SC4520

3A Step-Down Switching Regulator with Adjustable Switching Frequency POWER MANAGEMENT Description The SC4520 is a current mode switching regulator with an integrated switch and an adjustable frequency with enable function. The integrated switch allows for costeffective, low power solutions with a peak switch current of 3 amps. An adjustable high frequency of 100kHz to 600kHz provides for fast dynamic response and instantaneous duty cycle adjustment as the input varies, making the device ideal for CPE applications where the input is a wall plug power. Low shutdown current also makes the this device an excellent choice for portable applications where conserving battery life is of prime concern. SC4520 Features Wide operating voltage range: 4.4V to 24V Integrated 3 Amp switch 100kHz to 600kHz adjustable frequency operation Current mode control Precision enable threshold SO-8 EDP package. Lead-free product, fully WEEE and RoHS compliant Applications XDSL modems CPE equipment DC-DC point of load applications Portable equipment Digtial consumer electronics Typical Application Circuit D1 1 VIN Enable C3 2 5 8 BST IN EN FSET SW 3 6 C1 L1 VOUT SC4520 GND 4 FB COMP R1 7 D2 R2 C2 Rosc C4 R3 Revision: December 13, 2006 1 www.semtech.com SC4520 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. Exposure to Absolute Maximum rated conditions for extended periods of time may affect device reliability. Parameter Input Supply Voltage Boost Pin Above VSW Boost Pin Voltage EN Pin Voltage FB Pin Voltage FB Pin Current FSET Pin Voltage Thermal Impedance Junction to Ambient (1) Maximum Junction Temperature Storage Temperature Range Lead Temperature (Soldering) 10 sec ESD Rating (Human Body Model) Symbol VIN (VBST - VSW) V BST V EN V FB IFB V FS E T θJA TJ TSTG TLEAD ESD Limits -0.3 to +28 16 -0.3 to +32 -0.3 to +24 -0.3 to +6 1 +3 36.5 150 -65 to +150 300 2 Units V V V V V mA V °C/W °C °C °C kV Note: (1) Minimum pad size. Electrical Characteristics Unless specified: VIN = 12V, VCOMP = 0.8V, VBST = VIN + 5V, EN = tied to VIN, SW = open. TA = TJ = -40°C to 125°C. Parameter Operating Input Voltage Maximum Switch Current Limit Oscillator Frequency Oscillator Frequency Range Switch On Voltage Drop VIN Undervoltage Lockout VIN UVLO Hysteresis VIN Supply Current Standby Current Symbol VIN ISW fOSC fOSC VD(SW) VUVLO VHYST IQ IQ(OFF) Conditions Min Typ Max 24(1) Units V A 3.0 ROSC = 82.5k Ω 250 100 ISW = 3A 220 3.9 60 V FB = 1V V E N = 0V 3 250 5.5 4.4 300 350 600 kHz kHz mV V mV mA µA  2006 Semtech Corp. 2 www.semtech.com SC4520 POWER MANAGEMENT Electrical Characteristics (Cont.) Unless specified: VIN = 12V, VCOMP = 0.8V, VBST = VIN + 5V, EN = tied to VIN, SW = open. TA = TJ = -40°C to 125°C. PARAMETER FB Input Current Feedback Voltage Feedback Voltage Line Regulation FB to VCOMP Voltage Gain(2) FB to VCOMP Transconductance(2) VCOMP Pin Source Current VCOMP Pin Sink Current VCOMP Pin to Switch Current Transconductance VCOMP Pin Maximum Switching Threshold VCOMP OCP Threshold VCOMP Hiccup Retry Threshold Maximum Switch Duty Cycle SYMBOL IFB CONDITIONS MIN TYP -0.25 MAX -1 0.816 UNITS µA V mV/V V/V 0.784 4.4V < VIN < 24V(1) 0.9V ≤ VCOMP ≤ 2.0V ∆ ICOMP = ± 10µA VFB = 0.6V VFB = 1.0V VCOMP = 1.25V Duty cycle = 0% VCOMP rising VCOMP falling VCOMP = 1.2V, ISW = 400mA, ROSC = 0 85 150 500 0.8 +3 350 850 70 -70 4.3 0.6 2 0.25 1300 110 -110 µMho µA µA A/V V V V % Minimum Boost Voltage Above Switch(2) Boost Current ISW = 1A ISW = 3A Enable Input Threshold Voltage Enable Output Bias Current VETH IEOL IEOH EN = 50mV below threshold EN = 50mV below threshold 1.1 2.7 10 30 1.3 8 10 15 45 1.5 V mA V µA µA Notes: (1) The required minimum input voltage for a regulated output depends on the output voltage and load condition. (2) Guaranteed by design.  