SEMTECH SC2618

SC2618 Miniature PWM Controller for Buck Regulator POWER MANAGEMENT Description Features The SC2618 is a hysteretic mode PWM controller de- ‹ Operating Frequency up to 500kHz signed for high efficiency, low cost “Point of Load” appli- ‹ Input supply from 5V to 12V cations. ‹ 0.5A gate drive capability ‹ Internal soft start The hysteretic control scheme allows the use of ceramic ‹ Internal, trimmed bandgap reference (±1%) output capacitors without the need for compensation components and difficult calculations. ‹ Hiccup mode short circuit protection ‹ Input supply sequence protection An internal soft start prevents output voltage from start ‹ 6 lead SOT23 package up overshoot. A “Hiccup” short circuit protection provides protection against output short circuits as well as ad- Applications verse input supply sequencing. ‹ Graphics IC Power supplies ‹ Embedded, low cost, high efficiency converters Typical Application Circuit VIN BOOST Q1 VCC VO 5 6 Q2 DH BST 3 1 DL VCC 2 4 GND FB SC2618 GND GND 1 Revision: September 19, 2005 www.semtech.com SC2618 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 Sm ymbolMs aximu Unit IV nput Supply Voltage 1V 5 CC BV oost Supply Voltage 3V 0 BST FV B Voltage 8V FB DV L to GND -1 to V+V 0.3 DL CC DV H to GND -1 to V+V 0.3 DH BST OT perating Ambient Temperature Range -C 40 to +85 ° A OT perating Junction Temperature Range -C 40 to 125 ° J ST torage Temperature Range -C 65 to 150 ° STG LT ead Temperature (Soldering) 10s 3C 00 ° LEAD (1) Thermal Resistance Junction to Ambient 96 °C/W θ JA Thermal Resistance Junction to Case θ 6W 2 °C/ JC ED SD Rating (Human Body Model) E2 S kV Note: (1) 1 square inch of FR4, double sided, 1oz, minimum copper weight Electrical Characteristics Unless specified: V = V = 5V, V = 12V, V = V ; T = 25°C. Values in bold are over full operating temperature range. CC IN BST FB O A Pl arameter Ss ymbo Cn ondition Mp i Tx y Ms a Unit VV CC Supply Voltage CC 44 .75 1V VI CC Quiescent Current QVCC V = 5.0V, V = 0V, V=5 100mV 1A0 m CC REF FB VV CC Under Voltage Lockout U VCC 42 .0 45 . 4V . BV oost Supply Voltage BST 2V 9 BI oost Quiescent Current V=3 1.200V mA QBST FB Ss oft Start TimeT0 s 1s 0 u Output Voltage 10 .23713 .25 1.26 VO IO = 20mA; V = V V FB O 15 .225 1.27 LI oad Regulation O=1 0.2A to 4A % Line Regulation ±% 0.5 SV hort Circuit Trip Voltage SCT 1V .05 Mt inimum ON/OFF Time DEL 1s u FI B bias current FB VFB=1 1.25V -A u P, eak DH Sink/Source CurrentBV ST - DH = 4.5V DH - GND = 3.5 0.5 A DH - GND = 1.5V 50 mA P, eak DL Sink/Source CurrentBV ST - DL = 4.5V DL - GND = 3.5 0.5 A DL - GND = 1.5V 50 mA DC H, DL NonoverlappingTime =0 1000pF; Measured at DH/DL = 2V 205s n LOAD www.semtech.com  2004 Semtech Corp. 2 SC2618 POWER MANAGEMENT Pin Configuration Ordering Information Pe art Numbers Packag TOP VIEW (1)(2) SC2618SKTRT SOT23-6 DL 1 6 DH Notes: GND 2 5 BST (1) Only available in tape and reel packaging. A reel contains 3000 devices. VCC 3 4 FB (2) SC2618SKTRT is the lead free version. This product is fully WEEE and RoHS compliant. (SOT-23 6L) Pin Descriptions Pe in #Pn in Nam Pin Functio 1LD. Low side FET drive output 2D GN Analog and Power Ground, connect directly to ground plane, see layout guidelines. 3CV. C Chip Supply Input Voltage and Low Side FET drive supply 4BF. Feedback input 5TB. S Supply voltage for high gate driver 6HD. High Side FET drive output Block Diagram BST UVLO VCC SSOVER LEVEL SHIFT AND HIGH SIDE DRIVE RESTART DH LATCH SSRESET Q S Q R FAULT LATCH SHOOT-THRU SSBEGIN S Q CONTROL SOFT START VREF R Q VCC 200mV DRIVE SYNCHRONOUS FB LOGIC DL MOSFET DRIVE GND  2004 Semtech Corp. 3 www.semtech.com Toff=1us SC2618 POWER MANAGEMENT Theory of Operation The SC2618 is a hysteretic mode PWM controller. It uses soft turn on. If, however, one or more supply voltage rails a comparator to generate PWM wave with minimum on are late or absent, the output voltage will not rise and time and off time control. As shown in the block diagram, the part will behave as if there was a short circuit at the the output voltage is fed back to