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| LTC1430 High Power Step-Down Switching Regulator Controller FEATURES s s s s DESCRIPTIO s s s s s s s s High Power 5V to 3.xV Switching Controller: Can Exceed 10A Output All N-Channel External MOSFETs Constant Frequency Operation--Small Inductor Excellent Output Regulation: 1% Over Line, Load and Temperature Variations High Efficiency: Over 95% Possible Fixed Frequency Operation No Low Value Sense Resistor Needed Outputs Can Drive External FETs with Up to 10,000pF Gate Capacitance Quiescent Current: 350A Typ, 1A in Shutdown Fast Transient Response Adjustable or Fixed 3.3V Output Available in 8- and 16-Lead PDIP and SO Packages The LTC (R)1430 is a high power, high efficiency switching regulator controller optimized for 5V to 3.xV applications. It includes a precision internal reference and an internal feedback system that can provide output regulation of 1% over temperature, load current and line voltage shifts. The LTC1430 uses a synchronous switching architecture with two N-channel output devices, eliminating the need for a high power, high cost P-channel device. Additionally, it senses output current across the drain-source resistance of the upper N-channel FET, providing an adjustable current limit without an external low value sense resistor. The LTC1430 includes a fixed frequency PWM oscillator for low output ripple under virtually all operating conditions. The 200kHz free-running clock frequency can be externally adjusted from 100kHz to above 500kHz. The LTC1430 features low 350A quiescent current, allowing greater than 90% efficiency operation in converter designs from 1A to greater than 50A output current. Shutdown mode drops the LTC1430 supply current to 1A. , LTC and LT are registered trademarks of Linear Technology Corporation. Pentium is a registered trademark of Intel Corporation. APPLICATIO S s s s s Power Supply for P6 and Pentium(R) Microprocessors High Power 5V to 3.xV Regulators Local Regulation for Dual Voltage Logic Boards Low Voltage, High Current Battery Regulation TYPICAL APPLICATIO VIN 5V Typical 5V to 3.3V, 10A Application 100 + 100 1F PVCC2 1N4148 PVCC1 G1 IMAX LTC1430 IFB FREQSET SHDN COMP G2 PGND GND SENSE+ SENSE - FB NC 0.1F 1k 16k 0.1F + CIN 220F x4 EFFICIENCY (%) + 4.7F 0.1F 0.01F NC SHUTDOWN C1 220pF VCC SS M1A, M1B 2 IN PARALLEL L1, 2.8H + M2 COUT 330F x6 3.3V 10A RC 7.5k CC 4700pF LTC1430 * TA01 CIN: AVX-TPSE227M010R0100 COUT: AVX-TPSE337M006R0100 L1: ETQP6F1R6SFA M1A, M1B, M2: MOTOROLA MTD20N03HL U Efficiency TA = 25C VIN = 5V VOUT = 3.3V 90 80 70 60 50 40 0.1 1 LOAD CURRENT (A) 10 LTC1430 * TA02 U U 1 LTC1430 ABSOLUTE AXI U RATI GS Supply Voltage VCC ....................................................................... 9V PVCC1, 2 .............................................................. 13V Input Voltage IFB ......................................................... - 0.3V to 18V All Other Inputs ...................... - 0.3V to (VCC + 0.3V) PACKAGE/ORDER I FOR ATIO TOP VIEW G1 1 PVCC1 2 GND 3 FB 4 N8 PACKAGE 8-LEAD PDIP 8 7 6 5 G2 VCC /PVCC2 COMP SHDN ORDER PART NUMBER LTC1430CN8 LTC1430CS8 S8 PACKAGE 8-LEAD PLASTIC SO S8 PART MARKING 1430 TJMAX = 150C, JA = 100C/W (N8) TJMAX = 150C, JA = 150C/W (S8) Consult factory for Industrial and Military grade parts. The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V unless otherwise noted. (Note 2) SYMBOL VCC PVCC VOUT VFB VOUT PARAMETER Supply Voltage PVCC1, PVCC2 Output Voltage Feedback Voltage Output Load Regulation Output Line Regulation IVCC IPVCC fOSC Supply Current (VCC Only) Supply Current (PVCC) Internal Oscillator Frequency Figure 1 Figure 1, SENSE + and SENSE - Floating (LTC1430C) Figure 1, SENSE + and SENSE - Floating (LTC1430I) Figure 1, IOUT = 0A to 10A (Note 3) (LTC1430C) Figure 1, IOUT = 0A to 10A (Note 3) (LTC1430I) CONDITIONS q q q q q q ELECTRICAL CHARACTERISTICS Figure 1, VCC = 4.75V to 5.25V (Note 3) (LTC1430C) q Figure 1, VCC = 4.75V to 5.25V (Note 3) (LTC1430I) Figure 2, VSHDN = VCC VSHDN = 0V Figure 2, PVCC = 5V, VSHDN = VCC (Note 4) VSHDN = 0V FREQSET Floating (LTC1430C) FREQSET Floating (LTC1430I) q q q 2 U U W WW U W (Note 1) Operating Temperature Range LTC1430C .............................................. 0C to 70C LTC1430I ........................................... