LM5022

LM5022 60V Low Side Controller for Boost and SEPIC January 2007 LM5022 60V Low Side Controller for Boost and SEPIC General Description The LM5022 is a high voltage low-side N-channel MOSFET controller ideal for use in boost and SEPIC regulators. It contains all of the features needed to implement single ended primary topologies. Output voltage regulation is based on current-mode control, which eases the design of loop compensation while providing inherent input voltage feed-forward. The LM5022 includes a start-up regulator that operates over a wide input range of 6V to 60V. The PWM controller is designed for high speed capability including an oscillator frequency range up to 2 MHz and total propagation delays less than 100 ns. Additional features include an error amplifier, precision reference, line under-voltage lockout, cycle-by-cycle current limit, slope compensation, soft-start, external synchronization capability and thermal shutdown. The LM5022 is available in the MSOP-10 package. Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Internal 60V Startup Regulator 1A Peak MOSFET Gate Driver VIN Range 6V to 60V Duty Cycle Limit of 90% Programmable UVLO with Hysteresis Cycle-by-Cycle Current Limit External Synchronizable (AC-coupled) Single Resistor Oscillator Frequency Set Slope Compensation Adjustable Soft-start MSOP-10 Package Applications ■ Boost Converter ■ SEPIC Converter Typical Application 20212201 © 2007 National Semiconductor Corporation 202122 www.national.com LM5022 Connection Diagram 20212253 10-Lead MSOP NS Package Number MUB10A Ordering Information Part Number LM5022MM LM5022MMX NS Package Drawing MUB10A MUB10A Supplied As 1000 Units on Tape and Reel 3500 Units on Tape and Reel Pin Descriptions Pin(s) 1 2 Name VIN FB Description Source input voltage Feedback pin Error amplifier output and PWM comparator input Application Information Input to the start-up regulator. Operates from 6V to 60V. Inverting input to the internal voltage error amplifier. The noninverting input of the error amplifier connects to a 1.25V reference. The control loop compensation components connect between this pin and the FB pin. 3 4 5 6 7 COMP VCC OUT GND UVLO Output of the internal, high voltage linear This pin should be bypassed to the GND pin with a ceramic regulator. capacitor. Output of MOSFET gate driver System ground Input Under-Voltage Lock-out Current Sense input Set the start-up and shutdown levels by connecting this pin to the input voltage through a resistor divider. A 20 µA current source provides hysteresis. Input for the switch current used for current mode control and for current limiting. An external resistor connected from this pin to GND sets the oscillator frequency. This pin can also accept an AC-coupled input for synchronization from an external clock. An external capacitor placed from this pin to ground will be charged by a 10 µA current source, creating a ramp voltage to control the regulator start-up. Connect this pin to the gate of the external MOSFET. The gate driver has a 1A peak current capability. 8 CS 9 RT/SYNC Oscillator frequency adjust pin and synchronization input 10 SS Soft-start pin www.national.com 2 LM5022 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 GND VCC to GND RT/SYNC to GND OUT to GND All other pins to GND Power Dissipation Junction Temperature -0.3V to 65V -0.3V to 16V -0.3V to 5.5V -1.5V for < 100 ns -0.3V to 7V Internally Limited 150°C Storage Temperature Soldering Information Vapor Phase (60 sec.) Infrared (15 sec.) ESD Rating Human Body Model (Note 2) -65°C to +150°C 215°C 220°C 2 kV Operating Ranges Supply Voltage External Volatge at VCC Junction Temperature Range (Note 4) 6V to 60V 7.5V to 14V -40°C to +125°C 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. VIN = 24V and RT = 27.4 kΩ unless otherwise indicated. (Note 3) Symbol VFB Parameter FB Pin Voltage Conditions -40°C ≤ TJ ≤ 125°C 9V ≤ VIN ≤ 60V, ICC = 1 mA 6V ≤ VIN < 9V, VCC Pin Open Circuit OUT Pin Capacitance = 0 VCC = 10V VCC = 0V, (Note 4, 6) ICC = 0 mA, fSW < 200 kHz 6V ≤ VIN ≤ 8.5V VIN increasing VIN Decreasing VIN = 6.0V VIN = 8.0V VIN = 24.0V 15 Min 1.225 6.6 5 3.5 35 200 8.7 260 58 53 1.6 5 300 VIN = 60V VUVLO = 0V, VCC = Open Circuit 150 350 4 75 VFB = 1.5V VCOMP = 1V 5 17 500 450 V mV µA µA MHz dB mA Ω 4 Typ 1.250 7 Max 1.275 7.4 V Units V SYSTEM PARAMETERS START-UP REGULATOR VCC Regulation VCC VCC Regulation Supply Current VCC Current Limit Dropout Voltage Across Bypass Switch Bypass Switch Turn-off Threshold Bypass Switch Threshold Hysteresis VCC Pin Output Impedance 0 mA ≤ ICC ≤ 5 mA VCC Pin UVLO Rising Threshold VCC Pin UVLO Falling Hysteresis Start-up Regulator Leakage Shutdown Current Gain Bandwidth DC Gain COMP Pin Current Sink Capability Electrical Characteristics ICC ICC-LIM VIN - VCC VBYP-HI VBYP-HYS ZVCC VCC-HI VCC-HYS IVIN IIN-SD GBW ADC ICOMP UVLO VSD ISD-HYS CURRENT LIMIT tLIM-DLY VCS tBLK RCS mA mA mV V mV ERROR AMPLIFIER Shutdown Threshold Shutdown Hysteresis Current Source CS steps from 0V to 0.6V OUT transitions to 90% of VCC 1.22 16 1.25 20 1.28 24 V µA Delay from ILIM to Output Current Limit Threshold Voltage Leading Edge Blanking Time CS Pin Sink Impedance 30 0.45 0.5 65 40 0.55 75 ns V ns Ω Blanking active 3 www.national.com LM5022 Symbol SOFT-START ISS VSS-OFF OSCILLATOR Parameter Soft-start Current Source Soft-start to COMP Offset RT to GND = 84.5 kΩ Conditions Min 7 0.35 Typ 10 0.55 200 600 990 Max 13 0.75 230 675 1115 3.8 Units µA V kHz kHz kHz V (Note 5) (Note 5) (Note 5) 170 525 865 fSW VSYNC-HI RT to GND = 27.4 kΩ RT to GND = 16.2 kΩ Synchronization Rising Threshold PWM COMPARATOR tCOMP-DLY DMIN DMAX APWM VCOMP-OC ICOMP-SC VSLOPE MOSFET DRIVER VSAT-HI VSAT-LO tRISE tFALL TSD TSD-HYS θJA Output High Saturation Voltage (VCC – VOUT) Output Low Saturation Voltage (VOUT) OUT Pin Rise Time OUT Pin Fall Time Thermal Shutdown Threshold Thermal Shutdown Hysteresis Junction to Ambient Thermal Resistance MUB-10A Package IOUT = 50 mA IOUT = 100 mA OUT Pin load = 1 nF OUT Pin load = 1 nF 0.25 0.25 18 15 165 25 200 0.75 0.75 V V ns ns °C °C °C/W Delay from COMP to OUT Transition Minimum Duty Cycle Maximum Duty Cycle COMP to PWM Comparator Gain COMP Pin Open Circuit Voltage COMP Pin Short Circuit Current Slope Compensation Amplitude VFB = 0V VCOMP = 0V, VFB = 1.5V 4.3 0.6 80 VCOMP = 2V CS stepped from 0V to 0.4V VCOMP = 0V 90 95 0.33 5.2 1.1 105 6.1 1.5 130 25 0 ns % % V/V V mA mV SLOPE COMPENSATION THERMAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. The Recommended Operating Limits define the conditions within which 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 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. 