SC2604 POWER MANAGEMENT Features Description Input Voltage Range: 4.5V to 13.5V 1% Voltage Reference Accuracy Up to 95% Efficiency Input Disconnect FET Drive In-rush Current Control Internal Compensation Programmable Current Limit Programmable Soft Start 800mA Typical PWM Gate Drive 400kHz Switching Frequency Under Voltage Lockout <200uA Shutdown Current -40oC to +85oC Temperature Range MSOP-8 Package, Fully WEEE and RoHS Compliant Applications Simple PWM Boost Controller with Input Disconnect FET Drive Portable Devices Flat Panel TV TV Set Top Box Auxiliary Supplies Peripheral Card Supplies Industrial Power Supply High Density DC/DC Conversion The SC2604 is a versatile, low-cost, voltage-mode PWM controller designed for boost DC/DC power supply applications. It features input disconnect FET driver allowing power source and load separation at shutdown mode, which eliminates possible leakage current from source to load. Also, it prevents catastrophic failure when output is shorted during operation. The SC2604 also includes temperature compensated voltage reference, internal ramp, current limit comparator, internally compensated error amplifier, and floating driver with charge pump. Programmable soft start controls in-rush current and reduces output voltage overshoot. Hiccup mode over-current protection allows system autoretry and ease of trouble shooting. Internally compensated feedback loop makes power supply design simple, and eliminates the need for external compensation network. The SC2604 is available in MSOP-8 package with rated temperature range of -40oC to +85oC. Typical Application Circuit Figure 1. 12V to 25V/1A Boost Converter with Over Current Protection January 4, 2008 www.semtech.com SC2604 Pin Configuration Ordering Information Device Package SC2604MSTRT(1)(2) MSOP-8 SC2604EVB Evaluation Board Notes: (1) Available in tape and reel only. A reel contains 2,500 devices. (2) Available in lead-free package only. Device is fully WEEE and RoHS compliant. (8 - Pin MSOP) Marking Information Top Mark Bottom Mark nnnn=Part Number Code (Example AS00) - Reference Part No. Code for MSOP yyww=Date Code (Example: 0752) xxxx = Semtech Lot No. (Example: E901) xxxx = Semtech Lot No. (Example: 01-1) © 2007 Semtech Corp. www.semtech.com SC2604 Absolute Maximum Ratings Thermal Information VIN Supply Voltage ……………………………… -0.3 to 20V Junction to Ambient (1) ……………………………… 160°C/W CS Pin Voltage………………………………………-0.3 to 20V Junction to Case (1) ………………………………… GATE Pin Voltage……………………………………-0.3 to 20V Maximum Junction Temperature……………………… 150°C Storage Temperature ………………………… -45 to +150°C DRV Pin Voltage ……………………………………-0.3 to 25V Lead Temperature (Soldering) 10 sec ………………… 300°C OCP/EN Pin Voltage …………………………………-0.3 to 7V SS/VREF Pin Voltage …………………………………-0.3 to 7V FB Pin Voltage ………………………………………-0.3 to 7V Peak IR Reflow Temperature …………………………. 45°C/W 260°C Recommended Operating Conditions Input Voltage Range …………………………… 4.5V to 13.5V ESD Protection Level(2) ………………………………… 2000V Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES(1) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (2) Tested according to JEDEC standard JESD22-A114-B. Electrical Characteristics Unless otherwise noted, VIN = 12V, VO = 25V, -40°C < TA = TJ < 125°C. Parameter Conditions Min Typ Max Units 13.5 V 4.5 V Input Supply VIN Supply Voltage 4.5 VIN Start Voltage VIN Rising 4.2 VIN Start Hysteresis 400 VIN Supply Current Switching, GATE pin floating VIN Shutdown Current 6.0 mV 9.0 mA 200 µA 1.250 1.275 V 0.5 1.0 µA OCP/EN = Low Error Amplifier Feedback Voltage IO = 100mA Feedback Bias Current Error Amplifier Gain 1.225 VIN = 12V, VFB = VSS/VREF V/V 90 (1) Oscillator Oscillator Frequency 320 400 480 kHz Maximum Duty Cycle 86 90 % Internal Ramp Peak (2) 1.4 V Internal Ramp Valley (2) 0.4 V Regulation Load Regulation IO = 0.1A to 1A 0.5 % Line Regulation VIN = 5V to 13.5V, IO = 0.1A 1.0 % © 2007 Semtech Corp. www.semtech.com SC2604 Electrical Characteristics (Cont.) Unless otherwise noted, VIN = 12V, VO = 25V, -40°C < TA = TJ < 125°C. Parameter Conditions Min Typ Max Units Gate Source Current VIN = 12V, CGATE = 10nF 0.5 0.8 A Gate Sink Current VIN = 12V, CGATE = 10nF 0.5 0.8 A 55 µA PWM Switch Gate Drive PWM Switch Soft Start Soft Start Charge Current SS/VREF Threshold to Shutdown Switch SS/VREF Threshold to Turn-on Switch Pull down below this level to disable PWM Switch gate Pull above this level to enable PWM Switch gate 100 310 mV mV Disconnect Switch Gate Drive DRV Source Current DRV Sink Current VIN = 12V, VDRV = 15.5V 45 µA VIN = 12V, VDRV = 8V 45 µA Over Current Protection Current Limit Threshold OCP/EN Threshold VIN - CS 61 72 83 mV Pull down below this level to disable Disconnect FET gate 520 590 660 mV OCP/EN Charge Current 37 µA OCP/EN Discharge Current 1.0 µA CS Input Current 0.2 µA Note: (1). Guaranteed by Characterization (2). Guaranteed by design © 2007 Semtech Corp. www.semtech.com SC2604 Pin Descriptions Pin Pin Name 1 CS Current sense input (negative) 2 VIN Device supply voltage (also positive current sense input) 3 GATE PWM gate driver output for boost converter 4 GND Device ground 5 SS/VREF 6 FB 7 OCP/EN 8 DRV © 2007 Semtech Corp. Pin Function Soft start and reference voltage pin Error amplifier inverted input When a capacitor is tied to this pin, the maximum inrush current is controlled during start-up. The capacitor value also determines the off-time after the device has entered hiccup mode. Pulling this pin low can disable the linear and the switcher to turn off the circuit. Gate drive of input disconnect FET limiting system input current www.semtech.com SC2604 Block Diagram Figure 2. SC2604 Function Diagram © 2007 Semtech Corp. www.semtech.com SC2604 Typical Characteristics © 2007 Semtech Corp. www.semtech.com SC2604 Typical Characteristics (Cont.) © 2007 Semtech Corp. www.semtech.com SC2604 Applications Information PWM Control Loop The SC2604 is a voltage-mode PWM controller with a fixed switching frequency of 400kHz for use in high efficiency, boosted voltage, DC/DC power supplies. allow a complete shutdown of the output. Pulling the SS/ VREF pin below 0.1V only shuts the boost FET (Q2 in Figure 1) off and the output voltage will be (VIN-Vd). As shown in Figure 2, the PWM control loop of the SC2604 consists of a 400kHz oscillator, a PWM comparator, a voltage error amplifier, and a FET driver. The boost converter output voltage is fed back to FB (error amplifier negative) and is regulated to the reference voltage at SS/VREF pin. The error amplifier output is compared with the 400kHz ramp to generate a PWM wave, which is amplified and used to drive the boost FET (Q2 in Figure 1) for the converter. The PWM controller works with soft start and fault monitoring circuitry to meet application requirements. UVLO, Start-up, and Shutdown To initiate the SC2604, a supply voltage is applied to VIN. The DRV and GATE are held low. When VIN voltage exceeds UVLO (Under Voltage Lockout) threshold, typically 4.2V, an internal current source (37µA) begins to charge the OCP/EN pin capacitor. The OCP/EN voltage ramps from near ground to over 1.25V but the voltage between 0.625V and 1.25V provides the linear soft-start range for the disconnect FET (Q1). When the OCP/EN voltage is over 1.25V, the OCP hiccup is enabled, and SS/VREF pin is released. At this moment, another internal current source (55µA) begins to charge the SS/VREF pin capacitor. When the SS/VREF pin voltage reaches 0.5V, the error amplifier output will rise to 0.4V, then the PWM comparator begins to switch. The switching regulator output is slowly ramping up for a soft turn-on. The details of SC2604 startup timing is shown in Figure 3. If the supply voltage at VIN pin falls below UVLO threshold (3.8V typically) during a normal operation, the DRV pin is pulled low to cut off the supply power of the boost converter, while the OCP/EN pin capacitor is discharged with a 1µA internal current source. When the OCP/EN pin falls below 1.25V, the SS/VREF pin is forced to ground. This completely shuts down the boost conveter. Directly pulling the OCP/EN pin below 0.52V can also © 2007 Semtech Corp. Figure 3. Start-up Timing Diagram Hiccup Mode Short Circuit Protection Hiccup mode over-current protection is utilized in the SC2604. When an increasing load causes a voltage of 72mv to occur from VIN to CS then a current limit hiccup sequence is started. The sequence starts by pulling DRV low and discharging the OCP/EN pin with a 1µA current source. When the OCP/EN pin falls below 1.25V, the SS/ VREF pin is forced to ground (similar to the UVLO shutdown described in the last setion). When the voltage on the OCP/EN pin falls to near zero volt, the 1µA discharge current becomes a 37µA charging current and the OCP/EN pin starts to charge and DRV is enabled. When the OCP/EN voltage rises from 0.625V to 1.25V, the current in the disconnect FET is allowed to increase from zero to a maximum of 72mV/(Current Sense Resistor Value). If the over-current condition still exists when OCP/EN crosses 1.25V then the hiccup sequence will re-start. If there is no over-current as OCP/EN crosses 1.25V then the SS/VREF pin is released to rise and allow a www.semtech.com SC2604 Applications Information (Cont.) soft-start of the switching boost regulator. Capacitor at OCP/EN Pin - COCP/EN The DRV pin of the SC2604 is meant to drive an N-Channel FET that can disconnect the input supply in the event of an over-current condition. The OCP/EN capacitor becomes part of a hiccup oscillator that is charged with 37µA and discharged with 1µA to provide a low duty cycle for the FET Q1. As the current at start-up may hit its current