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C TOP VIEW G1 PVCC1 PGND GND SENSE - FB SENSE + 1 2 3 4 5 6 7 8 16 G2 15 PVCC2 14 VCC 13 IFB 12 IMAX 11 FREQSET 10 COMP 9 SS ORDER PART NUMBER LTC1430CN LTC1430CS LTC1430IS SHDN N PACKAGE 16-LEAD PDIP S PACKAGE 16-LEAD PLASTIC SO TJMAX = 150C, JA = 70C/W (N) TJMAX = 150C, JA = 110C/W (S) MIN 4 TYP MAX 8 13 UNITS V V V V V mV mV mV mV A A mA A 3.30 1.25 1.23 1.265 1.265 5 5 1 1 350 1 1.5 0.1 140 130 200 200 260 300 1.28 1.29 20 5 700 10 kHz kHz LTC1430 The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V unless otherwise noted. (Note 2) SYMBOL VIH VIL IIN AV gmV gmI IMAX ISS tr, ts tNOV DCMAX PARAMETER SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current Error Amplifier Open-Loop DC Gain Error Amplifier Transconductance ILIM Amplifier Transconductance IMAX Sink Current Soft-Start Source Current Driver Rise/Fall Time Driver Non-Overlap Time Maximum Duty Cycle (LTC1430I) (LTC1430C) (LTC1430I) (Note 5) VI(MAX) = VCC (LTC1430C) VI(MAX) = VCC (LTC1430I) VSS = 0 (LTC1430C) VSS = 0 (LTC1430I) Figure 3, PVCC1 = PVCC2 = 5V Figure 3, PVCC1 = PVCC2 = 5V VCOMP = VCC (LTC1430C) VCOMP = VCC, VFB = 0 (LTC1430I) VCOMP = VCC, VFB = 1.265V (LTC1430I) Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. Note 3: This parameter is guaranteed by correlation and is not tested directly. q q q q q q q ELECTRICAL CHARACTERISTICS CONDITIONS q q q q q MIN 2.4 TYP MAX 0.8 UNITS V V A dB Mho Mho Mho A A A A ns ns % % % 0.1 40 300 8 8 -8 -8 25 48 650 650 1300 12 12 -12 -12 80 130 90 83 90 88 1 1200 16 17 -16 -17 250 250 96 96 Note 4: Supply current in normal operation is dominated by the current needed to charge and discharge the external FET gates. This will vary with the LTC1430 operating frequency, operating voltage and the external FETs used. Note 5: The ILIM amplifier can sink but cannot source current. Under normal (not current limited) operation, the ILIM output current will be zero. 3 LTC1430 PI FU CTIO S G1 (Pin 1/Pin 1): Driver Output 1. Connect this pin to the gate of the upper N-channel MOSFET, M1. This output will swing from PVCC1 to PGND. It will always be low when G2 is high. PVCC1 (Pin 2/Pin 2): Power VCC for Driver 1. This is the power supply input for G1. G1 will swing from PGND to PVCC1. PVCC1 must be connected to a potential of at least PVCC + VGS(ON)(M1). This potential can be generated using an external supply or a simple charge pump connected to the switching node between the upper MOSFET and the lower MOSFET; see Applications Information for details. PGND (Pin 3/Pin 3): Power Ground. Both drivers return to this pin. It should be connected to a low impedance ground in close proximity to the source of M2. 8-lead parts have PGND and GND tied together at pin 3. GND (Pin 4/Pin 3): Signal Ground. All low power internal circuitry returns to this pin. To minimize regulation errors due to ground currents, GND should be connected to PGND right at the LTC1430. 8-lead parts have PGND and GND tied together internally at pin 3. SENSE -, FB, SENSE + (Pins 5, 6, 7/Pin 4): These three pins connect to the internal resistor divider and to the internal feedback node. To use the internal divider to set the output voltage to 3.3V, connect SENSE + to the positive terminal of the output capacitor and SENSE - to the negative terminal. FB should be left floating in applications that use the internal divider. To use an external resistor divider to set the output voltage, float SENSE + and SENSE - and connect the external resistor divider to FB. SHDN (Pin 8/Pin 5): Shutdown. A TTL compatible low level at SHDN for longer than 50s puts the LTC1430 into shutdown mode. In shutdown, G1 and G2 go low, all internal circuits are disabled and the quiescent current drops to 10A max. A TTL compatible high level at SHDN allows the part to operate normally. SS (Pin 9/NA): Soft-Start. The SS pin allows an external capacitor to be connected to implement a soft-start function. An external capacitor from SS to ground controls the start-up time and also compensates the current limit loop, allowing the LTC1430 to enter and exit current limit cleanly. See Applications Information for more details. 