LM2611

LM2611 1.4MHz Cuk Converter January 2002 LM2611 1.4MHz Cuk Converter General Description The LM2611 is a current mode, PWM inverting switching regulator. Operating from a 2.7 - 14V supply, it is capable of producing a regulated negative output voltage of up to −(36-VIN(MAX)). The LM2611 utilizes an input and output inductor, which enables low voltage ripple and RMS current on both the input and the output. With a switching frequency of 1.4MHz, the inductors and output capacitor can be physically small and low cost. High efficiency is achieved through the use of a low RDS(ON) FET. The LM2611 features a shutdown pin, which can be activated when the part is not needed to lower the Iq and save battery life. A negative feedback (NFB) pin provides a simple method of setting the output voltage, using just two resistors. Cycle-by-cycle current limiting and internal compensation further simplify the use of the LM2611. The LM2611 is available is a small SOT23-5 package. It comes in two grades: Grade A Current Limit RDS(ON) 1.2A 0.5Ω Grade B 0.9A 0.7Ω Features n n n n n n n n n 1.4MHz switching frequency Low RDS(ON) DMOS FET 1mVp-p output ripple −5V at 300mA from 5V input Better regulation than a charge pump Uses tiny capacitors and inductors Wide input range: 2.7V to 14V Low shutdown current: < 1uA 5-lead SOT-23 package Applications n n n n n MR Head Bias Digital camera CCD bias LCD bias GaAs FET bias Positive to negative conversion Typical Application Circuit 20018117 © 2002 National Semiconductor Corporation DS200181 www.national.com LM2611 Connection Diagram Top View 20018115 5-lead SOT-23 Package NS Package Number MF05A Ordering Information Order Number LM2611AMF LM2611AMFX LM2611BMF LM2611BMFX SOT23-5 MF05A Package Type NSC Package Drawing Supplied As 1K Tape and Reel 3K Tape and Reel 1K Tape and Reel 3K Tape and Reel Package ID S40A S40A S40B S40B Pin Description Pin 1 2 3 4 5 Name SW GND NFB SHDN VIN Analog and power ground. Negative feedback. Connect to output via external resistor divider to set output voltage. Shutdown control input. VIN = Device on. Ground = Device in shutdown. Analog and power input. Filter out high frequency noise with a 0.1 µF ceramic capacitor placed close to the pin. Function Drain of internal switch. Connect at the node of the input inductor and Cuk capacitor. Block Diagram 20018101 www.national.com 2 LM2611 Absolute Maximum Ratings (Note 1) ESD Susceptibility (Note 3) Human Body Model Machine Model 2kV 200V If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN SW Voltage NFB Voltage SHDN Voltage Maximum Junction Temperature Power Dissipation (Note 2) Lead Temperature 14.5V −0. 4V to 36V +0. 4V to −6V −0. 4V to 14.5V 125˚C Internally Limited 300˚C Operating Conditions Operating Junction Temperature Range (Note 4) Storage Temperature Supply Voltage θJA −40˚C to +125˚C −65˚C to +150˚C 2.7V to 14V 256˚C/W Electrical Characteristics Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40˚C to +85˚C) unless otherwise specified. VIN = 5.0V and IL = 0A, unless otherwise specified. Symbol VIN ISW RDSON SHDNTH ISHDN NFB INFB Iq Parameter Input Voltage Switch Current Limit Switch ON Resistance Shutdown Threshold Shutdown Pin Bias Current Negative Feedback Reference NFB Pin Bias Current Quiescent Current Grade A Grade B Grade A Grade B Device enabled Device disabled VSHDN = 0V VSHDN = 5V VIN = 3V VNFB =−1.23V VSHDN = 5V, Switching VSHDN = 5V, Not Switching VSHDN = 0V %VOUT/ ∆VIN fS DMAX IL Reference Line Regulation Switching Frequency Maximum Duty Cycle Switch Leakage Not Switching VSW = 5V 2.7V ≤ VIN ≤ 14V 1.0 82 −1.205 −2.7 0.0 0.0 −1.23 −4.7 1.8 270 0.024 0.02 1.4 88 1 1.8 1.0 −1.255 −6.7 3.5 500 1 V µA mA µA µA %/V MHz % µA 1.5 0.50 µA Conditions Min (Note 4) 2.7 1 0.7 1.2 0.9 0.5 0.7 0.65 0.9 Ω V Typ (Note 5) Max (Note 4) 14 2 Units V A Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance of various layouts. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 5: Typical numbers are at 25˚C and represent the expected value of the parameter. 3 www.national.com LM2611 Typical Performance Characteristics RDS(ON) vs VIN RDS(ON) Vs. Ambient Temperature VIN = 5V 20018112 20018145 Switch Current Limit vs. VIN Switch Current Limit vs Ambient Temperature VIN = 5V 20018111 20018143 Oscillator Frequency vs VIN Oscillator Frequency vs Ambient Temperature VIN = 5V 20018119 20018116 www.national.com 4 LM2611 Typical Performance Characteristics VNFB vs VIN TA = 25˚C, VOUT = −5V (Continued) VNFB vs Ambient Temperature VIN = 5V 20018107 20018124 INFB vs VIN TA = 25˚C, VOUT = −5V INFB vs Ambient Temperature VIN = 3.5V, VOUT = −5V 20018108 20018109 Iq vs Ambient Temperature (No Load) VSHUTDOWN vs Ambient Temperature VIN = 5V 20018144 20018110 5 www.national.com LM2611 Typical Performance Characteristics Efficiency vs. Load VOUT = −5V, VIN = 5V (Continued) Efficiency vs. VIN VOUT =− 5V, IOUT = 125mA 20018128 20018127 www.national.com 6 LM2611 Operation Cuk Converter 20018105 FIGURE 1. Operating Cycles of a Cuk Converter The LM2611 is a current mode, fixed frequency PWM switching regulator