LM3370TL-3008

LM3370 Dual Synchronous Step-Down DC-DC Converter with Dynamic Voltage Scaling Function January 4, 2008 LM3370 Dual Synchronous Step-Down DC-DC Converter with Dynamic Voltage Scaling Function General Description The LM3370 is a dual step-down DC-DC converter optimized for powering ultra-low voltage circuits from a single Li-Ion battery and input rail ranging from 2.7V to 5.5V. It provides two outputs with 600 mA load per channel. The output voltage range varies from 1V to 3.3V and can be dynamically controlled using the I2C compatible interface. This dynamic voltage scaling function allows processors to achieve maximum performance at the lowest power level. The I2C compatible interface can also be used to control auto PFM-PWM/PWM mode selection and other performance enhancing features. The LM3370 offers superior features and performance for portable systems with complex power management requirements. Automatic intelligent switching between PWM lownoise and PFM low-current mode offers improved system efficiency. Internal synchronous rectification enhances the converter efficiency without the use of further external devices. There is a power-on-reset function that monitors the level of the output voltage to avoid unexpected power losses. The independent enable pin for each output allows for simple and effective power sequencing. LM3370 is available in a 4 mm by 5 mm 16-lead non-pullback LLP and a 20-Bump micro SMD, 3.0 mm x 2.0 mm x 0.6 mm, package. A high switching frequency—2 MHz (typ)—allows use of tiny surface-mount components including a 2.2 µH inductor. Default fixed voltages for the 2 output voltages combination can be customized to fit system requirements by contacting National Semiconductor Corporation. Features ■ I2C compatible interface — VOUT1 = 1V to 2V in 50 mV steps — VOUT2 = 1.8V to 3.3V in 100 mV steps — Automatic PFM/PWM mode switching & Forced PWM mode for low noise operation — Spread Spectrum capability using I2C 600 mA load per channel 2 MHz PWM fixed switching frequency (typ.) Internal synchronous rectification for high efficiency Internal soft start Power-on-reset function for both outputs 2.7V ≤ VIN ≤ 5.5V Operates from a single Li-Ion cell or 3 cell NiMH/NiCd batteries and 3.3V/5.5V fixed rails 2.2 µH Inductor, 4.7 µF Input and 10 µF Output Capacitor per channel 16-lead LLP Package (4 mm x 5 mm x 0.8 mm) 20-Bump micro SMD Package (3.0 mm x 2.0 mm x 0.6 mm) ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Applications ■ ■ ■ ■ Baseband Processors Application Processors (Video, Audio) I/O Power FPGA Power and CPLD Typical Performance Curve 20167379 Dimensions: 3.0 mm x 2.0 mm x 0.60 mm 20167381 Efficiency vs. Output Current, VOUT1= 1.2V © 2008 National Semiconductor Corporation 201673 www.national.com LM3370 Typical Application 20167301 FIGURE 1. Typical Application Circuit Functional Block Diagram 20167302 FIGURE 2. Functional Diagram www.national.com 2 LM3370 Package Marking Information 16-Lead LLP Package 20167343 FIGURE 3. Top Marking 20167342 FIGURE 4. Top View Pin Descriptions (LLP) Pin # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Name VIN2 SW2 PGND2 VDD SGND PGND1 SW1 VIN1 FB1 SDA SCL nPOR1 nPOR2 EN1 EN2 FB2 Buck 2 Switch Pin Buck 2 Power Ground Signal supply voltage input, VDD must be equal or greater of the two inputs (VIN1 & VIN2) Signal GND Buck 1 Power Ground Buck 1 Switch Pin Power supply voltage input to PFET and NFET switches for Buck 1 Analog Feedback Input for Buck 1 I2C Compatible Data, a 2 kΩ pull up resistor is required I2C Compatible Clock, a 2 kΩ pull up resistor is required Power ON Reset for Buck 1, Open drain output Low when Buck 1 output is 92% of target output. A 100 kΩ pull up resistor is required Power ON Reset for Buck 2, Open drain output Low when Buck 2 output is 92% of target output. A 100 kΩ pull up resistor is required Buck 1 Enable Buck 2 Enable Analog feedback for Buck 2 Description Power supply voltage input to PFET and NFET switches for Buck 2 3 www.national.com LM3370 Package Marking Information (micro SMD) micro SMD Marking 20167379 Top View XY = Date Code TT = Die Run Traceability S = Switcher Family PSB = LM3370TL-3013 20167377 20167378 Top View Bottom View Pin Descriptions (micro SMD) Pin # A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4 Name SW1 VIN1 SGND FB1 PGND1 PGND1_S SDA SCL VDD SGND nPOR1 nPOR2 PGND2 PGND2_S EN2 EN1 SW2 VIN2 SGND FB2 Buck 1 Switch Pin Power supply voltage input to PFET and NFET switches for Buck 1 Signal GND Analog Feedback Input for Buck 1 Buck 1 Power Ground Buck 1 Power Ground Sense I2C Compatible Data, a 2 kΩ pull up resistor is required I2C Compatible Clock, a 2 kΩ pull up resistor is required Signal supply voltage input, VDD must be equal or greater of the two inputs ( VIN1 & VIN2) Signal GND Power ON Reset for Buck 1, Open drain output Low when Buck 1 output is 92% of target output. A 100 kΩ pull up resistor is required Power ON Reset for Buck 2, Open drain output Low when Buck 2 output is 92% of target output. A 100 kΩ pull up resistor is required Buck 2 Power Ground Buck 2 Power Ground Sense Buck 2 Enable Buck 1 Enable Buck 2 Switch Pin Power supply voltage input to PFET and NFET switches for Buck 2 Signal GND Analog feedback for Buck 2 Description www.national.com 4 LM3370 I2C Controlled Features Features Output Voltage Modes Parameter VOUT1 & VOUT2 Buck 1 & Buck 2 Mode can be controlled via I2C compatible by either forcing device in Auto mode or forced PWM mode Spread Spectrum capability via I2C compatible for noise reduction Comments Output voltage is controlled via I2C compatible Spread Spectrum Buck 1 & Buck 2 Ordering Information (LLP) Order Number LM3370SD-3013 LM3370SDX-3013 LM3370SD-3021 LM3370SDX-3021 LM3370SD-3416 LM3370SDX-3416 LM3370SD-3621 LM3370SDX-3621 LM3370SD-3806 LM3370SDX-3806 LM3370SD-4221 LM3370SDX-4221 1.8V & 3.3V 1.6V & 1.8V 1.5V & 3.3V 1.4V & 2.8V 1.2V & 3.3V Voltage Option 1.2V & 2.5V Package Marking S0003UB S0003UB S0003TB S0003TB S0003VB S0003VB S0004AB S0004AB S0003XB S0003XB S0003YB S0003YB Supplied As 1000 units, Tape-and-Reel 4500 units, Tape and Reel 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel 1000 units, Tape-and-Reel 4500 units, Tape-and-Reel (micro SMD) Order Number LM3370TL-3607 NOPB LM3370TLX-3607 NOPB LM3370TL-3008 NOPB LM3370TLX-3008 NOPB LM3370TL-3006 NOPB LM3370TLX-3006 NOPB LM3370TL-3806 NOPB LM3370TLX-3806 NOPB LM3370TL-3206 NOPB LM3370TLX-3206 NOPB LM3370TL-3022 NOPB LM3370TLX-3022 NOPB 1.2V & 1.85V 1.3V & 1.8V 1.6V & 1.8V 1.2V & 1.8V 1.2V & 2.0V Voltage Option 1.5V & 1.9V Package Marking SPSB SPSB SPTB SPTB SPUB SPUB SPVB SPVB SPXB SPXB STHB STHB Supplied As 1000 units, Tape-and-Reel 3000 units, Tape-and-Reel 1000 units, Tape-and-Reel 3000 units, Tape-and-Reel 1000 units, Tape-and-Reel 3000 units, Tape-and-Reel 1000 units, Tape-and-Reel 3000 units, Tape-and-Reel 1000 units, Tape-and-Reel 3000 units, Tape-and-Reel 1000 units, Tape-and-Reel 3000 units, Tape-and-Reel Note the LM3370TL-3607 has the following default output voltages where VOUT1 = 1.5V & VOUT2 = 1.9V 5 www.national.com LM3370 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN1 , VIN2 VDD to PGND & SGND −0.2V to 6V PGND to SGND -0.2V to +0.2V SDA, SCL, EN, EN2, nPOR1, nPOR2, SW1, SW2, FB1 & FB2 (GND - 0.2) to (VIN + 0.2V) Maximum Continuous Power Dissipation (PD_MAX) (Note 3) Internally Limited Junction Temperature (TJ-MAX) 125°C Storage Temperature Range −65°C to +150°C Maximum Lead Temperature (Soldering) (Note 4) ESD Ratings (Note 5) All Pins 2 kV HBM 200V MM (Notes 1, 2) Operating Ratings Input Voltage Range ((Note 10)) 2.7V to 5.5V Recommended Load Current Per 0 mA to 600 mA Channel Junction Temperature (TJ) Range −30°C to +125°C Ambient Temperature (TA) Range (Note −30°C to +85°C 6) Thermal Properties  θJA (20-Bump micro SMD) (Note 7) 26°C/W 50°C/W Junction-to-Ambient Thermal Resistance  θJA (LLP-16) Electrical Characteristics (Notes 2, 8, 10)Typical limits appearing in normal type apply for TJ = 25°C. Limits appearing in boldface type apply over the entire junction temperature range (TA = TJ = −30°C to +85°C). Unless otherwise noted, VIN1 = VIN2 = 3.6V. Symbol VFB VOUT Parameter Feedback Voltage Line Regulation Load Regulation IQ PFM