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: Device thermal limitations may limit usable range. Note 5: Specification applies to the oscillator frequency. Note 6: VCC provides bias for the internal gate drive and control circuits. www.national.com 4 LM5022 Typical Performance Characteristics Efficiency, VO = 40V Example Circuit BOM VFB vs. Temp (VIN = 24V) 20212203 20212255 VFB vs. VIN (TA = 25°C) VCC vs. VIN (TA = 25°C) 20212204 20212205 Max Duty Cycle vs. fSW (TA = 25°C) fSW vs. Temperature (RT = 16.2 kΩ) 20212206 20212207 5 www.national.com LM5022 RT vs. fSW (TA = 25°C) SS vs. Temperature 20212208 20212209 OUT Pin tRISE vs. Gate Capacitance OUT Pin tFALL vs. Gate Capacitance 20212210 20212211 www.national.com 6 LM5022 Block Diagram 20212212 7 www.national.com LM5022 Example Circuit 20212213 FIGURE 1. Design Example Schematic Applications Information OVERVIEW The LM5022 is a low-side N-channel MOSFET controller that contains all of the features needed to implement single ended power converter topologies. The LM5022 includes a highvoltage startup regulator that operates over a wide input range of 6V to 60V. The PWM controller is designed for high speed capability including an oscillator frequency range up to 2 MHz and total propagation delays less than 100 ns. Additional features include an error amplifier, precision reference, input under-voltage lockout, cycle-by-cycle current limit, slope compensation, soft-start, oscillator sync capability and thermal shutdown. The LM5022 is designed for current-mode control power converters that require a single drive output, such as boost and SEPIC topologies. The LM5022 provides all of the advantages of current-mode control including input voltage feedforward, cycle-by-cycle current limiting and simplified loop compensation. HIGH VOLTAGE START-UP REGULATOR The LM5022 contains an internal high-voltage startup regulator that allows the VIN pin to be connected directly to line voltages as high as 60V. The regulator output is internally current limited to 35 mA (typical). When power is applied, the regulator is enabled and sources current into an external capacitor, CF, connected to the VCC pin. The recommended capacitance range for CF is 0.1 µF to 100 µF. When the voltage on the VCC pin reaches the rising threshold of 5V, the controller output is enabled. The controller will remain enabled until VCC falls below 4.7V. In applications using a transformer, an auxiliary winding can be connected through a diode to the VCC pin. This winding should raise the VCC pin voltage to above 7.5V to shut off the internal startup regulator. Powering VCC from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller. The capacitance of CF must be high enough www.national.com 8 that it maintains the VCC voltage greater than the VCC UVLO falling threshold (4.7V) during the initial start-up. During a fault condition when the converter auxiliary winding is inactive, external current draw on the VCC line should be limited such that the power dissipated in the start-up regulator does not exceed the maximum power dissipation capability of the controller. An external start-up or other bias rail can be used instead of the internal start-up regulator by connecting the VCC and the VIN pins together and feeding the external bias voltage (7.5V to 14V) to the two pins. INPUT UNDER-VOLTAGE DETECTOR The LM5022 contains an input Under Voltage Lock Out (UVLO) circuit. UVLO is programmed by connecting the UVLO pin to the center point of an external voltage divider from VIN to GND. The resistor divider must be designed such that the voltage at the UVLO pin is greater than 1.25V when VIN is in the desired operating range. If the under voltage threshold is not met, all functions of the controller are disabled and the controller remains in a low power standby state. UVLO hysteresis is accomplished with an internal 20 µA current source that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 1.25V threshold the current source is turned off, causing the voltage at the UVLO pin to fall. The UVLO pin can also be used to implement a remote enable / disable function. If an external transistor pulls the UVLO pin below the 1.25V threshold, the converter will be disabled. This external shutdown method is shown in Figure 2. LM5022 To set the switching frequency, fSW, RT can be calculated from: 20212214 FIGURE 2. Enable/Disable Using UVLO ERROR AMPLIFIER An internal high gain error amplifier is provided within the LM5022. The amplifier’s non-inverting input is internally set to a fixed reference voltage of 1.25V. The inverting input is connected to the FB pin. In non-isolated applications such as the boost converter the output voltage, VO, is connected to the FB pin through a resistor divider. The control loop compensation components are connected between the COMP and FB pins. For most isolated applications the error amplifier function is implemented on the secondary side of the converter and the internal error amplifier is not used. The internal error amplifier is configured as an open drain output and can be disabled by connecting the FB pin to ground. An internal 5 kΩ pull-up resistor between a 5V reference and COMP can be used as the pull-up for an opto-coupler in isolated applications. CURRENT SENSING AND CURRENT LIMITING The LM5022 provides a cycle-by-cycle over current protection function. Current limit is accomplished by an internal current sense comparator. If the voltage at the current sense comparator input exceeds 0.5V, the MOSFET gate drive will be immediately terminated. A small RC filter, located near the controller, is recommended to filter noise from the current sense signal. The CS input has an internal MOSFET which discharges the CS pin capacitance at the conclusion of every cycle. The discharge device remains on an additional 65 ns after the beginning of the new cycle to attenuate leading edge ringing on the current sense signal. The LM5022 current sense and PWM comparators are very fast, and may respond to short duration noise pulses. Layout