LM2621_05

LM2621 Low Input Voltage, Step-Up DC-DC Converter March 2005 LM2621 Low Input Voltage, Step-Up DC-DC Converter General Description The LM2621 is a high efficiency, step-up DC-DC switching regulator for battery-powered and low input voltage systems. It accepts an input voltage between 1.2V and 14V and converts it into a regulated output voltage. The output voltage can be adjusted between 1.24V and 14V. It has an internal 0.17Ω N-Channel MOSFET power switch. Efficiencies up to 90% are achievable using the LM2621. The high switching frequency (adjustable up to 2MHz) of the LM2621 allows for tiny surface mount inductors and capacitors. Because of the unique constant-duty-cycle gated oscillator topology very high efficiencies are realized over a wide load range. The supply current is reduced to 80µA because of the BiCMOS process technology. In the shutdown mode, the supply current is less than 2.5µA. The LM2621 is available in a Mini-SO-8 package. This package uses half the board area of a standard 8-pin SO and has a height of just 1.09 mm. n n n n n n n n n 1.09 mm Package Height Up to 2 MHz Switching Frequency 1.2V to 14V Input Voltage 1.24V - 14V Adjustable Output Voltage Up to 1A Load Current 0.17 Ω Internal MOSFET Up to 90% Regulator Efficiency 80 µA Typical Operating Current < 2.5µA Guaranteed Supply Current In Shutdown Applications n n n n n n n n n n PDAs, Cellular Phones 2-Cell and 3-Cell Battery-Operated Equipment PCMCIA Cards, Memory Cards Flash Memory Programming TFT/LCD Applications 3.3V to 5.0V Conversion GPS Devices Two-Way Pagers Palmtop Computers Hand-Held Instruments Features n Small Mini-SO8 Package (Half the Footprint of Standard 8-Pin SO Package) Typical Application Circuit 10093412 © 2005 National Semiconductor Corporation DS100934 www.national.com LM2621 Connection Diagram Mini SO-8 (MM) Package 10093418 Top View Ordering Information Order Number LM2621MMX LM2621MM Package Type Mini SO-8 Mini SO-8 NSC Package Drawing MUA08A MUA08A Package Marking S06A S06A Supplied As 3000 Units on Tape and Reel 1000 Units on Tape and Reel www.national.com 2 LM2621 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. SW Pin Voltage BOOT, VDD, EN and FB Pins FREQ Pin θJA (Note 2) TJmax (Note 2) Storage Temperature Range Lead Temp. (Soldering, 5 sec) −0.5 V to 14.5V −0.5V to 10V 100µA 240˚C/W 150˚C −65˚C to +150˚C 260˚C Power Dissipation (TA=25˚C) (Note 2) ESD Rating (Note 3) 500mW 2kV Operating Conditions (Note 1) VDD Pin FB, EN Pins BOOT Pin Ambient Temperature (TA) 2.5V to 5V 0 to VDD 0 to 10V −40˚C to +85˚C Electrical Characteristics Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range of −40˚C to +85˚C. Unless otherwise specified: VDD= VOUT= 3.3V. Symbol VIN_ST VIN_OP VFB VOUT_MAX VHYST η Parameter Minimum Start-Up Supply Voltage (Note 4) Minimum Operating Supply Voltage (once started) FB Pin Voltage Maximum Output Voltage Hysteresis Voltage (Note 7) Efficiency VIN = 3.6V; VOUT = 5V; ILOAD = 500mA VIN = 2.5V; VOUT = 3.3V; ILOAD = 200mA D IDD ISD ICL RDS_ON Switch Duty Cycle Operating Quiescent Current (Note 6) Shutdown Quiescent Current (Note 7) Switch Peak Current Limit MOSFET Switch On Resistance EN Pin Voltage Low (Note 8) EN Pin Voltage High (Note 8) 0.7VDD FB Pin > 1.3V; EN Pin at VDD VDD, BOOT and SW Pins at 5.0V; EN Pin < 200mV Condition ILOAD = 0mA ILOAD = 0mA Typ 1.1 0.65 1.24 14 30 87 87 70 80 0.01 2.85 0.17 60 80 110 2.5 % 45 1.2028 1.2772 Min Max 1.2 Units V V V V mV % µA µA A Ω Enable Section VEN_LO VEN_HI 0.15VDD V V Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device outside of its rated operating conditions. Note 2: The maximum power dissipation must be derated at elevated temperatures and is dictated by Tjmax (maximum junction temperature), θJA (junction to ambient thermal resistance), and TA (ambient temperature). The maximum allowable power dissipation at any temperature is Pdmax = (Tjmax - TA)/ θJA or the number given in the Absolute Maximum Ratings, whichever is lower. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. For Pin 8 (SW) the ESD rating is 1.5 kV. Note 4: Output in regulation, VOUT = VOUT (NOMINAL) ± 5% Note 5: This is the hysteresis value of the internal comparator used for the gated-oscillator control scheme. Note 6: This is the current into the VDD pin. Note 7: This is the total current into pins VDD, BOOT, SW and FREQ. Note 8: When the EN pin is below VEN_LO, the regulator is shut down; when it is above VEN_HI, the regulator is operating. 3 www.national.com LM2621 Pin Description Pin 1 2 3 4 5 6 7 8 EN FREQ FB SGND VDD BOOT SW Name PGND Power Ground Active-Low Shutdown Input Frequency Adjust. An external resistor connected between this pin and Pin 6 (VDD) sets the switching frequency of the LM2621. Output Voltage Feedback Signal Ground Power Supply for Internal Circuitry Bootstrap Supply for the Gate Drive of Internal MOSFET Power Switch Drain of the Internal MOSFET Power Switch Function Typical Performance Characteristics Efficiency vs Load Current VOUT = 5.0V Efficiency vs Load Current VOUT = 3.3V 10093401 10093402 VFB vs Temperature IOP vs Temperature 10093403 10093404 www.national.com 4 LM2621 Typical Performance Characteristics ISD vs Temperature (Continued) ISD vs VDD 10093405 10093406 IOP vs VDD VIN_ST vs Load Current VOUT = 3.3V 10093407 10093408 Switching Frequency vs RFQ Peak Inductor Current vs Load Current 10093410 10093409 5 www.national.com LM2621 Typical Performance Characteristics Maximum Load Current vs Input Voltage (Continued) 10093411 Detailed Description OPERATING PRINCIPLE The LM2621 is designed to provide step-up DC-DC voltage regulation in battery-powered and low-input voltage systems. It combines a step-up switching regulator, N-channel power MOSFET, built-in current limit, thermal limit, and voltage reference in a single 8-pin MSOP package . The switching DC-DC regulator boosts an input voltage between 1.2V and 14V to a regulated output voltage between 1.24V and 14V. The LM2621 starts from a low 1.1V input and remains operational down to 0.65V. This device is optimized for use in cellular phones and other applications requiring a small size, low profile, as well as low quiescent current for maximum battery life during stand-by and shutdown. A high-efficiency gated-oscillator topology offers an output of up to 1A. Additional features include a built-in peak switch current limit, and thermal protection circuitry. 10093414 FIGURE 1. Functional Diagram www.national.com 6 LM2621 Detailed Description (Continued) GATED OSCILLATOR CONTROL SCHEME A unique gated oscillator control scheme enables the LM2621 to have an ultra-low quiescent current and provides a high efficiency over a wide load range. The switching frequency of the internal oscillator is programmable using an external resistor and can be set between 300 kHz and 2 MHz. This control scheme uses a hysteresis window to regulate the output voltage. When the output voltage is below the upper threshold of the window, the LM2621 switches continuously with a fixed duty cycle of 70% at the switching frequency selected by the user. During the first part of each switching cycle, the internal N-channel MOSFET switch is turned on. This causes the current to ramp up in the inductor and store energy. During the second part of each switching cycle, the MOSFET is turned off. The voltage across the inductor reverses and forces current through the diode to the output filter capacitor and the load. Thus when the LM2621 switches continuously, the output voltage starts to ramp up. When the output voltage hits the upper threshold of the window, the LM2621 stops switching completely. This causes the output voltage to droop because the energy stored in the output capacitor is depleted by the load. When the output voltage hits the lower threshold of the hysteresis window, the LM2621 starts switching continuously again causing the output voltage to ramp up towards the upper threshold. Figure 2 shows the switch voltage and output voltage waveforms. Because of this type of control scheme, the quiescent current is inherently very low. At light loads the gated oscillator control scheme offers a much higher efficiency compared to the conventional PWM control scheme. 