IQ SD ILIM RDS_ON (LLP) Quiescent Current “On” Quiescent Current “Off” Peak Switching Current Limit PFET NFET (Note 11) 2.7V ≤ VIN ≤ 5.5V IO = 10 mA, VOUT = 1.8V 100 mA ≤ IO ≤ 600 mA VIN = 3.6V, VOUT = 1.8V PFM Mode, Both Bucks ON EN1 = EN2 = 0V VIN = 3.6V VIN = 3.6V, ISW = 200 mA VIN = 3.6V, ISW = 200 mA VIN = 3.6V, ISW = 200 mA VIN = 3.6V, ISW = 200 mA 1.5 850 Conditions Min -3.5 0.031 0.0013 34 0.2 1200 390 240 350 170 2.0 0.01 1.0 50 mS (default) Can be pre-trimmd to 50 uS, 100 mS & 200 mS VOUT Rising VOUT Falling, 85% (default), Can be pre-trimmed to 70% or 94% 50 mS 3 1400 500 350 400 210 2.4 1 0.4 Typ Max +3.5 Units % %/V %/mA µA µA mA mΩ mΩ MHz µA V V RDS_ON PFET (micro SMD) NFET FOSC IEN VIL VIH nPOR1 & nPOR2 Delay Time POR Threshold Internal Oscillator Frequency Enable (EN) Input Current Enable Logic Low Enable Logic High nPOR1 = Power ON Reset for Buck 1 nPOR2 = Power ON Reset for Buck 2 Percentage of Target VOUT POWER ON RESET THRESHOLD/FUNCTION (POR) 94 85 % www.national.com 6 LM3370 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 guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 2: All voltages are with respect to the potential at the GND pin. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. The thermal shutdown engages at TJ = 150°C (typ.) and disengages at TJ = 140°C(typ.). Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note 1187: Leadless Leadframe Package (LLP) (AN-1187). Note 5: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. (MIL-STD-883 3015.7) The machine model is a 200 pF capacitor discharged directly into each pin. (EAIJ) Note 6: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 125ºC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to-ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). Note 7: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. The test board is a 4-layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm with a 2 x 1 array of thermal vias. Thickness of copper layers are 2/1/1/2oz. Junction-to-ambient thermal resistance is highly application and board-layout dependent. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues in board design. The value of θJA of this product can vary significantly, depending on PCB material, layout, and environmental conditions. In applications where high maximum power dissipation exists (high VIN, high IOUT), special care must be paid to thermal dissipation issues. For more information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Power Efficiency and Power Dissipation section of this datasheet. Note 8: Min. and Max are guaranteed by design, test and/or statistical analysis. All electrical characteristics having room-temperature limits are tested during production with TJ = 25°C. All hot and cold limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control. Note 9: Guaranteed by design. Note 10: Input voltage range for all voltage options is 2.7V to 5.5V. The voltage range recommended for the specified output voltages: VIN = 2.7V to 5.5V for 1V ≤ VOUT ≤ 1.7V and for VOUT = 1.8V or greater, VIN = VOUT + 1V or VIN,MIN = ILOAD * (RDSON_PFET + RDCR_INDUCTOR) + VOUT Note 11: Test condition: for VOUT less than 2.5V, VIN = 3.6V; for VOUT greater than or equal to 2.5V, VIN = VOUT + 1V. Dissipation Rating Table θJA 26°C/W (4-Layer Board) LLP-16 50°C/W (4-Layer Board) 20-bump micro SMD 1300 mW TA = 60°C Power Rating TA = 85°C Power Rating 1538 mW 800 mW 7 www.national.com LM3370 Typical Performance Characteristics LM3370SD/TL, Circuit of Figure 1, VIN = 3.6V, VOUT1 = 1.5V & VOUT2 = 2.5V, L = 2,2 µH (NR3015T2R2M), CIN = 4.7 µF (0805) and COUT = 10 µF (0805) & TA = 25°C, unless otherwise noted. IQ_PFM (Non Switching) Both Channels IQ_PWM (Non Switching) Both Channels 20167357 20167358 IQ_PWM (Switching) Both Channels IQ_SD (EN1 = EN2 = 0V) 20167349 20167359 RDS_ON (PFET) vs. Temperature VIN = 3.6V RDS_ON (NFET) vs. Temperature VIN = 3.6V 20167346 20167347 www.national.com 8 LM3370 RDS_ON (LLP) vs. VIN Current Limit vs. VIN 20167348 20167353 Switching Frequency vs. VIN Output Voltage vs. Output Current VIN = 3.6V (Forced PWM) 20167360 