4 U U U (16-Lead Package/8-Lead Package) COMP (Pin 10/Pin 6): External Compensation. The COMP pin is connected directly to the output of the error amplifier and the input of the PWM. An RC network is used at this node to compensate the feedback loop to provide optimum transient response. See Applications Information for compensation details. FREQSET (Pin 11/NA): Frequency Set. This pin is used to set the free running frequency of the internal oscillator. With the pin floating, the oscillator runs at about 200kHz. A resistor from FREQSET to ground will speed up the oscillator; a resistor to VCC will slow it down. See Applications Information for resistor selection details. IMAX (Pin 12/NA): Current Limit Set. IMAX sets the threshold for the internal current limit comparator. If IFB drops below IMAX with G1 on, the LTC1430 will go into current limit. IMAX has a 12A pull-down to GND. It can be adjusted with an external resistor to PVCC or an external voltage source. IFB (Pin 13/NA): Current Limit Sense. Connect to the switched node at the source of M1 and the drain of M2 through a 1k resistor. The 1k resistor is required to prevent voltage transients from damaging IFB. This pin can be taken up to 18V above GND without damage. VCC (Pin 14/Pin 7): Power Supply. All low power internal circuits draw their supply from this pin. Connect to a clean power supply, separate from the main PVCC supply at the drain of M1. This pin requires a 4.7F bypass capacitor. 8-lead parts have VCC and PVCC2 tied together at pin 7 and require a 10F bypass to GND. PVCC2 (Pin 15/Pin 7): Power VCC for Driver 2. This is the power supply input for G2. G2 will swing from GND to PVCC2. PVCC2 is usually connected to the main high power supply. The 8-lead parts have VCC and PVCC2 tied together at pin 7 and require a 10F bypass to GND. G2 (Pin 16/Pin 8): Driver Output 2. Connect this pin to the gate of the lower N-channel MOSFET, M2. This output will swing from PVCC2 to PGND. It will always be low when G1 is high. LTC1430 BLOCK DIAGRA SHDN FREQSET PVCC1 PWM COMP VCC 12A SS ILIM FB MIN MAX G1 PVCC2 G2 PGND IMAX 12A 40mV + 1.26V LTC1430 * BD TEST CIRCUITS PVCC = 5V + 100 1F PVCC2 1N4148 PVCC1 G1 IMAX LTC1430 IFB FREQSET SHDN COMP G2 PGND GND SENSE+ SENSE - FB NC 0.1F + 4.7F 0.1F 0.01F NC SHUTDOWN C1 220pF VCC SS RC 7.5k CC 4700pF M1A, M1B, M2: MOTOROLA MTD20N03HL CIN: AVX-TPSE227M010R0100 COUT: AVX-TPSE337M006R0100 Figure 1 5V VSHDN VCC PVCC 10F 0.1F SHDN VCC PVCC2 PVCC1 NC NC NC NC IMAX FREQSET COMP SS GND LTC1430 IFB G1 G2 FB NC NC NC PGND SENSE - SENSE+ LTC1430 * TC02 Figure 2 + W DELAY 50s INTERNAL SHUTDOWN - + + - IFB + 40mV FB 20.1k 12.4k SENSE + SENSE - + CIN 220F x4 FB MEASUREMENT LTC1430 SENSE 3.3V COUT 330F x6 SENSE - + M1A, M1B 2 IN PARALLEL 2.7H/15A VOUT NC 1.61k FB 1k M2 + NC LTC1430 * F01 VCC PVCC1 PVCC2 G1 10,000pF LTC1430 G2 PGND G2 RISE/FALL 10,000pF LTC1430 * TC03 G1 RISE/FALL GND Figure 3 5 LTC1430 APPLICATIO S I FOR ATIO OVERVIEW The LTC1430 is a voltage feedback PWM switching regulator controller (see Block Diagram) designed for use in high power, low voltage step-down (buck) converters. It includes an onboard PWM generator, a precision reference trimmed to 0.5%, two high power MOSFET gate drivers and all necessary feedback and control circuitry to form a complete switching regulator circuit. The PWM loop nominally runs at 200kHz. The 16-lead versions of the LTC1430 include a current limit sensing circuit that uses the upper external power MOSFET as a current sensing element, eliminating the need for an external sense resistor. Also included in the 16-lead version is an internal softstart feature that requires only a single external capacitor to operate. In addition, 16-lead parts feature an adjustable oscillator which can run at frequencies from 50kHz to beyond 500kHz, allowing added flexibility in external component selection. The 8-lead versions do not include current limit, internal soft-start or frequency adjustability. THEORY OF OPERATION Primary Feedback Loop The LTC1430 senses the output voltage of the circuit at the output capacitor with the SENSE+ and SENSE- pins and feeds this voltage back to the internal transconductance amplifier FB. FB compares the resistor-divided output voltage to the internal 1.26V reference and outputs an error signal to the PWM comparator. This is then compared to a fixed frequency sawtooth waveform generated by the internal oscillator to generate a pulse width modulated signal. This PWM signal is fed back to the external MOSFETs through G1 and G2, closing the loop. Loop compensation is achieved with an external compensation network at COMP, the output node of the FB transconductance amplifier. MIN, MAX Feedback Loops Two additional comparators in the feedback loop provide high speed fault correction in situations where the FB amplifier may not respond quickly enough. MIN