SC4905B

High Performance Voltage Mode PWM Controller POWER MANAGEMENT Description The SC4905A/B is a 10 pin BICMOS primary side voltage mode controller for use in Isolated DC-DC and off-line switching power supplies. It is a highly integrated solution, requiring few external components. The device features a high speed oscillator with integrated feed forward compensation, accurately programmable maximum duty cycle, voltage mode of operation, line voltage monitoring, supply UVLO, low start up current, low voltage current limit threshold and user accessible reference. The SC4905A/B device operates at a fixed frequency, highly desirable for Telecom applications. Features a separate SYNC pin which simplifies synchronization to an external clock. Feeding the oscillator of one device to the SYNC of another forces biphase operation (180 degrees apart) which reduces input ripple and filter size. The SC4905A has a typical turn-on threshold of 4.4V and the SC4905B has a typical turn-on threshold of about 11.6 volts. These devices are available in the 10 lead MSOP package. SC4905A/B Features Operation to 1MHz Accurate programmable maximum duty cycle Integrated oscillator/voltage feed forward compensation Line voltage monitoring External frequency synchronization Bi-phase mode of operation for ripple reduction Under 100µA start-up current Accessible reference voltage VDD undervoltage lockout -40°C to 105°C operating temperature 10-Pin MSOP Lead-free package available. Fully WEEE and RoHS compliant Applications Telecom equipment and power supplies Networking power supplies Power over LAN applications Industrial power supplies Isolated power supplies Typical Application Circuit Revision: January 4, 2006 1 www.semtech.com SC4905A/B POWER MANAGEMENT Absolute Maximum Ratings Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied. Parameter Supply Voltage Input Voltage (RC, ILIM) Input Voltage (VFF) Input Current (VFF) Input Voltage (FB) Output Current (REF) DC OUT Power Dissipation Storage Temperature Range Junction Temperature Lead Temperature (Soldering) 10 Sec. ESD Rating (Human Body Model) Symbol Maximum 18 -0.3 to VREF + 0.3 -0.3 to VREF + 0.7 2 -0.3 to VREF + 0.7 5 -0.3 to VREF + 0.3 180 Units V V V mA V mA V mW °C °C °C KV TSTG TJ TLEAD ESD -65 to +150 -55 to +150 +300 2 Electrical Characteristics Unless otherwise specified, VDD = 12V, VIN = 48V, ROSC = 499k, COSC = 220pF, RT = 280k, RM = 2k, RB = 8.25k, CVDD = 0.1uF, Ta = Tj = -40 to 105 °C. Parameter Supply Curent Section Startup Current IDD Active Line Under Voltage Lockout Start Threshold Hysteresis IIB (VFF) Oscillator Section Maximum Frequency CT Peak Voltage (1) Test Conditions Min Typ Max Unit VDD = UVLO Start - 1, VDD Comparator Off VDD = Comparator On, Oscillator Running 3.5 100 µA 4.2 mA Voltage measured at VFF pin 1.164 85 1.200 100 1.236 115 300 V mV nA VFF = 1.2V +/- 3% -300 VFF = 1.2V to 4.8V VFF = 1.2V VFF = 3.6V 0.8 1.0 1.2 3.6 200 1.2 MHz V V mV CT Valley Voltage Sync/CLOCK (1) Clock SYNC Threshold Sync Input Detect Time  2006 Semtech Corp. (1) .45 *VFF FSYNC > Fosc 2 .50 *VFF 50 .55 *VFF V nS www.semtech.com SC4905A/B POWER MANAGEMENT Electrical Characteristics (Cont.) Unless otherwise specified, VDD = 12V, VIN = 48V, ROSC = 499k, COSC = 220pF, RT = 280k, RM = 2k, RB = 8.25k, CVDD = 0.1uF, Ta = Tj = -40 to 105 °C. Parameter Sync/CLOCK (Cont.) Sync Frequency (1) Test Conditions Min Typ Max Unit 1.2 *FOSC Hz Current Limit Section Input Bias Current Current Limit Threshold Propagation Delay, ILIM to OUT (1) VREF Section VREF (A version) VREF (B version) VDD UVLO Section (A version) Start Threshold Hysteresis VDD UVLO Section (B version) Start Threshold Hysteresis Pulse Width Modulator Section FB Input current Minimum Duty Cycle Maximum Duty Cycle PWM Gain (1) Propagation Delay, PWM to OUT (1) Output Output VSAT Low Output VSAT High Rise Time Fall Time (1) (1) 0 170 50mV Overdrive 200 35 -2 230 µA mV ns 0 - 5mA 0 - 5mA -3% -3% 4 5 +3% +3% V V 4.1 4.4 200 4.6 300 V mV 11 11.6 3.6 12 4 V V VFB = 0V to Vref VFB < 500mV VDMAX = VFF , VFB = Vref VFF = 3.6 95 27.5 75 1 0 uA % % %/V ns IOUT = 1mA IOUT = 1mA COUT = 20pF COUT = 20pF VREF - 0.5 10 10 500 mV V ns ns (1) Note 1: Guaranteed by design. Not 100% tested in production.  2006 Semtech Corp. 3 www.semtech.com SC4905A/B POWER MANAGEMENT Pin Configuration (Top view) Ordering Information Part Number SC4905AIMSTR Package(1) Temp. Range (TA) VDD FB VFF DMAX RC (MSOP-10) REF OUT GND ILIM SYNC SC4905AIMSTRT(2) SC4905BIMSTR SC4905BIMSTRT(2) MSOP-10 -40°C to 105°C Notes: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free product. This product is fully WEEE and RoHS compliant.  