20167370 Efficiency vs. Output Current Forced PWM Mode, VOUT1 = 1.2V Efficiency vs. Output Current Forced PWM Mode, VOUT1 = 1.8V 20167362 20167363 9 www.national.com LM3370 Efficiency vs. Output Current Auto Mode, VOUT1 = 1.5V Efficiency vs. Output Current Auto Mode, VOUT2 = 1.9V 20167380 20167381 Efficiency vs. Output Current Auto Mode, VOUT2 = 3.3V Efficiency vs. Output Current Forced PWM Mode, VOUT2 = 3.3V 20167367 20167366 Typical Operation Waveform VIN = 3.6V, VOUT1 = 1.8V & VOUT2 = 1.8V Load = 400 mA Typical Operation Waveform VIN = 4.8V, VOUT1 = 1V & VOUT2 = 3.3V Load = 400 mA 20167320 20167321 www.national.com 10 LM3370 Typical Operation Waveform VIN = 3.6V, VOUT1 = 1.5V, VOUT2 = 2.5V, Load = 600 mA Each Start-up at PWM for BUCK1 ( VIN = 3.6V, VOUT = 1.5V, Load = 200 mA ) 20167327 20167322 Start-up at PWM for BUCK2 ( VIN = 3.6V, VOUT = 2.5V, Load = 200 mA ) Line Transient ( VOUT1 = 1.2V ) 20167325 20167330 Line Transient ( VOUT2 = 1.8V ) Load Transient in PFM MODE ( VOUT1 = 1.5V ) 20167331 20167332 11 www.national.com LM3370 Load Transient in PFM MODE ( VOUT1 = 1.5V ) Load Transient in PFM MODE ( VOUT1 = 1.8V ) 20167333 20167334 Load Transient in PFM MODE ( VOUT1 = 1.8V ) Load Transient in PWM MODE ( VIN = 3.6V, VOUT1 = 1.2V ) 20167335 20167338 Load Transient in PWM MODE ( VIN = 3.6V, VOUT1 = 1.5V ) Load Transient in PWM MODE ( VIN = 3.6V, VOUT2 = 2.5V ) 20167339 20167341 www.national.com 12 LM3370 Spread Spectrum Enabling ( VOUT Signal at 2 MHz) VOUT Stepping ( From 1.8V to 3.3V ) 20167371 20167375 VOUT Stepping ( From 3.3V to 1.8V ) 20167372 13 www.national.com LM3370 Operation Description DEVICE INFORMATION The LM3370, a dual high efficiency step down DC-DC converter, delivers regulated voltages from input rails between 2.7V to 5.5V. Using voltage mode architecture with synchronous rectification, the LM3370 has the ability to deliver up to 600 mA per channel. The performance is optimized for systems where efficiency and space are critical. There are three modes of operation depending on the current required—PWM, PFM, and shutdown. PWM mode handles loads of approximately 70 mA or higher with 90% efficiency or better. Lighter loads cause the device to automatically switch into PFM mode to maintain high efficiency with low supply current (IQ = 20 µA typ.) per channel. The LM3370 can operate up to a 100% duty cycle (PFET switch always on) for low drop out control of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage. Additional features include soft-start, under voltage lock out, current overload protection, and thermal overload protection. CIRCUIT OPERATION During the first portion of each switching cycle, the control block in the LM3370 turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of CURRENT LIMITING A current limit feature allows the LM3370 to protect itself and external components during overload conditions. PWM mode implements cycle-by-cycle current limiting using an internal comparator that trips at 1200 mA (typ.). If the outputs are shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until the inductor current falls below a low threshold, ensuring inductor has more time to decay, thereby preventing runaway. PFM OPERATION At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply current to maintain high efficiency. The part will automatically transition into PFM mode when either of two conditions are true, for a duration of 32 or more clock cycles: 1. The NFET current reaches zero. 2. The peak PFET switch current drops below the IMODE level . by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage across the load. PWM OPERATION During PWM operation the converter operates as a voltagemode controller with input voltage feed forward. This allows the converter to achieve excellent load and line regulation. The DC gain of the power stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is introduced. INTERNAL SYNCHRONOUS RECTIFICATION While in PWM mode, the LM3370 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. Supply current during this PFM mode is less than 20 µA per channel, which allows the part to achieve