considerations are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be located very close to the device and connected directly to the pins of the controller (CS and GND). If a current sense transformer is used, both leads of the transformer secondary should be routed to the sense resistor and the current sense filter network. The current sense resistor can be located between the source of the primary power MOSFET and power ground, but it must be a low inductance type. When designing with a current sense resistor all of the noise sensitive low-power ground connections should be connected together locally to the controller and a single connection should be made to the high current power ground (sense resistor ground point). OSCILLATOR, SHUTDOWN AND SYNC A single external resistor, RT, connected between the RT/ SYNC and GND pins sets the LM5022 oscillator frequency. The LM5022 can also be synchronized to an external clock. The external clock must have a higher frequency than the free running oscillator frequency set by the RT resistor. The clock signal should be capacitively coupled into the RT/SYNC pin with a 100 pF capacitor as shown in Figure 3. A peak voltage level greater than 3.8V at the RT/SYNC pin is required for detection of the sync pulse. The sync pulse width should be set between 15 ns to 150 ns by the external components. The RT resistor is always required, whether the oscillator is free running or externally synchronized. The voltage at the RT/ SYNC pin is internally regulated to 2V, and the typical delay from a logic high at the RT/SYNC pin to the rise of the OUT pin voltage is 120 ns. RT should be located very close to the device and connected directly to the pins of the controller (RT/ SYNC and GND). 20212254 FIGURE 3. Sync Operation PWM COMPARATOR AND SLOPE COMPENSATION The PWM comparator compares the current ramp signal with the error voltage derived from the error amplifier output. The error amplifier output voltage at the COMP pin is offset by 1.4V and then further attenuated by a 3:1 resistor divider. The PWM comparator polarity is such that 0V on the COMP pin will result in a zero duty cycle at the controller output. For duty cycles greater than 50%, current mode control circuits can experience sub-harmonic oscillation. By adding an additional fixed-slope voltage ramp signal (slope compensation) this oscillation can be avoided. Proper slope compensation damps the double pole associated with current mode control (see the Control Loop Compensation section) and eases the design of the control loop compensator. The LM5022 generates the slope compensation with a sawtooth-waveform current source with a slope of 45 µA x fSW, generated by the clock. (See Figure 4) This current flows through an internal 2 kΩ 9 www.national.com LM5022 resistor to create a minimum compensation ramp with a slope of 100 mV x fSW (typical). The slope of the compensation ramp increases when external resistance is added for filtering the current sense (RS1) or in the position RS2. As shown in Figure 4 and the block diagram, the sensed current slope and the compensation slope add together to create the signal used for current limiting and for the control loop itself. 20212216 FIGURE 4. Slope Compensation In peak current mode control the optimal slope compensation is proportional to the slope of the inductor current during the power switch off-time. For boost converters the inductor current slope while the MOSFET is off is (VO - VIN) / L. This relationship is combined with the requirements to set the peak current limit and is used to select RSNS and RS2 in the Design Considerations section. SOFT-START The soft-start feature allows the power converter output to gradually reach the initial steady state output voltage, thereby reducing start-up stresses and current surges. At power on, after the VCC and input under-voltage lockout thresholds are satisfied, an internal 10 µA current source charges an external capacitor connected to the SS pin. The capacitor voltage will ramp up slowly and will limit the COMP pin voltage and the switch current. MOSFET GATE DRIVER The LM5022 provides an internal gate driver through the OUT pin that can source and sink a peak current of 1A to control external, ground-referenced N-channel MOSFETs. THERMAL SHUTDOWN Internal thermal shutdown circuitry is provided to protect the LM5022 in the event that the maximum junction temperature is exceeded. When activated, typically at 165°C, the controller is forced into a low power standby state, disabling the output driver and the VCC regulator. After the temperature is reduced (typical hysteresis is 25°C) the VCC regulator will be re-enabled and the LM5022 will perform a soft-start. The following is a design procedure for selecting all the components for the boost converter circuit shown in Figure 1. The application is "in-cabin" automotive, meaning that the operating ambient temperature ranges from -20°C to 85°C. This circuit operates in continuous conduction mode (CCM), where inductor current stays above 0A at all times, and delivers an output voltage of 40.0V ±2% at a maximum output current of 0.5A. Additionally, the regulator must be able to handle a load transient of up to 0.5A while keeping VO within ±4%. The voltage input comes from the battery/alternator system of an automobile, where the standard range 9V to 16V and transients of up to 32V must not cause any malfunction. SWITCHING FREQUENCY The selection of switching frequency is based on the tradeoffs between size, cost, and efficiency. In general, a lower frequency means larger, more expensive inductors and capacitors will be needed. A higher switching frequency generally results in a smaller but less efficient solution, as the power MOSFET gate capacitances must be charged and discharged more often in a given amount of time. For this application, a frequency of 500 kHz was selected as a good compromise between the size of the inductor and efficiency. PCB area and component height are restricted in this application. Following the equation given for RT in the Applications Information section, a 33.2 kΩ 1% resistor should be used to switch at 500 kHz. MOSFET Selection of the power MOSFET is governed by tradeoffs between cost, size, and efficiency. Breaking down the losses in the MOSFET is one way to determine relative efficiencies between different devices. For