2006 Semtech Corp. 4 www.semtech.com SC4905A/B POWER MANAGEMENT Pin Descriptions VDD: The supply input for the device. Once VDD has exceeded the UVLO limit, the internal reference, oscillator, drivers and logic are powered up. This pin should be bypassed with a low ESR capacitance right at the IC pin to minimize noise problems, and to ensure proper operation. FB: Input to the PWM comparator with an offset voltage of 700mV. The feedback analog signal from the output of an error amplifier or an Optoisolator will be connected to this pin to provide regulation. VFF: The VFF pin provides the controller with a voltage proportional to the power supply input voltage to achieve feed-forward function. RM plus RB in conjunction with RT will set the Vff level (see page 1 circuit). VFF = Where VFF is the voltage at the VFF pin at a given Vin, frequency is in Hertz, resistance in ohms, and capacitance in farads. The recommended range if timing resistors is between 10 kohm and 500kohm and range of timing capacitors is between 100pF and 1000pF. Timing resistors less than 10 kohm should be avoided. Refer to layout guide lines on page 12 to achieve best results. SYNC: SYNC is a positive edge triggered input with a threshold precisely set to 0.5*VFF In the Bi-Phase operation mode SYNC pins should be connected to the Cosc (Timing Capacitors) of the other controller. This will force a 180° out of phase operation. (see page9). In a single controller operation, SYNC could be grounded or connected to an external synchronization clock with Frequency higher than the on board oscillator Frequency (see page 2). ILIM: Current sense input is provided via the ILIM pin. The current sense input from a sense resistor provides a pulse by pulse current limit by terminating the PWM pulse when the input is above 180mV. GND: Device power and analog ground. Careful attention should be paid to the layout of the ground planes (see page 12). OUT: The output is intended to drive an external FET driver or other high impedance circuit. The output voltage swings from GND to Vref with a typical output impedance of 500Ω. REF: The REF pin provides a 4 or 5V user accessible voltage reference. This pin should be decoupled with a 1µF capacitor. (R T + RB + RM ) (RB + RM ) × VIN DMAX: Programmable duty cycle is achieved via resistive divider from the VFF. The duty cycle percentage is set by the ratio of the divider RM, and RB (see page 1 circuit) from the VFF signal. When RM is shorted, maximum duty cycle of 100% is achieved. RM plus RB in conjunction with RT will also be used as the divider to set the Vff level. DutyCycle % = VDMAX VFF RC: The oscillator programming pin. The oscillator should be referenced to Vin to achieve the line feed forward function. Only two components are required to program the oscillator, a resistor ROSC (tied to the Vin and RC), and a capacitor COSC (tied to the RC and GND). Since the peak oscillator voltage is VFF, constant frequency operation is maintained over the full power supply. When the DMAX pin is shorted to the VFF pin, the oscillator can run at the largest duty cycle possible. Following formula can be used for a close approximation of the Oscillator Frequency. VFF    Vin −  2  ≅ (ROSC • COSC • VFF • 1.05) FOSC  2006 Semtech Corp. 5 www.semtech.com SC4905A/B POWER MANAGEMENT Block Diagram Marking Information SC4905AIMSTR Top Mark ALOA yyww SC4905BIMSTR Top Mark ALOB yyww Bottom Mark xxxx xxxx Bottom Mark xxxx xxxx yyww = Date Code (Example: 0012) xxxx = Semtech Lot No. (Example: E901 xxxx 01-1)  2006 Semtech Corp. 6 www.semtech.com SC4905A/B POWER MANAGEMENT Application Information THEORY OF OPERATION The SC4905 is a versatile 10 pin BICMOS primary side voltage mode controller optimized for applications requiring minimum space such as isolated DC-DC and off-line switching power supplies. The device contains all of the control and drive circuity required for isolated or non-isolated power supplies, where an external error amplifier is used. Fixed oscillator frequency up to 1MHz can be programmed by an external RC network. The SC4905 is a voltage mode controller, utilizing a feed forward scheme to accommodate for any variations in the input supply voltage resulting in a duty cycle adjustment. This feed forward