compares the feedback signal to a voltage 40mV (3%) below the 6 U internal reference. At this point, the MIN comparator overrides the FB amplifier and forces the loop to full duty cycle, set by the internal oscillator at about 90%. Similarly, the MAX comparator monitors the output voltage at 3% above the internal reference and forces the output to 0% duty cycle when tripped. These two comparators prevent extreme output perturbations with fast output transients, while allowing the main feedback loop to be optimally compensated for stability. Current Limit Loop The 16-lead LTC1430 devices include yet another feedback loop to control operation in current limit. The current limit loop is disabled in 8-lead devices. The ILIM amplifier monitors the voltage drop across external MOSFET M1 with the IFB pin during the portion of the cycle when G1 is high. It compares this voltage to the voltage at the IMAX pin. As the peak current rises, the drop across M1 due to its RDS(ON) increases. When IFB drops below IMAX, indicating that M1's drain current has exceeded the maximum level, ILIM starts to pull current out of the external soft-start capacitor, cutting the duty cycle and controlling the output current level. At the same time, the ILIM comparator generates a signal to disable the MIN comparator to prevent it from conflicting with the current limit circuit. If the internal feedback node drops below about 0.8V, indicating a severe output overload, the circuitry will force the internal oscillator to slow down by a factor of as much as 100. If desired, the turn on time of the current limit loop can be controlled by adjusting the size of the soft-start capacitor, allowing the LTC1430 to withstand short overcurrent conditions without limiting. By using the RDS(ON) of M1 to measure the output current, the current limit circuit eliminates the sense resistor that would otherwise be required and minimizes the number of components in the external high current path. Because power MOSFET RDS(ON) is not tightly controlled and varies with temperature, the LTC1430 current limit is not designed to be accurate; it is meant to prevent damage to the power supply circuitry during fault conditions. The actual current level where the limiting circuit begins to take effect may vary from unit to unit, depending on the power MOSFETs used. See Soft-Start and Current Limit for more details on current limit operation. W U U LTC1430 APPLICATIO S I FOR ATIO MOSFET Gate Drive Gate drive for the top N-channel MOSFET M1 is supplied from PVCC1. This supply must be above PVCC ( the main power supply input) by at least one power MOSFET VGS(ON) for efficient operation. An internal level shifter allows PVCC1 to operate at voltages above VCC and PVCC, up to 13V maximum. This higher voltage can be supplied with a separate supply, or it can be generated using a simple charge pump as shown in Figure 4. When using a separate PVCC1 supply, the PVCC input may exhibit a large inrush current if PVCC1 is present during power up. The 90% maximum duty cycle ensures that the charge pump will always provide sufficient gate drive to M1. Gate drive for the bottom MOSFET M2 is provided through PVCC2 for 16-lead devices or VCC/PVCC2 for 8-lead devices. PVCC2 can usually be driven directly from PVCC with 16-lead parts, although it can also be charge pumped or connected to an alternate supply if desired. The 8-lead parts require an RC filter from PVCC to ensure proper operation; see Input Supply Considerations. EXTERNAL COMPONENT SELECTION Power MOSFETs Two N-channel power MOSFETs are required for most LTC1430 circuits. These should be selected based primarily on threshold and on-resistance considerations; thermal dissipation is often a secondary concern in high PVCC OPTIONAL USE FOR PVCC 7V DZ 12V 1N5242 PVCC2 PVCC1 G1 1N4148 0.1F M1 L1 VOUT G2 + M2 COUT LTC1430 * F04 LTC1430 Figure 4. Doubling Charge Pump U efficiency designs. Required MOSFET threshold should be determined based on the available power supply voltages and/or the complexity of the gate drive charge pump scheme. In 5V input designs where an auxiliary 12V supply is available to power PVCC1 and PVCC2, standard MOSFETs with RDS(ON) specified at VGS = 5V or 6V can be used with good results. The current drawn from this supply varies with the MOSFETs used and the LTC1430's operating frequency, but is generally less than 50mA. LTC1430 designs that use a doubler charge pump to generate