initial value. As the soft-start input exceeds 0.796V the output of the power stage will be in regulation and the 50 μA current is deactivated. Note that the following conditions will reset the soft-start capacitor by discharging the SS input to ground with an internal current sink. • The Enable input being pulled low • Thermal shutdown condition • Internal Vcc UVLO (Approx 4.3V input to VIN) TRACKING SUPPLY DIVIDER OPTION The tracking function allows the module to be connected as a slave supply to a primary voltage rail (often the 3.3V system rail) where the slave module output voltage is lower than that of the master. Proper configuration allows the slave rail to power up coincident with the master rail such that the voltage difference between the rails during ramp-up is small (i.e.