this example, the SO-8 package provides a balance of a small footprint with good efficiency. Losses in the MOSFET can be broken down into conduction loss, gate charging loss, and switching loss. Conduction, or I2R loss, PC, is approximately: Design Considerations The most common circuit controlled by the LM5022 is a nonisolated boost regulator. The boost regulator steps up the input voltage and has a duty ratio D of: (VD is the forward voltage drop of the output diode) www.national.com 10 LM5022 The factor 1.3 accounts for the increase in MOSFET on resistance due to heating. Alternatively, the factor of 1.3 can be ignored and the maximum on resistance of the MOSFET can be used. Gate charging loss, PG, results from the current required to charge and discharge the gate capacitance of the power MOSFET and is approximated as: PG = VCC x QG x fSW QG is the total gate charge of the MOSFET. Gate charge loss differs from conduction and switching losses because the actual dissipation occurs in the LM5022 and not in the MOSFET itself. If no external bias is applied to the VCC pin, additional loss in the LM5022 IC occurs as the MOSFET driving current flows through the VCC regulator. This loss, PVCC, is estimated as: PVCC = (VIN – VCC) x QG x fSW Switching loss, PSW, occurs during the brief transition period as the MOSFET turns on and off. During the transition period both current and voltage are present in the channel of the MOSFET. The loss can be approximated as: PSW = 0.5 x VIN x [IO / (1 – D)] x (tR + tF) x fSW Where tR and tF are the rise and fall times of the MOSFET For this example, the maximum drain-to-source voltage applied across the MOSFET is VO plus the ringing due to parasitic inductance and capacitance. The maximum drive voltage at the gate of the high side MOSFET is VCC, or 7V typical. The MOSFET selected must be able to withstand 40V plus any ringing from drain to source, and be able to handle at least 7V plus ringing from gate to source. A minimum voltage rating of 50VD-S and 10VG-S MOSFET will be used. Comparing the losses in a spreadsheet leads to a 60VD-S rated MOSFET in SO-8 with an RDSON of 22 mΩ (the maximum vallue is 31 mΩ), a gate charge of 27 nC, and rise and falls times of 10 ns and 12 ns, respectively. OUTPUT DIODE The boost regulator requires an output diode D1 (see Figure 1) to carrying the inductor current during the MOSFET offtime. The most efficient choice for D1 is a Schottky diode due to low forward drop and near-zero reverse recovery time. D1 must be rated to handle the maximum output voltage plus any switching node ringing when the MOSFET is on. In practice, all switching converters have some ringing at the switching node due to the diode parasitic capacitance and the lead inductance. D1 must also be rated to handle the average output current, IO. The overall converter efficiency becomes more dependent on the selection of D1 at low duty cycles, where the boost diode carries the load current for an increasing percentage of the time. This power dissipation can be calculating by checking the typical diode forward voltage, VD, from the I-V curve on the diode's datasheet and then multiplying it by IO. Diode datasheets will also provide a typical junction-to-ambient thermal resistance, θJA, which can be used to estimate the operating die temperature of the Schottky. Multiplying the power dissipation (PD = IO x VD) by θJA gives the temperature rise. 11 The diode case size can then be selected to maintain the Schottky diode temperature below the operational maximum. In this example a Schottky diode rated to 60V and 1A will be suitable, as the maximum diode current will be 0.5A. A small case such as SOD-123 can be used if a small footprint is critical. Larger case sizes generally have lower θJA and lower forward voltage drop, so for better efficiency the larger SMA case size will be used. BOOST INDUCTOR The first criterion for selecting an inductor is the inductance itself. In fixed-frequency boost converters this value is based on the desired peak-to-peak ripple current, ΔiL, which flows in the inductor along with the average inductor current, IL. For a boost converter in CCM IL is greater than the average output current, IO. The two currents are related by the following expression: IL = IO / (1 – D) As with switching frequency, the inductance used is a tradeoff between size and cost. Larger inductance means lower input ripple current, however because the inductor is connected to the output during the off-time only there is a limit to the reduction in output ripple voltage. Lower inductance results in smaller, less expensive magnetics. An inductance that gives a ripple current of 30% to 50% of IL is a good starting point for a CCM boost converter. Minimum inductance should be calculated at the extremes of input voltage to find the operating condition with the highest requirement: By calculating in terms of amperes, volts, and megahertz, the inductance value will come out in micro henries. In order to ensure that the boost regulator operates in CCM a second equation is needed, and must also be evaluated at the corners of input voltage to find the minimum inductance required: By calculating in terms of volts, amps and megahertz the inductance value will come out in µH. For this design ΔiL will be set to 40% of the maximum IL. Duty cycle is evaluated first at VIN(MIN) and at VIN(MAX). Second, the average inductor current is evaluated at the two input voltages. Third, the inductor ripple current is determined. Finally, the inductance can be calculated, and a standard inductor value selected that meets all the criteria. Inductance for Minimum Input Voltage DVIN(MIN) = (40 – 9.0 + 0.5) / (40 + 0.5) = 78% IL-VIN(MIN) = 0.5 / (1 – 0.78) = 2.3A ΔiL = 0.4 x 2.3A = 0.92A www.national.com LM5022 Inductance for Maximum Input Voltage DVIN(MAX) = (40 - 16 + 0.5) / (40 + 0.5) = 60% IL-VIN(MIAX) = 0.5 / (1 – 0.6) = 1.25A ΔiL = 0.4 x 1.25A = 0.5A selected based on their capacitance, CO, their equivalent series resistance (ESR) and their RMS or AC current rating. The magnitude of ΔVO is comprised of three parts, and in steady state the ripple voltage during the on-time is equal to the ripple voltage during the off-time. For simplicity the analysis will be performed for the MOSFET turning off (off-time) only. The first part of the ripple voltage is the surge created