the comparator nega- output, that is, it will go into hiccup mode. It will remain in tive input and compared to a setting voltage. If the out- this mode until all the necessary voltage rails are present, put voltage is below its set point, the top gate drive will at which time normal operation will start. turn on and remain on until the minimum on time has If the supply voltage Vcc falls below UVLO threshold dur- expired AND the output voltage has risen above the set ing normal operation, the soft start capacitor begins to point. Similarly, if the output voltage is above its set point, discharge. When the voltage reaches the setting level, top gate drive will be turned off and bottom gate will turn the PWM controller control the switching regulator out- on and stay on until the minimum off time has expired put to ramp down slowly for a soft turn off. AND the output is below its set point. Because of this minimum time control scheme, the comparator hyster- Hiccup Mode Shor Hiccup Mode Shor Hiccup Mode Shor Hiccup Mode Shor Hiccup Mode Short Cir t Cir t Cir t Cir t Circuit Pr cuit Pr cuit Pr cuit Pr cuit Pro o o o ot t t t tection ection ection ection ection esis is not required and is internally set to zero. Short circuit protection is implemented by comparing the feedback node with the soft start and reference volt- Switching F Switching F Switching F Switching F Switching Freq req req req requency vs. Duty Cy uency vs. Duty Cy uency vs. Duty Cy uency vs. Duty Cy uency vs. Duty Cycle cle cle cle cle age. If the FB voltage falls more than about 200mV be- This control scheme will force a buck converter to oper- low it’s correct voltage the short circuit latch is set which ate either at minimum on time, or minimum off time, or immediately disables the top drive. If the short circuit both. The SC2618 has minimum time of 1us. Its switch- occurred during soft start, the soft start capacitor con- ing frequency, peaking at 500kHz, can be found in Fig. 1 tinues to charge to it’s full voltage before discharging as long as the voltage ratio of the buck converter has towards zero. If the short occurred during steady state been decided. operation, the internal soft start capacitor begins to dis- charge immediately, at the same rate as it charged, to- 500 wards zero. Once the soft start capacitor reaches its minimum voltage, the latches are cleared and a normal 400 soft start is attempted, if the short circuit condition has not been cleared, the fault latch will again be set once 300 the soft start voltage reaches about 200mV and the cycle 200 will repeat until the short is cleared. It is also possible, with large output capacitor values, 100 that the output voltage will be unable to rise fast enough to accurately follow the soft start voltage and a short 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 circuit trip may result. During soft start the bottom gate Duty Cycle drive is disabled to prevent discharging the output ca- pacitor and so the output capacitor will retain the volt- Fig. 1. Switching frequency vs. duty cycle age achieved before the trip and will charge to a higher voltage during the next hiccup cycle. In this way the out- put voltage will achieve the set point value but may take UVL UVL UVL UVL UVLO, Star O, Star O, Star O, Star O, Start up and Shut do t up and Shut do t up and Shut do t up and Shut do t up and Shut down wn wn wn wn several hiccup cycles to do so. To initiate the SC2618, supply voltages are applied to Vcc and BST pins. The top gate (DH) and bottom gate (DL) are held low until Vcc exceed UVLO threshold, typi- cally 4.5V. At that point, the internal soft start capacitor begins to charge, the top gate remains low, and the bot- tom gate is pulled high to turn on the bottom FET. When the voltage at soft start cap reaches a setting level, the top and bottom gates of PWM controller begin to switch. The switching regulator output is slowly ramping up for a www.semtech.com  2004 Semtech Corp. 4 Ton=1us F (kHz) SC2618 POWER MANAGEMENT Component Selection General design guideline of switching