high efficiency under extremely light load conditions. When the output drops below the ‘low’ PFM threshold, the cycle repeats to restore the output voltage to ∼1.2% above the nominal PWM output voltage. If the load current should increase during PFM mode (see Figure 5) causing the output voltage to fall below the ‘low2’ PFM threshold, the part will automatically transition into fixedfrequency PWM mode. During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output FETs such that the output voltage ramps between 0.8% and 1.6% (typical) above the nominal PWM output voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PFET power switch is turned on. It remains on until the output voltage exceeds the ‘high’ PFM threshold or the peak current exceeds the I PFM level set for PFM mode. The typical peak current in PFM mode is: IPFM = 115 mA + VIN/57Ω Once the PFET power switch is turned off, the NFET power switch is turned on until the inductor current ramps to zero. When the NFET zero-current condition is detected, the NFET power switch is turned off. If the output voltage is below the ‘high’ PFM comparator threshold (see Figure 5), the PFET switch is again turned on and the cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM threshold, the NFET switch is turned on briefly to ramp the inductor current to zero and then both output switches are turned off and the part enters an extremely low power mode. www.national.com 14 LM3370 FORCED PWM MODE The LM3370 auto mode can be bypassed by forcing the device to operate in PWM mode, this can be implemented through the I2C compatible interface, see Table 1. SOFT-START The LM3370 has a soft start circuit that limits in-rush current during start up. Soft start is activated only if EN goes from logic low to logic high after VIN reaches 2.7V. LDO - LOW DROP OUT OPERATION The LM3370 can operate at 100% duty cycle (no switching, PFET switch completely on) for low drop out support of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage. The minimum input voltage needed to support the output voltage is VIN,MIN = ILOAD*(RDSON,PFET + RINDUCTOR) + VOUT • ILOAD load current • RDSON/PFET drain to source resistance of PFET switch in the triode region • RINDUCTOR inductor resistance 20167303 FIGURE 5. Operation in PFM Mode and Transfer to PWM Mode 15 www.national.com LM3370 I2C Compatible Interface Electrical Specifications Unless otherwise noted, VBATT = 2.7V to 5.5V. Typical values and limits appearing in normal type apply for TJ = 25°C. Limits appearing in boldface type apply over the entire junction temperature range for operation, −30°C to +125°C. (Notes 2, 8, 9) Symbol FCLK tBF tHOLD tCLKLP tCLKHP tSU tDATAHLD tCLKSU TSU TTRANS Clock Frequency Bus-Free Time between Start and Stop Hold Time Repeated Start Condition CLK Low Period CLK High Period Set Up Time Repeated Start Condition Data Hold Time Data Set Up Time Set Up Time for Start Condition Maximum Pulse Width of Spikes that Must be Suppressed by the Input Filter of Both DATA & CLK signals. I2C Logic High Level (Note 10) (Note 10) (Note 10) (Note 10) (Note 10) (Note 10) (Note 10) (Note 10) (Note 10) 1.3 0.6 1.3 0.6 0.6 200 200 0.6 50 Parameter Conditions Min Typ Max 400 Units kHz µS µS µS µS µS nS nS µS nS VDD_I2C 1 VIN V I2C Compatible Interface In I2C compatible mode, the SCL pin is used for the I2C clock and the SDA pin is used for the I2C data. Both these signals need a pull-up resistor according to I2C specification. The values of the pull-up resistor are determined by the capacitance of the bus (typ. ∼1.8k). Signal timing specifications are according to the I2C bus specification. Maximum frequency is 400 kHz. I2C COMPATIBLE DATA VALIDITY The data on SDA line must be stable during the HIGH period of the clock signal (SCL). In other words, state of the data line can only be changed when CLK is LOW. 20167306 I2C COMPATIBLE START AND STOP CONDITIONS START and STOP bits classify the beginning and the end of the I2C session. START condition is defined as SDA signal transitioning from HIGH to LOW while SCL line is HIGH. STOP condition is defined as the SDA