10093415 FIGURE 2. Typical Step-Up Regulator Waveforms LOW VOLTAGE START-UP The LM2621 can start-up from input voltages as low as 1.1V. On start-up, the control circuitry switches the N-channel MOSFET continuously at 70% duty cycle until the output voltage reaches 2.5V. After this output voltage is reached, the normal step-up regulator feedback and gated oscillator control scheme take over. Once the device is in regulation it can operate down to a 0.65V input, since the internal power for the IC can be boot-strapped from the output using the VDD pin. SHUTDOWN The LM2621 features a shutdown mode that reduces the quiescent current to less than a guaranteed 2.5µA over temperature. This extends the life of the battery in battery powered applications. During shutdown, all feedback and control circuitry is turned off. The regulator’s output voltage drops to one diode drop below the input voltage. Entry into the shutdown mode is controlled by the active-low logic input pin EN (Pin 2). When the logic input to this pin pulled below 0.15VDD, the device goes into shutdown mode. The logic input to this pin should be above 0.7VDD for the device to work in normal step-up mode. OUTPUT VOLTAGE RIPPLE FREQUENCY A major component of the output voltage ripple is due to the hysteresis used in the gated oscillator control scheme. The frequency of this voltage ripple is proportional to the load current. The frequency of this ripple does not necessitate the use of larger inductors and capacitors however, since the size of these components is determined by the switching frequency of the oscillator which can be set upto 2MHz using an external resistor. INTERNAL CURRENT LIMIT AND THERMAL PROTECTION An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85A typical, approximately 4A maximum) so as not to come into effect during normal operating conditions. An internal thermal protection circuitry disables the MOSFET power switch when the junction temperature (TJ) exceeds about 160˚C. The switch is re-enabled when TJ drops below approximately 135˚C. 7 www.national.com LM2621 Design Procedure SETTING THE OUTPUT VOLTAGE The output voltage of the step-up regulator can be set between 1.24V and 14V by connecting a feedback resistive divider made of RF1 and RF2. The resistor values are selected as follows: RF2 = RF1 /[(VOUT/ 1.24) −1] A value of 150kΩ is suggested for RF1. Then, RF2 can be selected using the above equation. A 39pF capacitor (CF1) connected across RF1 helps in feeding back most of the AC ripple at VOUT to the FB pin. This helps reduce the peak-topeak output voltage ripple as well as improve the efficiency of the step-up regulator, because a set hysteresis of 30mV at the FB pin is used for the gated oscillator control scheme. BOOTSTRAPPING When the output voltage (VOUT) is between 2.5V and 5.0V a bootstrapped operation is suggested. This is achieved by connecting the VDD pin (Pin 6) to VOUT. However if the VOUT is outside this range, the VDD pin should be connected to a voltage source whose range is between 2.5V and 5V. This can be the input voltage (VIN), VOUT stepped down using a linear regulator, or a different voltage source available in the system. This is referred to as non-bootstrapped operation. The maximum acceptable voltage at the BOOT pin (Pin 7) is 10V. SETTING THE SWITCHING FREQUENCY The switching frequency of the oscillator is selected by choosing an external resistor (RFQ) connected between FREQ and VDD pins. See the graph titled „Switching Frequency vs RFQ“ in the Typical Operating Characteristics section of the datasheet for choosing the RFQ value to achieve the desired switching frequency. A high switching frequency allows the use of very small surface mount inductors and capacitors and results in a very small solution size. A switching frequency between 300kHz and 2MHz is recommended. INDUCTOR SELECTION The LM2621’s high switching frequency enables the use of a small surface mount inductor. A 6.8µH shielded inductor is suggested. The inductor should have a saturation current rating higher than the peak current it will experience during circuit operation (see graph titled „Peak Inductor Current vs. Load Current“ in the