as the output diode D1 turns on. At this point inductor/diode current is at the peak value, and the ripple voltage increase can be calculated as: ΔVO1 = IPK x ESR The second portion of the ripple voltage is the increase due to the charging of CO through the output diode. This portion can be approximated as: Maximum average inductor current occurs at VIN(MIN), and the corresponding inductor ripple current is 0.92AP-P. Selecting an inductance that exceeds the ripple current requirement at VIN(MIN) and the requirement to stay in CCM for VIN(MAX) provides a tradeoff that allows smaller magnetics at the cost of higher ripple current at maximum input voltage. For this example, a 33 µH inductor will satisfy these requirements. The second criterion for selecting an inductor is the peak current carrying capability. This is the level above which the inductor will saturate. In saturation the inductance can drop off severely, resulting in higher peak current that may overheat the inductor or push the converter into current limit. In a boost converter, peak current, IPK, is equal to the maximum average inductor current plus one half of the ripple current. First, the current ripple must be determined under the conditions that give maximum average inductor current: ΔVO2 = (IO / CO) x (D / fSW) The final portion of the ripple voltage is a decrease due to the flow of the diode/inductor current through the output capacitor’s ESR. This decrease can be calculated as: ΔVO3 = ΔiL x ESR The total change in output voltage is then: ΔVO = ΔVO1 + ΔVO2 - ΔVO3 The combination of two positive terms and one negative term may yield an output voltage ripple with a net rise or a net fall during the converter off-time. The ESR of the output capacitor (s) has a strong influence on the slope and direction of ΔVO. Capacitors with high ESR such as tantalum and aluminum electrolytic create an output voltage ripple that is dominated by ΔVO1 and ΔVO3, with a shape shown in Figure 5. Ceramic capacitors, in contrast, have very low ESR and lower capacitance. The shape of the output ripple voltage is dominated by ΔVO2, with a shape shown in Figure 6. Maximum average inductor current occurs at VIN(MIN). Using the selected inductance of 33 µH yields the following: ΔiL = (9 x 0.78) / (0.5 x 33) = 425 mAP-P The highest peak inductor current over all operating conditions is therefore: IPK = IL + 0.5 x ΔiL = 2.3 + 0.213 = 2.51A Hence an inductor must be selected that has a peak current rating greater than 2.5A and an average current rating greater than 2.3A. One possibility is an off-the-shelf 33 µH ±20% inductor that can handle a peak current of 3.2A and an average current of 3.4A. Finally, the inductor current ripple is recalculated at the maximum input voltage: ΔiL-VIN(MAX) = (16 x 0.6) / (0.5 x 33) = 0.58AP-P OUTPUT CAPACITOR The output capacitor in a boost regulator supplies current to the load during the MOSFET on-time and also filters the AC portion of the load current during the off-time. This capacitor determines the steady state output voltage ripple, ΔVO, a critical parameter for all voltage regulators. Output capacitors are www.national.com 12 20212226 FIGURE 5. ΔVO Using High ESR Capacitors LM5022 The RMS current through the output capacitor(s) can be estimated using the following, worst-case equation: The highest RMS current occurs at minimum input voltage. For this example the maximum output capacitor RMS current is: IO-RMS(MAX) = 1.13 x 2.3 x (0.78 x 0.22)0.5 = 1.08ARMS These 2220 case size devices are capable of sustaining RMS currents of over 3A each, making them more than adequate for this application. VCC DECOUPLING CAPACITOR The VCC pin should be decoupled with a ceramic capacitor placed as close as possible to the VCC and GND pins of the LM5022. The decoupling capacitor should have a minimum X5R or X7R type dielectric to ensure that the capacitance remains stable over voltage and temperature, and be rated to a minimum of 470 nF. One good choice is a 1.0 µF device with X7R dielectric and 1206 case size rated to 25V. INPUT CAPACITOR The input capacitors to a boost regulator control the input voltage ripple, ΔVIN, hold up the input voltage during load transients, and prevent impedance mismatch (also called power supply interaction) between the LM5022 and the inductance of the input leads. Selection of input capacitors is based on their capacitance, ESR, and RMS current rating. The minimum value of ESR can be selected based on the maximum output current transient, ISTEP, using the following expression: 20212227 FIGURE 6. ΔVO Using Low ESR Capacitors For this example the small size and high temperature rating of ceramic capacitors make them a good choice. The output ripple voltage waveform of Figure 6 is assumed, and the capacitance will be selected first. The desired ΔVO is ±2% of 40V, or 0.8VP-P. Beginning with the calculation for ΔVO2, the required minimum capacitance is: CO-MIN = (IO / ΔVO) x (DMAX / fSW) CO-MIN = (0.5 / 0.8) x (0.77 / 5 x 105) = 0.96 µF The next higher standard 20% capacitor value is 1.0 µF, however to provide margin for component tolerance and load transients two capacitors rated 4.7 µF each will be used. Ceramic capacitors rated 4.7 µF ±20% are available from many manufacturers. The minimum quality dielectric that is suitable for switching power supply output capacitors is X5R, while X7R (or better) is preferred. Careful attention must be paid to the DC voltage rating and case size, as ceramic capacitors can lose 60% or more of their rated capacitance at the maximum DC voltage. This is the reason that ceramic capacitors are often de-rated to 50% of their capacitance at their working voltage. The output capacitors for this example will have a 100V rating in a 2220 case size. The typical ESR of the selected capacitors is 3 mΩ each, and in parallel is approximately 1.5 mΩ. The worst-case value for ΔVO1 occurs during the peak current at minimum input voltage: ΔVO1 = 2.5 x 0.0015 = 4 mV The worst-case capacitor charging ripple occurs at maximum duty cycle: ΔVO2 = (0.5 / 9.4 x 10-6) x (0.77 / 5 x 105) = 82 mV Finally, the worst-case value for ΔVO3 occurs when inductor ripple current is highest, at maximum input voltage: ΔVO3 = 0.58 x 0.0015 = 1 mV (negligible) The output voltage ripple can be estimated by summing the three terms: ΔVO = 4 mV + 82 mV - 1 mV = 85 mV 13 For this example the