gate drive for M1 and run from PVCC voltages below 7V cannot provide enough gate drive voltage to fully enhance standard power MOSFETs. When run from 5V, a doubler circuit may work with standard MOSFETs, but the MOSFET RON may be quite high, raising the dissipation in the FETs and costing efficiency. Logic level FETs are a better choice for 5V PVCC systems; they can be fully enhanced with a doubler charge pump and will operate at maximum efficiency. Doubler designs running from PVCC voltages near 4V will begin to run into efficiency problems even with logic level FETs; such designs should be built with tripler charge pumps (see Figure 5) or with newer, super low threshold MOSFETs. Note that doubler charge pump designs running from more than 7V and all tripler charge pump designs should include a zener clamp diode DZ at PVCC1 to prevent transients from exceeding the absolute maximum rating at that pin. DZ 12V 1N5242 1N5817 1N5817 0.1F 0.1F M1 L1 VOUT G2 PVCC 1N5817 10F PVCC2 PVCC1 G1 W U U + M2 COUT LTC1430 * F05 LTC1430 Figure 5. Tripling Charge Pump 7 LTC1430 APPLICATIO S I FOR ATIO Once the threshold voltage has been selected, RON should be chosen based on input and output voltage, allowable power dissipation and maximum required output current. In a typical LTC1430 buck converter circuit operating in continuous mode, the average inductor current is equal to the output load current. This current is always flowing through either M1 or M2 with the power dissipation split up according to the duty cycle: VOUT VIN V DC (M2) = 1 - OUT VIN (VIN - VOUT ) = VIN DC (M1) = The RON required for a given conduction loss can now be calculated by rearranging the relation P = I2R: RON (M1) = = PMAX (M1) DC (M1) * I MAX2 VIN * PMAX (M1) VOUT * I MAX2 RON (M2) = = PMAX (M2) DC (M2) * IMAX2 VIN * PMAX (M2) (VIN - VOUT ) * I MAX2 PMAX should be calculated based primarily on required efficiency. A typical high efficiency circuit designed for 5V in, 3.3V at 10A out might require no more than 3% efficiency loss at full load for each MOSFET. Assuming roughly 90% efficiency at this current level, this gives a PMAX value of (3.3V * 10A/0.9) * 0.03 = 1.1W per FET and a required RON of: 5V * 1.1W RON (M1) = = 0.017 3.3V * 10A2 5V * 1.1W RON (M2) = = 0.032 (5V - 3.3V) * 10A2 8 U Note that the required RON for M2 is roughly twice that of M1 in this example. This application might specify a single 0.03 device for M2 and parallel two more of the same devices to form M1. Note also that while the required RON values suggest large MOSFETs, the dissipation numbers are only 1.1W per device or less--large TO-220 packages and heat sinks are not necessarily required in high efficiency applications. Siliconix Si4410DY (in SO-8) and Motorola MTD20N03HL (in DPAK) are two small, surface mount devices with RON values of 0.03 or below with 5V of gate drive; both work well in LTC1430 circuits with up to 10A output current. A higher PMAX value will generally decrease MOSFET cost and circuit efficiency and increase MOSFET heat sink requirements. Inductor The inductor is often the largest component in an LTC1430 design and should be chosen carefully. Inductor value and type should be chosen based on output slew rate requirements and expected peak current. Inductor value is primarily controlled by the required current slew rate. The maximum rate of rise of the current in the inductor is set by its value, the input-to-output voltage differential and the maximum duty cycle of the LTC1430. In a typical 5V to 3.3V application, the maximum rise time will be: 90% * W U U (VIN - VOUT ) L AMPS 1.53A I = SECOND s L where L is the inductor value in H. A 2H inductor would have a 0.76A/s rise time in this application, resulting in a 6.5s delay in responding to a 5A load current step. During this 6.5s, the difference between the inductor current and the output current must be made up by the output capacitor, causing a temporary droop at the output. To minimize this effect, the inductor value should usually be in the 1H to 5H range for most typical 5V to 3.xV LTC1430 circuits. Different combinations of input and output voltages and expected loads may require different values. Once the required value is known, the inductor core type can be chosen based on peak current and efficiency requirements. Peak current in the inductor will be equal to the maximum output load current added to half the peakto- peak inductor ripple current. Ripple current