power supplies can Induct Induct Inductor and MOSFET or and MOSFET or and MOSFETs s s Induct Inductor and MOSFET or and MOSFETs s be applied to the component selection for SC2618. The selection of inductor and MOSFETs should meet ther- mal requirement because they are power loss dominant components. Pick an inductor with as high inductance Output Filter Components Output Filter Components Output Filter Components Output Filter Components Output Filter Components as possible if without adding extra cost and size. Higher The purpose of soft start is to control inductor current inductance, lower ripple current, smaller core loss and during start up, this in turn controls the amount of ex- higher efficiency. However, too high inductance slows cess charge the output capacitor will receive and there- down output transient response. It is recommended to fore the output voltage overshoot that will occur at the choose the inductance that gives the inductor ripple cur- end of soft start. rent to be approximate 20% of maximum load current. VIN So choose inductor value from: Q1 5 V O L = ⋅V ⋅(1− ) O I ⋅ f V L VO O osc IN Q2 The MOSFETs are selected from their Rdson, gate charge, Co and package. The SC2618 provides 0.5A gate drive cur- GND GND rent. To drive a 25nC gate charge MOSFET gives 25nC/ 0.5A=50ns switching time. The switching time ts contrib- 2618-02 utes to the top MOSFET switching loss: Fig. 2. Output filter components P = I ⋅V ⋅ t ⋅ f S O IN S OSC V P V O There is no switching loss for the bottom MOSFET be- cause of its zero voltage switching. The conduction losses V REF v C of the top and bottom MOSFETs are given by: 0 2 P = I ⋅ R ⋅ D C _TOP O dson I L 2 i P = I ⋅ R ⋅(1− D) L C _ BOT O dson 0 If the requirement of total power losses for each MOSFET t t is given, the above equations can be used to calculate SS P t the values of Rdson and gate charge can be calculated Fig. 3. Soft start, inductor current and overshoot using above equations, then the devices can be deter- mined accordingly. The solution should ensure the MOSFET is within its maximum junction temperature at Normally, controllers with an external soft start capaci- highest ambient temperature. tor, have soft start set so long that overshoot is unde- tectable. Since the soft start for the SC2618 is internal, Output Capacitor(s) Output Capacitor(s) Output Capacitor(s) Output Capacitor(s) Output Capacitor(s) it is necessarily limited and the maximum values of L and The output capacitors should be selected to meet both Co should meet the constraints of: output ripple and transient response criteria. The output 2 capacitor ESR causes output ripple VRIPPLE during the t ⋅ X SS S L ⋅C = O inductor ripple current flowing in. To meet output ripple 50 criteria, the ESR value should be: where tss is soft start time, 100us typically, and Xs is the percentage overshoot allowable. L⋅ f ⋅V OSC RIPPLE R < ESR V O V ⋅(1− ) O V IN  2004 Semtech Corp. 5 www.semtech.com SC2618 POWER MANAGEMENT Component Selection (Cont.) The output capacitor ESR also causes output voltage tran- Divider Ratio Divider Ratio Divider Ratio Divider Ratio Divider Ratio sient VT during a transient load current IT flowing in. To The top resistor of the voltage divider can be chosen meet output transient criteria, the ESR value should be: from 5k to 15k. Then the bottom resistor is found from V 1.25V T R < R = ⋅ R ESR bot top I V −1.25V T O To meet both criteria, the smaller one of above two ESRs where 1.25V is the internal reference voltage of the is required. SC2618. The output capacitor value also contributes to load tran- sient response. Based on a worst case where the induc- tor energy 100% dumps to the output capacitor during the load transient, the capacitance then can be calcu- lated by: 2 I T C > L⋅ 2 V T Input Capacitor Input Capacitor Input Capacitor Input Capacitor Input Capacitor The input capacitor should be chosen to handle the RMS ripple current of a synchronous buck converter. This value is given by: 2 2 I = (1− D)⋅ I + D ⋅(I − I ) RMS IN o IN where Io is the load current, IIN