with a −1.23V reference that makes it ideal for use in a Cuk converter. The Cuk converter inverts the input and can step up or step down the absolute value. Using inductors on both the input and output, the Cuk converter produces very little input and output current ripple. This is a significant advantage over other inverting topologies such as the buck-boost and flyback. The operating states of the Cuk converter are shown in Figure 1. During the first cycle, the transistor switch is closed and the diode is open. L1 is charged by the source and L2 is charged by CCUK, while the output current is provided by L2. In the second cycle, L1 charges CCUK and L2 discharges through the load. By applying the volt-second balance to either of the inductors, the relationship of VOUT to the duty cycle (D) is found to be: 20018103 FIGURE 2. Voltage and Current Waveforms in Inductor L1 of a Cuk Converter The following sections review the steady-state design of the LM2611 Cuk converter. Output and Input Inductor The voltage and current waveforms of inductor L2 are shown in Figure 3. During the first cycle of operation, when the switch is closed, VIN is applied across L2. When the switch opens, VOUT is applied across L2. Figure 2 and Figure 3 show the steady-state voltage and current waveforms for L1 and L2, respectively. Referring to Figure 1 (a), when the switch is closed, VIN is applied across L1. In the next cycle, the switch opens and the diode becomes forward biased, and VOUT is applied across L1 (the voltage across CCUK is VIN − VOUT. 20018104 FIGURE 3. Voltage and Current Waveforms in Inductor L2 of a Cuk Converter 7 www.national.com LM2611 Operation (Continued) The following equations define values given in Figure 2 and Figure 3: IL2 = IOUT 20018126 ) LEVEL 3 FIGURE 5. IOUT(MAX) vs VIN using 1oz. copper layout. See Figure 14 for the test circuit. Use these equations to choose correct core sizes for the inductors. The design of the LM2611’s internal compensation assumes L1 and L2 are equal to 10 - 22 µH, thus it is recommended to stay within this range. Switch Current Limit The LM2611 incorporates a separate current limit comparator, making current limit independent of any other variables. The current limit comparator measures the switch current versus a reference that represents current limit. If at any time the switch current surpasses the current limit, the switch opens until the next switching period. To determine the maximum load for a given set of conditions, both the input and output inductor currents must be considered. The switch current is equal to iL1 + iL2, and is drawn in Figure 4. In summary: Input Capacitor The input current waveform to a Cuk converter is continuous and triangular, as shown in Figure 2. The input inductor insures that the input capacitor sees fairly low ripple currents. However, as the input inductor gets smaller, the input ripple goes up. The RMS current in the input capacitor is given by: The input capacitor should be capable of handling the RMS current. Although the input capacitor is not so critical in a Cuk converter, a 10µF or higher value good quality capacitor prevents any impedance interactions with the input supply. A 0.1µF or 1µF ceramic bypass capacitor is also recommended on the VIN pin (pin 5) of the IC. This capacitor must be connected very close to pin 5 (within 0.2 inches). Output Capacitor Like the input current, the output current is also continuous, triangular, and has low ripple (see IL2 in Figure 3). The output capacitor must be rated to handle its RMS current: iSW(PEAK) must be less than the current limit (1.2A typical), but will also be limited by the thermal resistivity of the LM2611’s SOT23-5 package (θJA = 265˚C/W). Figure 5 shows the maximum output current vs. input voltage that can be expected from a typical layout using 1oz. copper (no heatsink or fan), it is limited by thermal shutdown rather than current limit. 20018102 FIGURE 4. Switch Current Waveform in a Cuk Converter. The peak value is equal to the sum of the average currents through L1 and L2 and the average-to-peak current ripples through L1 and L2. www.national.com 8 For example, ICOUT(RMS) can range from 30mA to 180mA with 10µH ≤ L1,2 ≤ 22µH, −10V ≤ VOUT ≤ −3.3V, and 2.7V ≤ VIN ≤ 30V (VIN may be 30V if using separate power and analog supplies, see Split Supply Operation in the APPLICATIONS section). The worst case conditions are with L1,2, VOUT(MAX), and VIN(MAX). Many capacitor technologies will provide this level of RMS current, but ceramic capacitors are ideally suited for the LM2611. Ceramic capacitors provide a good combination of capacitance and equivalent series resistance (ESR) to keep the zero formed by the capacitance and ESR at high frequencies. The ESR zero is calculated as: LM2611 Operation (Continued) A general rule of thumb is to keep fESR > 80kHz for LM2611 Cuk designs. Low ESR tantalum capacitors will usually be rated for at least 180mA in a voltage rating of 10V or above. However the ESR in a tantalum capacitor (even in a low ESR tantalum capacitor) is much higher than in a ceramic capacitor and could place fESR low enough to cause the LM2611 to run unstable. Improving Transient Response/Compensation The compensator in the LM2611 is internal. However, a zero-pole pair can be added to the open loop frequency response by inserting a feed forward capacitor, CFF, in parallel to the top feedback resistor (RFB1). Phase margin and bandwidth can be improved with the added zero-pole pair. This inturn will improve the transient response to a step load change (see Figure 6 and Figure 7). The position of the zero-pole pair is a function of the feedback resistors and the capacitor value: 20018120 ) LEVEL 3 FIGURE 6. 