is set by the LTC1430 APPLICATIO S I FOR ATIO inductor value, the input and output voltage and the operating frequency. If the efficiency is high and can be approximately equal to 1, the ripple current is approximately equal to: I = (VIN - VOUT ) * DC fOSC * L VOUT VIN DC = fOSC = LTC1430 oscillator frequency L = inductor value Solving this equation with our typical 5V to 3.3V application, we get: 1.7 * 0.66 = 2.8AP -P 200kHz * 2H Peak inductor current at 10A load: 10A + 2.8A = 11.4A 2 The inductor core must be adequate to withstand this peak current without saturating, and the copper resistance in the winding should be kept as low as possible to minimize resistive power loss. Note that the current may rise above this maximum level in circuits under current limit or under fault conditions in unlimited circuits; the inductor should be sized to withstand this additional current. Input and Output Capacitors A typical LTC1430 design puts significant demands on both the input and output capacitors. Under normal steady load operation, a buck converter like the LTC1430 draws square waves of current from the input supply at the switching frequency, with the peak value equal to the output current and the minimum value near zero. Most of this current must come from the input bypass capacitor, since few raw supplies can provide the current slew rate to feed such a load directly. The resulting RMS current flow in the input capacitor will heat it up, causing premature capacitor failure in extreme cases. Maximum RMS current occurs with 50% PWM duty cycle, giving an RMS current U value equal to IOUT/2. A low ESR input capacitor with an adequate ripple current rating must be used to ensure reliable operation. Note that capacitor manufacturers' ripple current ratings are often based on only 2000 hours (3 months) lifetime; further derating of the input capacitor ripple current beyond the manufacturer's specification is recommended to extend the useful life of the circuit. The output capacitor in a buck converter sees much less ripple current under steady-state conditions than the input capacitor. Peak-to-peak current is equal to that in the inductor, usually a fraction of the total load current. Output capacitor duty places a premium not on power dissipation but on ESR. During an output load transient, the output capacitor must supply all of the additional load current demanded by the load until the LTC1430 can adjust the inductor current to the new value. ESR in the output capacitor results in a step in the output voltage equal to the ESR value multiplied by the change in load current. A 5A load step with a 0.05 ESR output capacitor will result in a 250mV output voltage shift; this is a 7.6% output voltage shift for a 3.3V supply! Because of the strong relationship between output capacitor ESR and output load transient response, the output capacitor is usually chosen for ESR, not for capacitance value; a capacitor with suitable ESR will usually have a larger capacitance value than is needed to control steady-state output ripple. Electrolytic capacitors rated for use in switching power supplies with specified ripple current ratings and ESR can be used effectively in LTC1430 applications. OS-CON electrolytic capacitors from Sanyo give excellent performance and have a very high performance/size ratio for an electrolytic capacitor. Surface mount applications can use either electrolytic or dry tantalum capacitors. Tantalum capacitors must be surge tested and specified for use in switching power supplies; low cost, generic tantalums are known to have very short lives followed by explosive deaths in switching power supply applications. AVX TPS series surface mount devices are popular tantalum capacitors that work well in LTC1430 applications. A common way to lower ESR and raise ripple current capability is to parallel several capacitors. A typical LTC1430 application might require an input capacitor with a 5A ripple current capacity and 2% output shift with a 10A output load step, which requires a 0.007 output capacitor ESR. Sanyo W U U 9 LTC1430 APPLICATIO S I FOR ATIO OS-CON part number 10SA220M (220F/10V) capacitors feature 2.3A allowable ripple current at 85C and 0.035 ESR; three in parallel at the input and six at the output will meet the above requirements. Input Supply Considerations/Charge Pump The 16-lead LTC1430 requires four supply voltages to operate: PVCC for the main power input, PVCC1 and PVCC2 for MOSFET gate drive and a clean, low ripple VCC for the LTC1430 internal circuitry (Figure 6). In many applications, PVCC and PVCC2 can be tied together and fed from a common high power supply, provided that the supply voltage is high enough to fully enhance the gate of external MOSFET M2. This can be the 5V system supply if a logic level MOSFET is used for M2. VCC can usually be filtered with an RC from this same high power supply; the low quiescent current (typically 350A) allows the use of relatively large filter resistors and correspondingly small filter capacitors. 