transitioning from LOW to HIGH while SCL is HIGH. The I2C master always generates START and STOP bits. The I2C bus is considered to be busy after START condition and free after STOP condition. During data transmission, I2C master can generate repeated START conditions. First START and repeated START conditions are equivalent, function-wise. 20167307 www.national.com 16 LM3370 TRANSFERRING DATA Every byte put on the SDA line must be eight bits long, with the most significant bit (MSB) being transferred first. The number of bytes that can be transmitted per transfer is unrestricted. Each byte of data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated by the master. The transmitter releases the SDA line (HIGH) during the acknowledge clock pulse. The receiver must pull down the SDA line during the 9th clock pulse, signifying an acknowledge. A receiver which has been addressed must generate an acknowledge after each byte has been received. After the START condition, I2C master sends a chip address. This address is seven bits long followed by an eighth bit which is a data direction bit (R/W). For the eighth bit, a “0” indicates a WRITE and a “1” indicates a READ. The second byte selects the register to which the data will be written. The third byte contains data to write to the selected register. I2C Compatible Write Cycle 20167309 W = write (SDA = “0”) r = read (SDA = “1”) ack = acknowledge (SDA pulled down by either master or slave) rs = repeated startxx=36h However, if a READ function is to be accomplished, a WRITE function must precede the READ function, as shown in the read cycle waveform. I2C Compatible Read Cycle 20167310 17 www.national.com LM3370 Device Register Information Register Information Register Name Control Buck 1 Buck 2 Location 00 01 02 Type R/W R/W R/W Control signal for Buck 1 and Buck 2 Output setting & Mode selection for Buck 1 Output setting & Mode selection for Buck 2 and POR disable Function I2C CHIP ADDRESS INFORMATION 20167308 REGISTER 00 20167311 www.national.com 18 LM3370 REGISTER 01 20167312 REGISTER 02 20167313 19 www.national.com LM3370 TABLE 1. Output Selection Table via I2C Programing Buck Output Voltage Selection Codes Data Code 00000 00001 00010 00011 00100 00101 00110 00111 01000 01001 01010 01011 01100 01101 01110 01111 10000 10001 10010 10011 10100 10101 10110 10111 11000 11001 * Can be trimmed at the factory at 1.85V or 1.9V using the same trim code. Buck_1 (V) NA NA NA NA NA 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 Buck_2 (V) NA 1.8 1.85 or 1.9* 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 NA NA NA NA NA NA NA NA NA www.national.com 20 LM3370 Application Information SETTING OUTPUT VOLTAGE VIA I2C Compatible The outputs of the LM3370 can be programmed through Buck 1 & Buck 2 registers via I2C. Buck 1 output voltage can be dynamically adjusted between 1V to 2V in 50 mV steps and Buck 2 output voltage can be adjusted between 1.8V to 3.3V in 100 mV steps. Finer adjustments to the output of Buck 2 can be achieved with the placement of a resistor betweeen VOUT2 and the FB2 pin. Typically by placing a 20KΩ resistor, R, between these nodes will result in the programmed Output Voltage increasing by approximately 45mV,ΔVTYP . ΔVTYP= R × 500mV / 234KΩ Please refer to for programming the desire output voltage. If the I2C compatible feature is not used, the default output voltage will be the pre-trimmed voltage. For example, LM3370SD-3021 refers to 1.2V for Buck 1 and 3.3V for Buck 2. VDD Pin VDD is the power supply to the internal control circuit, if VDD pin is not tied to VIN during normal operating condition, VDD must be set equal or greater of the two inputs ( VIN1 or VIN2 ). An optional capacitor can be used for better noise immunity at VDD pin or when VDD is not tied to either VIN pins. Additionally, for reasons of noise suppression, it is advisable to tie the EN1/EN2 pins to VDD rather than VIN . SDA, SCL Pins When not using I2C the SDA and SCL pins should be tied directly to the VDD pin. Micro-Stepping: The Micro-Stepping feature minimizes output voltage overshoot/undershoot during large output