Typical Performance Characteristics section). Less than 100mΩ ESR is suggested for high efficiency. Open-core inductors cause flux linkage with circuit components and interfere with the normal operation of the circuit. They should be avoided. For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce the core losses. To minimize radiated noise, use a toroid, pot core or shielded core inductor. The inductor should be connected to the SW pin as close to the IC as possible. See Table 1 for a list of the inductor manufacturers. OUTPUT DIODE SELECTION A Schottky diode should be used for the output diode. The forward current rating of the diode should be higher than the load current, and the reverse voltage rating must be higher than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer. Table 1 shows a list of the diode manufacturers. INPUT AND OUTPUT FILTER CAPACITORS SELECTION Tantalum chip capacitors are recommended for the input and output filter capacitors. A 22µF capacitor is suggested for the input filter capacitor. It should have a DC working voltage rating higher than the maximum input voltage. A 68µF tantalum capacitor is suggested for the output capacitor. The DC working voltage rating should be greater than the output voltage. Very high ESR values ( > 3Ω) should be avoided. Table 1 shows a list of the capacitor manufacturers. TABLE 1. Suggested Manufacturers List Inductors Coilcraft Tel: (800) 322-2645 Fax: (708) 639-1469 Coiltronics Tel: (407) 241-7876 Fax: (407) 241-9339 Pulse Engineering Tel: (619) 674-8100 Fax: (619) 674-8262 Capacitors Sprague/ Vishay Tel: (207) 324-4140 Fax: (207) 324-7223 Kemet Tel: (864) 963-6300 Fax: (864) 963-6521 Nichicon Tel: (847) 843-7500 Fax: (847) 843-2798 Diodes Motorola Tel: (800) 521-6274 Fax: (602) 244-6609 International Rectifier (IR) Tel: (310) 322-3331 Fax: (310) 322-3332 General Semiconductor Tel: (516) 847-3222 Fax: (516) 847-3150 and output filter capacitors and the PGND and SGND pins of LM2621 should be connected using short, direct and wide traces. The voltage feedback network (RF1, RF2, and CF1) should be kept very close to the FB pin. Noisy traces, such as from the SW pin, should be kept away from the FB and VDD pins. The traces that run between Vout and the FB pin of the IC should be kept away from the inductor flux. Always provide sufficient copper area to dissipate the heat due to power loss in the circuitry and prevent the thermal protection circuitry in the IC from shutting the IC down. PC BOARD LAYOUT High switching frequencies and high peak currents make a proper layout of the PC board an important part of design. Poor design can cause excessive EMI and ground-bounce, both of which can cause malfunction and loss of regulation by corrupting voltage feedback signal and injecting noise into the control section. Power components - such as the inductor, input and output filter capacitors, and output diode - should be placed as close to the regulator IC as possible, and their traces should be kept short, direct and wide. The ground pins of the input www.national.com 8 LM2621 Application examples EXAMPLE 1. 5V/0.5A Step-Up Regulator 10093412 U1 C1 C2 D1 L National Vishay/Sprague Vishay/Sprague Motorola Coilcraft LM2621MM 595D226X06R3B2T, Tantalum 595D686X0010C2T, Tantalum MBRS140T3 DT1608C-682 EXAMPLE 2. 2mm Tall 5V/0.2A Step-Up Regulator for Low Profile Applications 10093417 U1 C1 C2 D1 L National Vishay/Sprague Vishay/Sprague Motorola Vishay/Dale LM2621MM 592D156X06R3B2T, Tantalum 592D336X06R3C2T, Tantalum MBRS140T3 ILS-3825-03 9 www.national.com LM2621 EXAMPLE 3. 3.3V/0.5A SEPIC Regulator 10093422 U1 C1 C2 D1 L1, L2 CS National Vishay/Sprague Vishay/Sprague Motorola Coilcraft Vishay/Vitramon LM2621MM 595D226X06R3B2T, Tantalum 595D686X0010C2T, Tantalum MBRS140T3 DT1608C-682 VJ1210Y105M , Ceramic www.national.com 10 LM2621 Low Input Voltage, Step-Up DC-DC Converter Physical Dimensions unless otherwise noted inches (millimeters) 8-Lead Mini SO-8 (MM) NS Package Number MUA08A For Order Numbers, refer to the table in the "Ordering Information" section of this document. 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