2006 Semtech Corp. 3 www.semtech.com SC4520 POWER MANAGEMENT Pin Configurations Ordering Information Part Number (1)(2) SC4520SETRT P ackag e SO-8 EDP EVALUATION BOARD TOP VIEW BST IN SW GND 1 2 3 4 8 7 6 5 FSET COMP FB EN S C 4520E V B Notes: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead-free product. This product is WEEE and RoHS compliant. (SO-8 EDP) Pin Descriptions Pin # 1 2 3 4 5 Pin Name Pin Function BST IN SW GND EN This pin provides power to the internal NPN switch. The minimum turn on voltage for this switch is 2.7V. Pin IN delivers all power required by control and power circuitry. This pin sees high di/dt during switching. A decoupling capacitor should be attached to this pin as close as possible. Pin SW is the emitter of the internal switch. The external freewheeling diode should be connected as close as possible to this pin. All voltages are measured with respect to this pin. The decoupling capacitor and the freewheeling diode should be connected to GND as short as possible. This is the chip enable input. The regulator is switched on if EN is high, and it is off if EN is low. The regulator is in standby mode when EN is low, and the input supply current is reduced to a few microamperes. Feedback input for adjustable output controllers. Thi s i s the output of the i nternal error ampli fi er and i nput of the peak current comparator. A compensation network is connected to this pin to achieve the specified performance. Frequency setting pin. An external resistor connected from this pin to GND, sets the oscillator frequency. 6 7 8 - FB COMP FS E T THERMAL P ad for heatsi nki ng purposes. C onnect to ground plane usi ng multi ple vi as. Not connected PAD internally.  2006 Semtech Corp. 4 www.semtech.com SC4520 POWER MANAGEMENT Block Diagram + + SLOPE Is + ISEN IN 40m COMP FB BST - + EA PWM S R Q POWER TRANSISTOR SW Is 1V EN REFERENCE UVLO SOFT START HICCUP 0.7V SLOPE SLOPE COMP FB OL GND FSET SYNC OSCILLATOR FREQUENCY CLK Typical Characteristic - OCP Limit SC4519H OCP Limit vs Duty Cycle Rosc = 0 7 6.5 6 5.5 Current Limit (A) 5 4.5 4 3.5 3 2.5 2 0 20 40 60 80 100 Duty Cycle (%) ILIM @-40C ILIM @25C ILIM @125C  2006 Semtech Corp. 5 www.semtech.com SC4520 POWER MANAGEMENT Application Information The SC4520 is a current mode buck converter regulator. SC4520 has an internal fixed-frequency clock. The SC4520 uses two feedback loops that control the duty cycle of the internal power switch. The error amplifier functions like that of the voltage mode converter. The output of the error amplifier works as a switch current reference. This technique effectively removes one of the double poles in the voltage mode system. With this, it is much simpler to compensate a current mode converter to have better performance. The current sense amplifier in the SC4520 monitors the switch current during each cycle. Overcurrent protection (OCP) is triggered when the current limit exceeds the upper limit of 3A, detected by a voltage on COMP greater than about 2V. When an OCP fault is detected, the switch is turned off and the external COMP capacitor is discharged at the rate of dv/dt = 3µA/ Ccomp. Once the COMP voltage has fallen below 250mV, the part enters a normal startup cycle. Ccomp is the total capacitance value attached to COMP. In the case of sustained overcurrent or dead-short, the part will continually cycle through the retry sequence as described above, at a rate dependent on the value of Ccomp. During start up, the voltage on COMP rises roughly at the rate of dv/dt = 120µA/Ccomp. Therefore, the retry time for a sustained overcurrent can be approximately calculated as: Tretry 2V 2V = Ccomp • + Ccomp • 120uA 3uA Oscillator The external resistor connected to the FSET pin sets the PWM frequency from 100kHz to 600kHz. Frequency vs ROSC 700 600 500 Frequency - kHz 400 300 200 100 0 