is the input average cur- rent, and D is the duty cycle. Choosing low ESR input capacitors will help maximize ripple rating for a given size. Bootstrap Circuit Bootstrap Circuit Bootstrap Circuit Bootstrap Circuit Bootstrap Circuit The SC2618 uses an external bootstrap circuit to pro- vide a voltage at BST pin for the top MOSFET drive. This voltage is held up by a bootstrap capacitor. Typically, it is recommended to use a 1uF ceramic capacitor with 25V rating and a commonly available diode IN4148 for the bootstrap circuit. Filter for Vcc Supply Power Filter for Vcc Supply Power Filter for Vcc Supply Power Filter for Vcc Supply Power Filter for Vcc Supply Power For the pin of Vcc, it is recommended to use a 1uF/25V ceramic capacitor for decoupling. In addition, place a small resistor (10 ohm) in between Vcc pin and the sup- ply power for noise reduction. www.semtech.com  2004 Semtech Corp. 6 SC2618 POWER MANAGEMENT Layout Guidelines Careful attention to layout requirements are necessary grounds for the rest of the system and c) minimize source for successful implementation of the SC2618 controller. ringing, resulting in more reliable gate switching signals. High currents switching at up to 500kHz are present in Cin should consist of the bulk input capacitors and must the application and their effect on ground plane voltage be supplemented with one or more ceramic decoupling differentials must be understood and minimized. capacitors as close as possible to the FETs 1). The high power parts of the circuit should be laid out 3). The connection between the junction of Q1, Q2 and first. A ground plane should be used, the number and the output inductor should be a wide trace or copper position of ground plane interruptions should be such as region. It should be as short as practical. Since this con- to not unnecessarily compromise ground plane integrity. nection has fast voltage transitions, keeping this con- Isolated or semi-isolated areas of the ground plane may nection short will minimize EMI. The connection between be deliberately introduced to constrain ground currents the output inductor and the output capacitors should be to particular areas, for example the input capacitor and a wide trace or copper area, there are no fast voltage or bottom FET ground. current transitions in this connection and length is not so important, however adding unnecessary impedance 2). The loop formed by the Input Capacitor(s) (Cin), the will reduce efficiency. Top FET (Q1) and the Bottom FET (Q2) must be kept as small as possible. 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 a) reduce EMI, b) lower ground injection currents, resulting in electrically “cleaner” VPWR IN VCC IN VBST IN 10uF 10 U1 Q1 5 6 BST DH 3 1 Cin Vout VCC DL 2 4 L GND FB Cout Q2 SC2618 0.1uF GND  2004 Semtech Corp. 7 www.semtech.com SC2618 POWER MANAGEMENT Layout Guidelines (Cont.) Currents in Power Section Vin + Vout + www.semtech.com  2004 Semtech Corp. 8 SC2618 POWER MANAGEMENT Outline Drawing - SOT23-6 DIMENSIONS INCHES MILLIMETERS A DIM MIN NOM MAX MIN NOM MAX A - .057 0.90 - 1.45 e1 .035 D A1 .000 - .006 0.00 - 0.15 A2 .035 .045 .051 .90 1.15 1.30 b .010 - .020 0.25 - 0.50 N 2X E/2 c .003 - .009 0.08 - 0.22 EI E D .110 .114 .118 2.80 2.90 3.00 E1 .060 .063 .069 1.50 1.60 1.75 12 E .110 BSC 2.80 BSC ccc C e .037 BSC 0.95 BSC e1 .075 BSC 1.90 BSC 2X N/2 TIPS L .012 .018 .024 0.30 0.45 0.60 e (.024) (0.60) L1 B N 6 6 01 - - 0° 10° 0° 10° aaa .004 0.10 bbb .008 0.20 D ccc .008 0.20 aaa C A2 A SEATING H H PLANE A1 C c bxN GAGE PLANE bbb C A-B D 0.25 L 01 (L1) DETAIL A SEE DETAIL A SIDE VIEW NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS A -A- ND T -B- O BE DETERMINED AT DATUM PLANE -H- 3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. Minimum Land Pattern - SOT23-6 X DI DIMENSI MENSIO ONS NS DI DIM M I INCHES NCHES MIL MILL LI IMETERS METERS C C (.098) (2.50) G G .055 1.40 (C) G Z P P .037 0.95 X X .024 0.60 Y Y Y .043 1.10 Z Z .141 3.60 P NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804  2004 Semtech Corp. 9 www.semtech.com