transients. If Microstepping is enabled through I2C, the output voltage automatically ramps at 50 mV per step for Buck 1 and 100 mV per step for Buck 2. The steps are summarized as follow: Buck 1: 50 mV/step and 32 µs/step Buck 2: 100 mV/step and 32 µs/step For example if changing Buck 1 voltage from 1V to 1.8V yields 20 steps [(1.8 - 1)/ 0.05 = 20]. This translates to 640μs [(20 x 32 µs) = 640 µs] needed to reach the final target voltage. INDUCTOR SELECTION There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor current ripple is small enough to achieve the desired output voltage ripple. There are two methods to choose the inductor current rating. method 1: The total current is the sum of the load and the inductor ripple current. This can be written as method 2: A more conservative approach is to choose an inductor that can handle the maximum current limit of 1400 mA. Given a peak-to-peak current ripple (IPP) the inductor needs to be at least A 2.2 µH inductor with a saturation current rating of at least 1400 mA is recommended for most applications. The inductor’s resistance should be less than around 0.2Ω for good efficiency. Table 2 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor is suggested. For noise critical applications, a toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with overlapping footprints of both types for design flexibility. This allows substitution of a low-noise toroidal inductor, in the event that noise from low-cost bobbin models is unacceptable. Below are some suggested inductor manufacturers include but are not limited to: TABLE 2. Suggested Inductors and Suppliers Model DO3314-222 LPO3310-222 SD3114-2R2 NR3015T2R2M VLF3010AT2R2M1R0 TDK Cooper NR3010T2R2M Taiyo Yuden Vendor Coilcraft Dimensions (mm) 3.3 x 3.3 x 1.4 3.3 x 3.3 x 1.0 3.1 x 3.1 x 1.4 3.0 x 3.0 x 1.0 3.0 x 3.0 x 1.5 2.6 x 2.8 x 1.0 ISAT 1.6A 1.1A 1.48A 1.1A 1.48A 1.0A INPUT CAPACITOR SELECTION A ceramic input capacitor of 4.7 μF, 6.3V is sufficient for most applications. A larger value may be used for improved input voltage filtering. The input filter capacitor supplies current to the PFET switch of the LM3370 in the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor's low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with a surge current rating sufficient for the power-up surge from the input power source. The power-up surge current is approximately the capacitor’s value (µF) times the voltage rise rate (V/µs). The input current ripple can be calculated as: • • • • ILOAD load current VIN input voltage L inductor f switching frequency OUTPUT CAPACITOR SELECTION DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. DC 21 www.national.com LM3370 bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested from them as part of the capacitor selection process. The output filter capacitor smoothes out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these functions. The output ripple voltage can be calculated as: Voltage peak-to-peak ripple due to capacitance = VOUT) or 85% (falling VOUT) of the desire output. The inherent delay between the output (at 94% of VOUT) to the time at which the nPOR is enabled is about 50 ms. A pull up resistor of 100 kΩ at nPOR pin is required. Please refer to the electrical specification table for other timing options. The diagram below illustrates the timing response of the POR function. Voltage peak-to-peak ripple due to ESR = VPP-ESR = IPP*RESR Voltage peak-to-peak ripple, root mean squared = 20167319 Note that the output ripple is dependent on the current ripple and the equivalent series resistance of the output capacitor (RESR). The RESR is frequency dependent (as well as temperature dependent); make sure that the frequency of the RESR given is the same order of magnitude as the switching frequency. TABLE 3. Suggested Capacitors and Their Suppliers Model 4.7 µF for CIN C1608X5R0J475 C2012X5R0J475 JMK212BJ475 GRM21BR60J475 GRM219R60J475KE19D Ceramic, X5R, 6.3V Rating Ceramic, X5R, 6.3V Rating Ceramic, X5R, 6.3V Rating Ceramic, X5R, 6.3V Rating Ceramic, X5R, 6.3V Rating 0603 0805 0805 0805 0805 (Thin)