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 Rosc - kOhm UVLO When the EN pin is pulled and held above 1.8V, the voltage on Pin IN determines the operation of the SC4520. As VIN increases during power up, the internal circuit senses VIN and keeps the power transistor off until VIN reaches 4.4V. Load Current The peak current IPEAK in the switch is internally limited. For a specific application, the allowed load current IOMAX will change if the input voltage drifts away from the original design as given for continuous current mode: IOMAX = 3 − VO ⋅ (1 − D) 2 ⋅ L ⋅ fs Figure 1 shows the voltage on COMP during a sustained overcurrent condition. Where: fS = switching frequency, VO= output voltage and D = duty ratio, VO / VIN VIN= input voltage. Figure 1. Voltage on COMP for Startup and OCP Enable Pulling and holding the EN pin below 0.4V activates the shut down mode of the SC4520 which reduces the input supply current to less than 150µA. During the shut down mode, the switch is turned off. The SC4520 is turned on if the EN pin is pulled high.  2006 Semtech Corp. 6 www.semtech.com SC4520 POWER MANAGEMENT Application Information (Cont.) Figure 2 shows the theoretical maximum load current for the specific cases. In a real application, however, the allowed maximum load current also depends on the layout and the air cooling condition. Therefore, the maximum load current may need to be degraded according to the thermal situation of the application. L= VO ⋅ (VI − VO ) VI ⋅ f s ⋅ δ ⋅ IOMAX Maximum Load Current vs Input Voltage L=10uH 2.900 2.880 2.860 2.840 2.820 2.800 2.780 2.760 2.740 2.720 2.700 4 6 8 10 12 14 16 18 Vi (V) Where: fs = switching frequency, δ = ratio of the peak to peak inductor current to the output load current and VO = output voltage. The peak to peak inductor current is: I p − p = δ • I OMAX Iomax (A) Vo=2.5V Vo=3.3V Vo=5V After the required inductor value is selected, the proper selection of the core material is based on the peak inductor current and efficiency specifications. The core must be able to handle the peak inductor current IPEAK without saturation and produce low core loss during the high frequency operation. IPEAK = IOMAX + Ip − p 2 Figure 2. Theoretical maximum load current curves Inductor Selection The factors for selecting the inductor include its cost, efficiency, size and EMI. For a typical SC4520 application, the inductor selection is mainly based on its value, saturation current and DC resistance. Increasing the inductor value will decrease the ripple level of the output voltage while the output transient response will be degraded. Low value inductors offer small size and fast transient responses while they allow large ripple currents, poor efficiencies and require more output capacitance for low output ripple. The inductor should be able to handle the peak current without saturating and its copper resistance in the winding should be as low as possible to minimize its resistive power loss. A good trade-off among its size, loss and cost is to set the inductor ripple current to be within 15% to 30% of the maximum output current. The inductor value can be determined according to its operating point under its continuous mode and the switching frequency as follows: The power loss for the inductor includes its core loss and copper loss. If possible, the winding resistance should be minimized to reduce inductor’s copper loss. The core must be able to handle the peak inductor current IPEAK without saturation and produce low core loss during the high frequency operation. The core loss can be found in the manufacturer’s datasheet. The inductor’s copper loss can be estimated as follows: PCOPPER = I 2LRMS ⋅ R WINDING Where: ILRMS is the RMS current in the inductor. This current can be calculated as follows: ILRMS = IOMAX ⋅ 1 + 12 ⋅δ 12 