100 and 4.7F usually provide adequate filtering for VCC. VCC PVCC2 PVCC1 PVCC G1 M1 L1 VOUT INTERNAL CIRCUITRY G2 + M2 COUT LTC1430 * F06 LTC1430 (16-LEAD) Figure 6. 16-Lead Power Supplies The 8-lead versions of the LTC1430 have the PVCC2 and VCC pins tied together inside the package (Figure 7). This pin, brought out as VCC/PVCC2, has the same low ripple requirements as the 16-lead part, but must also be able to supply the gate drive current to M2. This can be obtained by using a larger RC filter from the PVCC pin; 22 and 10F work well here. The 10F capacitor must be VERY close to the part (preferably right underneath the unit) or output regulation may suffer. 10 U For both versions of the LTC1430, PVCC1 must be higher than PVCC by at least one external MOSFET VGS(ON) to fully enhance the gate of M1. This higher voltage can be provided with a separate supply (typically 12V) which should power up after PVCC, or it can be generated with a simple charge pump (Figure 4). The charge pump consists of a 1N4148 diode from PVCC to PVCC1 and a 0.1F capacitor from PVCC1 to the switching node at the drain of M2. This circuit provides 2PVCC - VF to PVCC1 while M1 is ON and PVCC - VF while M1 is OFF where VF is the ON voltage of the 1N4148 diode. Ringing at the drain of M2 can cause transients above 2PVCC at PVCC1; if PVCC is higher than 7V, a 12V zener diode should be included from PVCC1 to PGND to prevent transients from damaging the circuitry at PVCC2 or the gate of M1. More complex charge pumps can be constructed with the 16-lead versions of the LTC1430 to provide additional voltages for use with standard threshold MOSFETs or very low PVCC voltages. A tripling charge pump (Figure 5) can provide 2PVCC and 3PVCC voltages. These can be connected to PVCC2 and PVCC1 respectively, allowing standard threshold MOSFETs to be used with 5V at PVCC or 5V logic level threshold MOSFETs to be used with 3.3V at PVCC. VCC can be driven from the same potential as PVCC2, allowing the entire system to run from a single 3.3V supply. Tripling charge pumps require the use of Schottky diodes to minimize forward drop across the diodes at start-up. The tripling charge pump circuit will tend to rectify any ringing at the drain of M2 and can provide well more than 3PVCC at PVCC1; all tripling (or higher multiplying factor) circuits should include a 12V zener clamp diode DZ to prevent overvoltage at PVCC1. VCC /PVCC2 PVCC1 PVCC G1 M1 L1 INTERNAL CIRCUITRY G2 VOUT W U U + M2 COUT LTC1430 * F07 LTC1430 (8-LEAD) Figure 7. 8-Lead Power Supplies LTC1430 APPLICATIO S I FOR ATIO Compensation and Transient Response The LTC1430 voltage feedback loop is compensated at the COMP pin; this is the output node of the internal gm error amplifier. The loop can generally be compensated properly with an RC network from COMP to GND and an additional small C from COMP to GND (Figure 8). Loop stability is affected by inductor and output capacitor values and by other factors. Optimum loop response can be obtained by using a network analyzer to find the loop poles and zeros; nearly as effective and a lot easier is to empirically tweak the RC values until the transient recovery looks right with an output load step. Table 1 shows recommended compensation components for 5V to 3.3V applications based on the inductor and output capacitor values. The values were calculated using multiple paralleled 330F AVX TPS series surface mount tantalum capacitors as the output capacitor. Table 1. Recommended Compensation Network for 5V to 3.3V Application Using Multiple 330F AVX Output Capacitors L1 (H) 1 1 1 1 2.7 2.7 2.7 2.7 5.6 5.6 5.6 5.6 10 10 10 10 COUT (F) 990 1980 4950 9900 990 1980 4950 9900 990 1980 4950 9900 990 1980 4950 9900 RC (k) 1.8 3.6 9.1 18 3.6 7.5 18 39 9.1 18 47 91 18 39 91 180 CC (F) 0.022 0.01 0.0047 0.0022 0.01 0.0047 0.0022 0.001 0.0047 0.0022 820pF 470pF 