action results in an improved dynamic performance of the converter. The SC4905 also provides a programmable maximum duty cycle to prevent core saturation when a transformer is used. As an added level of protection, SC4905 provides a cycle by cycle peak current limit during an over current condition. SUPPLY A single supply, VDD is used to provide the bias for the internal reference, oscillator, drivers, and logic circuitry of the SC4905. PWM CONTROLLER The SC4905 is a BICMOS primary side voltage mode controller for use in isolated DC-DC and off-line switching power supplies. It is a highly integrated solution, requiring few external components. The device features a high speed oscillator with integrated feed forward compensation, accurately programmable maximum duty cycle, voltage mode of operation, line voltage monitoring, supply UVLO, low start-up current, low voltage current limit threshold and user accessible reference. Two voltage options are available for the SC4905. The SC4905A version has a typical VDD under voltage of 4.4V, and a 4V reference, while the SC4905B version provides a 11.6V VDD UVLO, and a 5V reference. The Oscillator frequency is programmed by a resistor and a capacitor network connected to the line supply voltage . Any variations in the input supply voltage result in a duty cycle adjustment, provided by the change of the oscillator peak voltage via the VFF pin. This feed forward action provides an immediate duty cycle adjustment while maintaining a constant oscillator frequency. A maximum duty cycle can be programmed by connecting a resistor divider from the VFF to the DMAX pin. The scaling of the VFF signal will set the maximum duty cycle percentage. An external error amplifier will provide the error signal to the FB pin of the SC4905. A current sense input is provided via the ILIM pin. The current sense input from a sense resistor is used for the peak current limit comparator. Once VDD has exceeded the UVLO (VDD under voltage lock out) limit, the internal reference, oscillator, drivers and logic are powered up. SYNC is a positive edge triggered input with a threshold set to 0.5*VFF. By connecting a faster external control signal to the SYNC pin, the internal oscillator frequency will be synchronized to the positive edge of the external control signal. In a single controller operation, SYNC could be grounded or connected to an external synchronization clock within the SYNC frequency (see page 3). In the Bi-Phase operation mode a very unique oscillator is utilized to allow two SC4905 to be synchronized together and work out of phase. This feature is setup by simple connection of the SYNC input to the RC pin of the other part. The fastest oscillator automatically becomes the master, forcing the two PWMs to operate out of phase. This feature minimizes the input and output ripples, and reduces stress on the capacitors. Device S C 4905A S C 4905B Typical V dd U V LO 4.4V 11.6V Typical Reference Voltage 4V 5V  2006 Semtech Corp. 7 www.semtech.com SC4905A/B POWER MANAGEMENT Application Information (Cont.) VDD UNDER VOLTAGE LOCK OUT According to the application, and the voltages available, the SC4905A (UVLO = 4.4V), or the SC4905B (UVLO = 11.6V) can be used to provide the VDD undervoltage lock out function to ensure the converters controlled start up. Before the VDD UVLO has been reached, the internal reference, oscillator, OUT driver, and logic are disabled. REFERENCE A 4V (SC4905A) or a 5V(SC4905B) reference voltage is available that can be used to source a typical current up to 5mA to the external circuitry. The REF can be used to provide the feed back circuitry with a regulated bias. OSCILLATOR The oscillator frequency is set by connecting a RC network as shown below. Vin Since the Rosc is referenced to the input supply voltage, any variation in the supply is directly translated into a variation in the duty cycle, while maintaining the fixed frequency operation. Following equation can be used to calculate the oscillator frequency: FOSC VFF    Vin −  2  ≅ (ROSC • COSC • VFF • 1.05) The recommended range if timing resistors is between 10 kohm and 500kohm and range of timing capacitors is between 100pF and 1000pF. Timing resistors less than 10 kohm should be avoided. SC4905 U1 RT 280k Rosc 499k 1 2 3 4 VDD FB VFF DMAX RC REF OUT GND ILIM SYNC 10 9 8 7 6 RM 2k 5 RB 8.25k Cosc 220p, 16V The oscillator has a ramp voltage that will track the voltage at the VFF pin (1.2V