Output Capacitor Selection Basically there are two major factors to consider in selecting the type and quantity of the output capacitors. The first one is the required ESR (Equivalent Series Resistance) which should be low enough to reduce the output voltage deviation during load changes. The second one is the required capacitance, which should be high enough to hold up the output voltage. Before the SC4520 regulates the inductor current to a new value during a  2006 Semtech Corp. 7 www.semtech.com SC4520 POWER MANAGEMENT Application Information (Cont.) l oad transient, the output capacitor delivers all the additional current needed by the load. The ESR and ESL of the output capacitor, the loop parasitic inductance between the output capacitor and the load combined with inductor ripple current are all major contributors to the output voltage ripple. Surface mount ceramic capacitors are recommended. Input Capacitor Selection The input capacitor selection is based on its ripple current level, required capacitance and voltage rating. This capacitor must be able to provide the ripple current drawn by the converter. For the continuous conduction mode, the RMS value of the input capacitor current ICIN(RMS) can be calculated from: ICIN (RMS) = I OMAX ⋅ VO ⋅ (VI − VO ) V 2I TW = 1 ⋅ D max fs Where: fs = the switching frequency and Dmax = maximum duty ratio, 0.85 for the SC4520. The required minimum capacitance for the boost capacitor will be: C boost = IB ⋅ TW VD Where: IB = the boost current and VD= discharge ripple voltage. With fs = 600kHz, VD = 0.5V and IB = 0.045A, the required minimum capacitance for the boost capacitor is: Cboost = IB 1 0.045 1 ⋅ ⋅ Dmax = ⋅ ⋅ 0.85 = 128nF VD fs 0.5 600k This current gives the capacitor’s power loss through its RCIN(ESR) as follows: PCIN = I2CIN (RMS) • R CIN(ESR) The input ripple voltage mainly depends on the input capacitor’s ESR and its capacitance for a given load, input voltage and output voltage. Assuming that the input current of the converter is constant, the required input capacitance for a given voltage ripple can be calculated by: C IN = IOMAX ⋅ D ⋅ (1 − D) fs ⋅ ( ∆ VI − IOMAX ⋅ R CIN (ESR) ) The internal driver of the switch requires a minimum 2.7V to fully turn on that switch to reduce its conduction loss. If the output voltage is less than 2.7V, the boost capacitor can be connected to either the input side or an independent supply with a decoupling capacitor. But the Pin BST should not see a voltage higher than its maximum rating. Freewheeling Diode Selection This diode conducts during the switch’s off-time. The diode should have enough current capability for full load and short circuit conditions without any thermal concerns. Its maximum repetitive reverse block voltage has to be higher than the input voltage of the SC4520. A low forward conduction drop is also required to increase the overall efficiency. The freewheeling diode should be turned on and off fast with minimum reverse recovery because the SC4520 is designed for high frequency applications. SS23 Schottky rectifier is recommended for certain applications. The average current of the diode, ID_AVG can be calculated by: Where: ∆VI = the given input voltage ripple. Because the input capacitor is exposed to the large surge current, attention is needed for the input capacitor. If tantalum capacitors are used at the input side of the converter, one needs to ensure that the RMS and surge ratings are not exceeded. For generic tantalum capacitors, it is suggested to derate their voltage ratings at a ratio of about two to protect these input capacitors. Boost Capacitor and its Supply Source Selection The boost capacitor selection is based on its discharge ripple voltage, worst case conduction time and boost current. The worst case conduction time T w c an