maximum load step is equal to the load current, or 0.5A. The maximum permissable ΔVIN during load transients is 4%P-P. ΔVIN and duty cycle are taken at minimum input voltage to give the worst-case value: ESRMIN = [(1 – 0.77) x 0.36] / (2 x 0.5) = 83 mΩ The minimum input capacitance can be selected based on ΔVIN, based on the drop in VIN during a load transient, or based on prevention of power supply interaction. In general, the requirement for greatest capacitance comes from the power supply interaction. The inductance and resistance of the input source must be estimated, and if this information is not available, they can be assumed to be 1 µH and 0.1Ω, respectively. Minimum capacitance is then estimated as: As with ESR, the worst-case, highest minimum capacitance calculation comes at the minimum input voltage. Using the default estimates for LS and RS, minimum capacitance is: www.national.com LM5022 The next highest standard 20% capacitor value is 6.8 µF, but because the actual input source impedance and resistance are not known, two 4.7 µF capacitors will be used. In general, doubling the calculated value of input capacitance provides a good safety margin. The final calculation is for the RMS current. For boost converters operating in CCM this can be estimated as: IRMS = 0.29 x ΔiL(MAX) From the inductor section, maximum inductor ripple current is 0.58A, hence the input capacitor(s) must be rated to handle 0.29 x 0.58 = 170 mARMS. The input capacitors can be ceramic, tantalum, aluminum, or almost any type, however the low capacitance requirement makes ceramic capacitors particularly attractive. As with the output capacitors, the minimum quality dielectric used should X5R, with X7R or better preferred. The voltage rating for input capacitors need not be as conservative as the output capacitors, as the need for capacitance decreases as input voltage increases. For this example, the capacitor selected will be 4.7 µF ±20%, rated to 50V, in the 1812 case size. The RMS current rating of these capacitors is over 2A each, more than enough for this application. CURRENT SENSE FILTER Parasitic circuit capacitance, inductance and gate drive current create a spike in the current sense voltage at the point where Q1 turns on. In order to prevent this spike from terminating the on-time prematurely, every circuit should have a low-pass filter that consists of CCS and RS1, shown in Figure 1. The time constant of this filter should be long enough to reduce the parasitic spike without significantly affecting the shape of the actual current sense voltage. The recommended range for RS1 is between 10Ω and 500Ω, and the recommended range for CCS is between 100 pF and 2.2 nF. For this example, the values of RS1 and CCS will be 100Ω and 1 nF, respectively. RSNS, RS2 AND CURRENT LIMIT The current sensing resistor RSNS is used for steady state regulation of the inductor current and to sense over-current conditions. The slope compensation resistor is used to ensure control loop stability, and both resistors affect the current limit threshold. The RSNS value selected must be low enough to keep the power dissipation to a minimum, yet high enough to provide good signal-to-noise ratio for the current sensing circuitry. RSNS, and RS2 should be set so that the current limit comparator, with a threshold of 0.5V, trips before the sensed current exceeds the peak current rating of the inductor, without limiting the output power in steady state. For this example the peak current, at VIN(MIN), is 2.5A, while the inductor itself is rated to 3.2A. The threshold for current limit, ILIM, is set slightly between these two values to account for tolerance of the circuit components, at a level of 3.0A. The required resistor calculation must take into account both the switch current through RSNS and the compensation ramp current flowing through the internal 2 kΩ, RS1 and RS2 resistors. RSNS should be selected first because it is a power resistor with more limited selection. The following equation should be evaluated at VIN(MIN), when duty cycle is highest: L in µH, fSW in MHz The closest 5% value is 100 mΩ. Power dissipation in RSNS can be estimated by calculating the average current. The worst-case average current through RSNS occurs at minimum input voltage/maximum duty cycle and can be calculated as: PCS = [(0.5 / 0.22)2 x 0.1] x 0.78 = 0.4W For this example a 0.1Ω ±1%, thick-film chip resistor in a 1210 case size rated to 0.5W will be used. With RSNS selected, RS2 can be determined using the following expression: The closest 1% tolerance value is 3.57 kΩ. CONTROL LOOP COMPENSATION The LM5022 uses peak current-mode PWM control to correct changes in output voltage due to line and load transients. Peak current-mode provides inherent cycle-by-cycle current limiting, improved line transient response, and easier control loop compensation. The control loop is comprised of two parts. The first is the power stage, which consists of the pulse width modulator, output filter, and the load. The second part is the error amplifier, which is an op-amp configured as an inverting amplifier. Figure 7 shows the regulator control loop components. www.national.com 14 LM5022 The sampling double pole quality factor is: The sampling double corner frequency is: ωn = π x fSW 20212234 The natural inductor current slope is: Sn = RSNS x VIN / L FIGURE 7. Power Stage and Error Amp One popular method for selecting the compensation components is to create Bode plots of gain and phase for the power stage and error amplifier. Combined, they make the overall bandwidth and phase margin of the regulator easy to determine. Software tools such as Excel, MathCAD, and Matlab are useful for observing how changes in compensation or the power stage affect system gain and phase. The power stage in a CCM peak current mode boost converter consists of the DC gain, APS, a single low frequency pole, fLFP, the ESR zero, fZESR, a right-half plane zero, fRHP, and a double pole resulting from the sampling of the peak current. The power stage transfer function (also called the Control-to-Output transfer function) can be written: The external ramp slope is: Se = 45 µA x (2000 + RS1 + RS2)] x fSW In the equation for APS, DC gain is highest when input voltage and output current are at the maximum. In this the example those conditions are VIN = 16V and IO = 500 mA. DC gain is 44 dB. The low frequency pole fP = 2πωP is at 423Hz, the ESR zero fZ = 2πωZ is at 5.6 MHz, and the righthalf plane zero fRHP = 2πωRHP is at 61 kHz. The sampling double-pole occurs at one-half of the switching frequency. Proper selection of slope compensation (via RS2) is most evident the sampling double pole. A well-selected RS2 value eliminates peaking in the gain and reduces the rate of change of the phase lag. Gain and phase plots for the power stage are shown in Figure 8. Where the DC gain is defined as: Where: RO = VO / IO The system ESR zero is: The low frequency pole is: 20212241 The right-half plane zero is: 15 www.national.com LM5022 3. 