limit threshold, the ramp rate of the current must be slow enough to allow the output capacitor to be fully charged to a voltage one diode drop Vd less than input voltage VIN. To guarantee a successful start-up at no load, the value of the capacitor at the OCP/EN pin has to satisfy the following formula: It should be understood that sufficiently fast ramp rates on the OCP/EN pin and the SS/VREF pin can trigger a hiccup event because of the charging current demanded by the boost regulator output capacitor. Disconnect FET Selection Setting the Output Voltage In Figure 1, an external resistive divider R7 and R8 with its center tap tied to the FB pin sets the output voltage. In some applications, a RC branch (R6, C12 in the Typical Schematic on page 12) will be needed for loop stability. Maximum Duty Cycle The maximum duty cycle, Dmax defines the upper limit of power conversion ratio Calculating Current Sense Resistor Current sense resistor is placed at the input to sense inductor peak current of the boost regulator. The value of the resistor can be calculated by where IPEAK is the allowed boost inductor peak current. In many applications, a noise filter circuit (R1=200, C10=10nF in the Typical Schematic on page 12) may be needed for the input current sensing. © 2007 Semtech Corp. The floating driving voltage of DRV pin drops slightly as the supply voltage VIN is below 7.5V (Typical Characteristics on page 8), where a FET with low gate threshold voltage (VGS(TH)) has to be used for the disconnect FET. In a 5V input application, a FET with VGS(TH)=2V, such as FDD6672A from Fairchild, is needed. Layout Guidelines Careful attentions to layout requirements are necessary for successful implementation of the SC2604 PWM controller. High currents switching at 400kHz are present in the application and their effect on ground plane voltage differentials must be understood and minimized. 1) The high power parts of the circuit should be laid out first. A ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. Isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example the input capacitor and bottom Schottky ground. 2) The loop formed by the output Capacitor(s) (COUT ), the FET (Q1), the current sensing resistor, and the Schottky (D1) must be kept as small as possible, as shown on the layout diagram in Figure 4. This loop contains all the high current, fast transition switching. Connections should be as wide and as short as possible to minimize loop inductance. Minimizing this loop area will reduce EMI, www.semtech.com 10 SC2604 Applications Information (Cont.) lower ground injection currents, resulting in electrically “cleaner” grounds for the rest of the system and minimize source ringing, resulting in more reliable gate switching signals. 3) The connection between the junction of Q1, D1 and the output capacitor should be a wide trace or copper region. It should be as short as practical. Since this connection has fast voltage transitions, keeping this connection short will minimize EMI. 4) The Output Capacitor(s) (COUT ) should be located as close to the load as possible, fast transient load currents are supplied by COUT only, and connections between COUT and the load must be short, wide copper areas to minimize inductance and resistance. 5) The SC2604 is best placed over an isolated ground plane area. The soft-start capacitor and the Vin decoupling capacitor should also connected to this ground pad area. This isolated ground area should be connected to the main ground by a trace that runs from the GND pin to the ground side of the output capacitor. If this is not possible, the GND pin may be connected to the ground path between the Output Capacitor and the CIN, Q1, D1 loop. Under no circumstances should GND be returned to a ground inside the CIN, Q1, D1 loop. 6) Input voltage of the SC2604 should be supplied from the power rail through a 1Ω resistor, the Vin pin should be decoupled directly to GND by a 0.1µF~1µF ceramic capacitor, trace lengths should be as short as possible. Note: Heavy lines indicate the critical loop carrying high pulsating current. The inductance of the loop needs to be minimized. Figure 4. SC2604 Layout Diagram © 2007 Semtech Corp. www.semtech.com 11 SC2604 Applications Information (Cont.) Typical application schematic with 12V input and 25V/1.5A output Note: A small Schottky diode (Da) may be required in some applications to clamp negative spike at the GATE pin. Bill of materials © 2007 Semtech Corp. www.semtech.com 12 SC2604 Applications Information (Cont.) © 2007 Semtech Corp. Start up Inductor current and DRV pin voltage at OCP www.semtech.com 13 SC2604 Outline Drawing - MSOP-8 Land Pattern - MSOP-8 Contact Information Semtech Corporation Power Mangement Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 © 2007 Semtech Corp. www.semtech.com 14