be estimated as follows:  2006 Semtech Corp. 8 ID- AVG = Iomax ⋅ (I − D) www.semtech.com SC4520 POWER MANAGEMENT Application Information (Cont.) Thermal Considerations There are three major power dissipation sources for the SC4520. The internal switch conduction loss, its switching loss due to the high frequency switching actions and the base drive boost circuit loss. These losses can be estimated as: Ptotal = Io ⋅ R on ⋅ D + 10.8 ⋅ 10 −3 ⋅ Io ⋅ VI + 2 Where: RL – Load and C – Output capacitor. The goal of the compensation design is to shape the loop to have a high DC gain, high bandwidth, enough phase margin, and high attenuation for high frequency noises. Figure 3 gives a typical compensation network which offers 2 poles and 1 zero to the power stage: 1 IN EN 8 FSET 4 C4 C5 R3 D2 R2 GND BST BST 2 5 FB 7 COMP 10 ⋅ Io ⋅ D ⋅ (Vboost ) 1000 Where: IO = load current; Ron = on-equivalent resistance of the switch; VBOOST = input voltage or output based on the boost circuit connection. The junction temperature of the SC4520 can be further determined by: TJ = TA + θJA ⋅ Ptotal SC452 SC4520 SW 3 6 L1 Vout R1 C θ JA is the thermal resistance from junction to ambient. Its value is a function of the IC package, the application layout and the air cooling system. The freewheeling diode also contributes a significant portion of the total converter loss. This loss should be minimized to increase the converter efficiency by using Schottky diodes with low forward drop (VF). Pdiode = VF ⋅ Io ⋅ (1 − D) Figure 3. Compensation network provides 2 poles and 1 zero. The compensation network gives the following characteristics: s R2 ωZ GCOMP (s) = ω1 ⋅ ⋅ gm ⋅ s R1 + R2 s ⋅ (1 + ) ωP2 1+ Loop Compensation Design The SC4520 has an internal error amplifier and requires a compensation network to connect between the COMP pin and GND pin as shown in Figure 3. The compensation network includes C4, C5 and R3. R1 and R2 are used to program the output voltage according to: VO = 1.2 • (1 + R1 ) R2 Where: ω1 = ωZ = ωP2 = 1 C4 + C5 1 R3 ⋅ C4 C4 + C5 R3 ⋅ C4 ⋅ C5 Assuming the power stage ESR (equivalent series resistance) zero is an order of magnitude higher than the closed loop bandwidth, which is typically one tenth of the switching frequency, the power stage control to output transfer function with the current loop closed (Ridley model) for the SC4520 will be as follows: G VD (s) = 4.3 ⋅ R L s 1+ 1 RL ⋅ C The loop gain will be given by: T(s) = GCOMP (s) ⋅ GVD (s) = 3.655 ⋅ 10 −3 s 1+ RL R2 1 ωZ ⋅ ⋅ ⋅ C 4 R1 + R2 s (1 + s ) ⋅ (1 + s ) ωP1 ωP2 Where: ωp1 = 1 RL ⋅ C  2006 Semtech Corp. 9 www.semtech.com SC4520 POWER MANAGEMENT Application Information (Cont.) One integrator is added at origin to increase the DC gain. ωZ is used to cancel the power stage pole ωP1 so that the loop gain has –20dB/dec rate when it reaches 0dB line. ωP2 is placed at half switching frequency to reject high frequency switching noises. Figure 4 gives the asymptotic diagrams of the power stage with current loop closed and its loop gain. 2. Start the PCB layout by placing the power components first. Arrange the power circuit to achieve a clean power flow route. Put all power connections on one side of the PCB with wide copper filled areas if possible. 3. The VIN bypass capacitor should be placed next to the VIN and GND pins. 4. The trace connecting the feedback resistors to the output should be short, direct and far away from any noise sources such as switching node and switching components. 5. Minimize the loop including input capacitor, the SC4520 and freewheeling diode D2. This loop passes high di/dt current. Make sure the trace width is wide enough to reduce copper losses in this loop. 6. Maximize the trace width of the loop connecting the inductor, freewheeling diode D 2 a nd the output capacitor. 