4. 5. 6. 7. 20212297 Calculate the negative of A and convert it to a linear gain: By setting the mid-band gain of the error amplifier to the negative of the power stage gain at f0dB, the control loop gain will equal 0 dB at that frequency. For this example, -16 dB = 0.15V/V. Select the resistance of the top feedback divider resistor RFB2: This value is arbitrary, however selecting a resistance between 10 kΩ and 100 kΩ will lead to practical values of R1, C1 and C2. For this example, RFB2 = 20 kΩ 1%. Set R1 = A x RFB2: For this example: R1 = 0.15 x 20000 = 3 kΩ Select a frequency for the compensation zero, fZ1: The suggested placement for this zero is at the low frequency pole of the power stage, fLFP = ωLFP / 2π. For this example, fZ1 = fLFP = 423Hz Set FIGURE 8. Power Stage Gain and Phase The single pole causes a roll-off in the gain of -20 dB/decade at lower frequency. The combination of the RHP zero and sampling double pole maintain the slope out to beyond the switching frequency. The phase tends towards -90° at lower frequency but then increases to -180° and beyond from the RHP zero and the sampling double pole. The effect of the ESR zero is not seen because its frequency is several decades above the switching frequency. The combination of increasing gain and decreasing phase makes converters with RHP zeroes difficult to compensate. Setting the overall control loop bandwidth to 1/3 to 1/10 of the RHP zero frequency minimizes these negative effects, but requires a compromise in the control loop bandwidth. If this loop were left uncompensated, the bandwidth would be 89 kHz and the phase margin -54°. The converter would oscillate, and therefore is compensated using the error amplifier and a few passive components. The transfer function of the compensation block, GEA, can be derived by treating the error amplifier as an inverting op-amp with input impedance ZI and feedback impedance ZF. The majority of applications will require a Type II, or two-pole onezero amplifier, shown in Figure 7. The LaPlace domain transfer function for this Type II network is given by the following: 8. 9. For this example, C2 = 125 nF Select a frequency for the compensation pole, fP1: The suggested placement for this pole is at one-fifth of the switching frequency. For this example, fP1 = 100 kHz Set For this example, C1 = 530 pF 10. Plug the closest 1% tolerance values for RFB2 and R1, then the closest 10% values for C1 and C2 into GEA and model the error amp: The open-loop gain and bandwidth of the LM5022’s internal error amplifier are 75 dB and 4 MHz, respectively. Their effect on GEA can be modeled using the following expression: Many techniques exist for selecting the compensation component values. The following method is based upon setting the mid-band gain of the error amplifier transfer function first and then positioning the compensation zero and pole: 1. Determine the desired control loop bandwidth: The control loop bandwidth, f0dB, is the point at which the total control loop gain (H = GPS x GEA) is equal to 0 dB. For this example, a low bandwidth of 10 kHz, or approximately 1/6th of the RHP zero frequency, is chosen because of the wide variation in input voltage. 2. Determine the gain of the power stage at f0dB: This value, A, can be read graphically from the gain plot of GPS or calculated by replacing the ‘s’ terms in GPS with ‘2πf0dB’. For this example the gain at 10 kHz is approximately 16 dB. ADC is a linear gain, the linear equivalent of 75 dB is approximately 5600V/V. C1 = 560 pF 10%, C2 = 120 nF 10%, R1 = 3.01 kΩ 1% 11. Plot or evaluate the actual error amplifier transfer function: www.national.com 16 LM5022 20212248 20212299 FIGURE 10. Overall Loop Gain and Phase The bandwidth of this example circuit at VIN = 16V is 10.5 kHz, with a phase margin of 66°. 13. Re-evaluate at the corners of input voltage and output current: Boost converters exhibit significant change in their loop response when VIN and IO change. With the compensation fixed, the total control loop gain and phase should be checked to ensure a minimum phase margin of 45° over both line and load. Efficiency Calculations A reasonable estimation for the efficiency of a boost regulator controlled by the LM5022 can be obtained by adding together the loss is each current carrying element and using the equation: 20212298 FIGURE 9. Error Amplifier Gain and Phase 12. Plot or evaluate the complete control loop transfer function: The complete control loop transfer function is obtained by multiplying the power stage and error amplifier functions together. The bandwidth and phase margin can then be read graphically or evaluated numerically. The following shows an efficiency calculation to complement the circuit design from the Design Considerations section. Output power for this circuit is 40V x 0.5A = 20W. Input voltage is assumed to be 13.8V, and the calculations used assume that the converter runs in CCM. Duty cycle for VIN = 13.8V is 66%, and the average inductor current is 1.5A. CHIP OPERATING LOSS This term accounts for the current drawn at the VIN pin. This current, IIN, drives the logic circuitry and the power MOSFETs. The gate driving loss term from the power MOSFET section of Design Considerations is included in the chip operating loss. For the LM5022, IIN is equal to the steady state operating current, ICC, plus the MOSFET driving current, IGC. Power is lost as this current passes through the internal linear regulator of the LM5022. IGC = QG X fSW IGC = 27 nC x 500 kHz = 13.5 mA ICC is typically 3.5 mA, taken from the Electrical Characteristics table. Chip Operating Loss is then: 20212249 PQ = VIN X (IQ + IGC) 17 www.national.com LM5022 PQ = 13.8 X (3.5m + 13.5m) = 235 