130mA to 400mA Transient Response of the circuit in Figure 10 with CFF = 1nF (1) (2) The optimal position for this zero-pole pair will vary with circuit parameters such as D, IOUT, COUT, L1, L2, and CCUK. For most cases, placing the zero at 34 krad/s (5.4 kHz) is effective (this corresponds to the values on the front page schematic). Notice how the pole position, ωp, is dependant on the feedback resistors RFB1 and RFB2, and therefore also dependant on the output voltage. As the output voltage becomes closer to −1.26V, the pole moves towards the zero, tending to cancel it out. If the absolute magnitude of the output voltage is less than 3.3V, adding the zero-pole pair will not have much effect on the response. 20018122 ) LEVEL 3 FIGURE 7. 130mA to 400mA Transient Response of the circuit in Figure 10 with CFF disconnected Hysteric Mode As the output current decreases, there will come a point when the energy stored in the Cuk capacitor is more than the energy required by the load. The excess energy is absorbed by the output capacitor, causing the output voltage to increase out of regulation. The LM2611 detects when this happens and enters a pulse skipping, or hysteretic mode. In hysteretic mode, the output voltage ripple will increase, as illustrated in Figure 8 and Figure 9. 20018121 FIGURE 8. The LM2611 in PWM mode has very low ripple 9 www.national.com LM2611 Operation (Continued) 20018123 FIGURE 9. At low loads, the LM2611 enters a pluse-skipping mode. The output ripple slightly increases in this mode. Thermal Shutdown If the junction temperature of the LM2611 exceeds 163˚C, it will enter thermal shutdown. In thermal shutdown, the part deactivates the driver and the switch turns off. The switch remains off until the junction temperature drops to 155˚C, at which point the part begins switching again. It will typically take 10ms for the junction temperature to drop from 163˚C to 155˚C with the switch off. Application Circuits 20018114 FIGURE 10. LM2611 Operating with Separate Power and Biasing Supplies Split Supply Operation The LM2611 may be operated with separate power and bias supplies. In the circuit shown in Figure 10, VIN is the power supply that the regulated voltage is derived from, and VDD is a low current supply used to bias the LM2611. Conditions for the supplies are: 2.7V ≤ VDD ≤ 14V 0V ≤ VIN ≤ (36 − |VOUT|)V www.national.com 10 As the input voltage increases, the maximum output current capability increases, as depicted in Figure 5. Using a separate, higher voltage supply for power conversion enables the LM2611 to provide higher output currents than it would with a single supply that is limited in voltage by VIN(MAX). LM2611 Application Circuits Shutdown/Soft Start (Continued) A soft start circuit is used in switching power supplies to limit the input inrush current upon start-up. Without a soft-start circuit, the inrush current can be several times the steady-state load current, and thus apply unnecessary stress to the input source. The LM2611 does not have soft-start circuitry, but implementing the circuit in Figure 11 will lower the peak inrush current. The SHDN pin is coupled to the output through CSS. The LM2611 is toggled between shutdown and run states while the output slowly decreases to its steady-state value. The energy required to reach steady-state is spread over a longer time and the input current spikes decrease (see Figure 12 and Figure 13). 20018125 FIGURE 11. LM2611 Soft Start Circuit 20018142 20018141 FIGURE 12. Start-Up Waveforms with Soft Start Circuit FIGURE 13. Start-Up Waveforms without Soft Start Circuit 11 www.national.com LM2611 Application Circuits (Continued) High Duty Cycle/Load Current Operation The circuit in Figure 14 is used for high duty cycles (D > 0.5) and high load currents (see Figure 5). The duty cycle will begin to increase beyond 50% as the input voltage drops below the absolute magnitude of the output voltage. RFB3 and CFF2 are added to the feedback network to introduce a low frequency lag compensation (pole-zero pair) necessary to stabilize the circuit under the combination of high duty cycle and high load currents. 20018129 FIGURE 14. LM2611 High Current Schematic www.national.com 12 LM2611 1.4MHz Cuk Converter Physical Dimensions unless otherwise noted inches (millimeters) 5-lead SOT-23 Package NS Package Number MF05A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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