7. Connect the ground of the feedback divider and the compensation components directly to the GND pin of the SC4520 by using a separate ground trace. 8. Connect Pin 4 to a large copper area to remove the IC heat and increase the power capability of the SC4520. A few feedthrough holes are required to connect this large copper area to a ground plane to further improve the thermal environment of the SC4520. The traces attached to other pins should be as wide as possible for the same purpose. Loop gain T(s) ωp1 Power stage ωC ωZ ωP2 Figure 4. Asymptotic diagrams of power stage with current loop closed and its loop gain. The design guidelines for the SC4520 applications are as following: 1. Set the loop gain crossover corner frequency ωC for given switching corner frequency ωC = 2πfC 2. Place an integrator at the origin to increase DC and low frequency gains. 3. Select ωZ such that it is placed at ωP1 to obtain a -20dB/dec rate to go across the 0dB line. 4. Place a high frequency compensator pole ωP2 (ωP2 = πfs) to get the maximum attenuation of the switching ripple and high frequency noise with the adequate phase lag at ωC. Layout Guidelines: In order to achieve optimal electrical and thermal performance for high frequency converters, special attention must be paid to the PCB layouts. The goal of layout optimization is to identify the high di/dt loops and minimize them. The following guidelines should be used to ensure proper operation of the converters. 1. A ground plane is suggested to minimize switching noises and trace losses and maximize heat transferring.  2006 Semtech Corp. 10 www.semtech.com SC4520 POWER MANAGEMENT Application Information (Cont.) Design Example: 16V to 5V at 2A VIN =16V 2 C3 10u 4.75k R4 8 Rosc=0 5 IN EN C1 0.22u 1 BST 3 D3 L1 Vo=5V R1 52.3k C2 10u SW FB 8.2uH 6 7 C4 3.3n C5 180p SC4520 GND 4 FSET COMP R2 10k R3 3.4k D2 Bill of Materials Item 1 2 3 4 5 6 7 8 9 10 11 12 13 Qty 1 2 1 1 1 1 1 1 1 1 1 1 1 C1 C 2, C 3 C4 C5 D1 D2 L1 R1 R2 R3 R4 Rosc U1 Reference Value 0.22uF, 25V, 0805, X7R 10u, 1210, X5R, 25V 3.3n, 0805, X7R, 25V 180pF 1N4148WS, SOD-323 S S 33 8.2uH 52.3K 10k 3.4k 4.75k 0 S C 4520 Semtech Fairchild P/N: SS33 COOPER P/N:DR125-8R2 Part No./Manufacturer Vishay Panasonic Vishay Unless specified, all resistors have 1% precision with 0603 package. Resistors are +/-1% and all capacitors are +/-20%  2006 Semtech Corp. 11 www.semtech.com SC4520 POWER MANAGEMENT Outline Drawing - SOIC-8L EDP A N 2X E/2 E1 E 1 ccc C 2X N/2 TIPS 2 e/2 B D aaa C SEATING PLANE A2 A C bxN bbb F EXPOSED PAD H H GAGE PLANE 0.25 L (L1) h A1 C A-B D e D DIM A A1 A2 b c D E1 E e F H h L L1 N 01 aaa bbb ccc DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX .053 .069 .000 .005 .049 .065 .020 .012 .007 .010 .189 .193 .197 .150 .154 .157 .236 BSC .050 BSC .116 .120 .130 .085 .095 .099 .010 .020 .016 .028 .041 (.041) 8 0° 8° .004 .010 .008 1.35 1.75 0.13 0.00 1.65 1.25 0.31 0.51 0.25 0.17 4.80 4.90 5.00 3.80 3.90 4.00 6.00 BSC 1.27 BSC 2.95 3.05 3.30 2.15 2.41 2.51 0.25 0.50 0.40 0.72 1.04 (1.05) 8 0° 8° 0.10 0.25 0.20 h c 01 SEE DETAIL SIDE VIEW NOTES: 1. A DETAIL A CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 4. REFERENCE JEDEC STD MS-012, VARIATION BA. Land Pattern - SOIC-8L EDP E D SOLDER MASK DIM (C) F G Z C D E F G P X Y Z DIMENSIONS INCHES MILLIMETERS (.205) .134 .201 .101 .118 .050 .024 .087 .291 (5.20) 3.40 5.10 2.56 3.00 1.27 0.60 2.20 7.40 Y THERMAL VIA Ø 0.36mm NOTES: P X 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 2. REFERENCE IPC-SM-782A, RLP NO. 300A. 3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD SHALL BE CONNECTED TO A SYSTEM GROUND PLANE. FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR FUNCTIONAL PERFORMANCE OF THE DEVICE. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804  2006 Semtech Corp. 12 www.semtech.com