mW MOSFET SWITCHING LOSS PSW = 0.5 x VIN x IL x (tR + tF) x fSW PSW = 0.5 x 13.8 x 1.5 x (10 ns + 12 ns) x 5 x 105 = 114 mW MOSFET AND RSNS CONDUCTION LOSS PC = D x (IL2 x (RDSON x 1.3 + RSNS)) PC = 0.66 x (1.52 x (0.029 + 0.1)) = 192 mW OUTPUT DIODE LOSS The average output diode current is equal to IO, or 0.5A. The estimated forward drop, VD, is 0.5V. The output diode loss is therefore: PD1 = IO x VD PD1 = 0.5 x 0.5 = 0.25W INPUT CAPACITOR LOSS This term represents the loss as input ripple current passes through the ESR of the input capacitor bank. In this equation ‘n’ is the number of capacitors in parallel. The 4.7 µF input capacitors selected have a combined ESR of approximately 1.5 mΩ, and ΔiL for a 13.8V input is 0.55A: PDCR = 1.52 x 0.04 = 90 mW Core loss in the inductor is estimated to be equal to the DCR loss, adding an additional 90 mW to the total inductor loss. TOTAL LOSS PLOSS = Sum of All Loss Terms = 972 mW EFFICIENCY η = 20 / (20 + 0.972) = 95% Layout Considerations To produce an optimal power solution with the LM5022, good layout and design of the PCB are as important as the component selection. The following are several guidelines to aid in creating a good layout. FILTER CAPACITORS The low-value ceramic filter capacitors are most effective when the inductance of the current loops that they filter is minimized. Place CINX as close as possible to the VIN and GND pins of the LM5022. Place COX close to the load, and CF next to the VCC and GND pins of the LM5022. SENSE LINES The top of RSNS should be connected to the CS pin with a separate trace made as short as possible. Route this trace away from the inductor and the switch node (where D1, Q1, and L1 connect). For the voltage loop, keep RFB1/2 close to the LM5022 and run a trace from as close as possible to the positive side of COX to RFB2. As with the CS line, the FB line should be routed away from the inductor and the switch node. These measures minimize the length of high impedance lines and reduce noise pickup. COMPACT LAYOUT Parasitic inductance can be reduced by keeping the power path components close together and keeping the area of the loops that high currents travel small. Short, thick traces or copper pours (shapes) are best. In particular, the switch node should be just large enough to connect all the components together without excessive heating from the current it carries. The LM5022 (boost converter) operates in two distinct cycles whose high current paths are shown in Figure 11: IIN-RMS = 0.29 x ΔiL = 0.29 x 0.55 = 0.16A PCIN = [0.162 x 0.0015] / 2 = 0.02 mW (negligible) OUTPUT CAPACITOR LOSS This term is calculated using the same method as the input capacitor loss, substituting the output capacitor RMS current for VIN = 13.8V. The output capacitors' combined ESR is also approximately 1.5 mΩ. IO-RMS = 1.13 x 1.5 x (0.66 x 0.34)0.5 = 0.8A PCO = [0.8 x 0.0015] / 2 = 0.6 mW BOOST INDUCTOR LOSS The typical DCR of the selected inductor is 40 mΩ. PDCR = IL2 x DCR 20212252 FIGURE 11. Boost Converter Current Loops www.national.com 18 LM5022 The dark grey, inner loops represents the high current paths during the MOSFET on-time. The light grey, outer loop represents the high current path during the off-time. GROUND PLANE AND SHAPE ROUTING The diagram of Figure 11 is also useful for analyzing the flow of continuous current vs. the flow of pulsating currents. The circuit paths with current flow during both the on-time and offtime are considered to be continuous current, while those that carry current during the on-time or off-time only are pulsating currents. Preference in routing should be given to the pulsating current paths, as these are the portions of the circuit most likely to emit EMI. The ground plane of a PCB is a conductor and return path, and it is susceptible to noise injection just as any other circuit path. The continuous current paths on the ground net can be routed on the system ground plane with less risk of injecting noise into other circuits. The path between the input source, input capacitor and the MOSFET and the path between the output capacitor and the load are examples of continuous current paths. In contrast, the path between the grounded side of the power switch and the negative output capacitor terminal carries a large pulsating current. This path should be routed with a short, thick shape, preferably on the component side of the PCB. Multiple vias in parallel should be used right at the negative pads of the input and output capacitors to connect the component side shapes to the ground plane. Vias should not be placed directly at the grounded side of the MOSFET (or RSNS) as they tend to inject noise into the ground plane. A second pulsating current loop that is often ignored but must be kept small is the gate drive loop formed by the OUT and VCC pins, Q1, RSNS and capacitor CF. 19 www.national.com LM5022 BOM for Example Circuit ID U1 Q1 D1 L1 Cin1, Cin2 Co1, Co2 Cf Cinx Cox C1 C2 Css Ccs R1 Rfb1 Rfb2 Rs1 Rs2 Rsns Rt Ruv1 Ruv2 Part Number LM5022 Si4850EY CMSH2-60M SLF12575T-M3R2 C4532X7R1H475M C5750X7R2A475M C2012X7R1E105K C2012X7R2A104M VJ0805A561KXXAT VJ0805Y124KXXAT VJ0805Y103KXXAT VJ0805Y102KXXAT CRCW08053011F CRCW08056490F CRCW08052002F CRCW0805101J CRCW08053571F ERJL14KF10C CRCW08053322F CRCW08052611F CRCW08051002F Type Low-Side Controller MOSFET Schottky Diode Inductor Capacitor Capacitor Capacitor Capacitor Capacitor Capacitor Capacitor Capacitor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Size MSOP-10 SO-8 SMA 12.5 x 12.5 x 7.5 mm 1812 2220 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 1210 0805 0805 0805 Parameters 60V 60V, 31mΩ, 27nC 60V, 2A 33µH, 3.2A, 40mΩ 4.7µF, 50V, 3mΩ 4.7µF,100V, 3mΩ 1µF, 25V 100nF, 100V 560pF 10% 120nF 10% 10nF 10% 1nF 10% 3.01kΩ 1% 649Ω 1% 20kΩ 1% 100Ω 5% 3.57kΩ 1% 100mΩ, 1%, 0.5W 33.2kΩ 1% 2.61kΩ 1% 10kΩ 1% Qty 1 1 1 1 2 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 Vendor NSC Vishay Central Semi TDK TDK TDK TDK TDK Vishay Vishay Vishay Vishay Vishay Vishay Vishay Vishay Vishay Panasonic Vishay Vishay Vishay www.national.com 20 LM5022 Physical Dimensions inches (millimeters) unless otherwise noted 10-Lead MSOP Package NS Package Number MUB10A 21 www.national.com LM5022 60V Low Side Controller for Boost and SEPIC Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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