0.0022 0.001 470pF 220pF C1 (pF) 820 470 150 82 470 220 82 39 150 82 33 15 82 39 15 10 Output transient response is set by three major factors: the time constant of the inductor and the output capacitor, the more impact on overall transient recovery time than the third; unless the loop compensation is way off, more improvement can be had by optimizing the inductor and the output capacitor than by fiddling with the loop com- U pensation components. In general, a smaller value inductor will improve transient response at the expense of ripple and inductor core saturation rating. Minimizing output capacitor ESR will also help optimize output transient response. See Input and Output Capacitors for more information. LTC1430 COMP RC CC GND SGND C1 LTC1430 * F08 W U U Figure 8. Compensation Pin Hook-Up Soft-Start and Current Limit The 16-lead versions of the LTC1430 include a soft-start circuit at the SS pin; this circuit is used both for initial startup and during current limit operation. The soft-start and current limit circuitry is disabled in 8-lead versions. SS requires an external capacitor to GND with the value determined by the required soft-start time. An internal 12A current source is included to charge the external capacitor. Soft-start functions by clamping the maximum voltage that the COMP pin can swing to, thereby controlling the duty cycle (Figure 9). The LTC1430 will begin to operate at low duty cycle as the SS pin rises to about 2V below VCC. As SS continues to rise, the duty cycle will increase until the error amplifier takes over and begins to regulate the output. When SS reaches 1V below VCC the LTC1430 will be in full operation. An internal switch shorts the SS pin to GND during shutdown. The LTC1430 detects the output current by watching the voltage at IFB while M1 is ON. The ILIM amplifier compares this voltage to the voltage at IMAX (Figure 10). In the ON state, M1 has a known resistance; by calculating backwards, the voltage generated at IFB by the maximum output current in M1 can be determined. As IFB falls below IMAX, ILIM will begin to sink current from the soft-start pin, 11 LTC1430 APPLICATI S I FOR ATIO causing the voltage at SS to fall. As SS falls, it will limit the output duty cycle, limiting the current at the output. Eventually the system will reach equilibrium, where the pull-up current at the SS pin matches the pull-down current in the ILIM amplifier; the LTC1430 will stay in this state until the overcurrent condition disappears. At this time IFB will rise, ILIM will stop sinking current and the internal pull-up will recharge the soft-start capacitor, restoring normal operation. Note that the IFB pin requires an external 1k series resistor to prevent voltage transients at the drain of M2 from damaging internal structures. The ILIM amplifier pulls current out of SS in proportion to the difference between IFB and IMAX. Under mild overload conditions, the SS pin will fall gradually, creating a time delay before current limit takes effect. Very short, mild overloads may not trip the current limit circuit at all. Longer overload conditions will allow the SS pin to reach a steady level, and the output will remain at a reduced voltage until the overload is removed. Serious overloads will generate a larger overdrive at ILIM, allowing it to pull SS down more quickly and preventing damage to the output components. The ILIM amplifier output is disabled when M1 is OFF to prevent the low IFB voltage in this condition from activating the current limit. It is re-enabled a fixed 170ns after M1 COMP VCC LTC1430 FB 12A SS CSS LTC1430 * F09 Figure 9. Soft-Start Clamps COMP Pin 12 U turns on; this allows for the IFB node to slew back high and the ILIM amplifier to settle to the correct value. As the LTC1430 goes deeper into current limit, it will reach a point where the M1 on-time needs to be cut to below 170ns to control the output current. This conflicts with the minimum settling time needed for proper operation of the ILIM amplifier. At this point, a secondary current limit circuit begins to reduce the internal oscillator frequency, lengthening the off-time of M1 while the on-time remains constant at 170ns. This further reduces the duty cycle, allowing the LTC1430 to maintain control over the output current. Under extreme output overloads or short circuits, the ILIM amplifier will pull the SS pin more than 2V below VCC in a single switching cycle, cutting the duty cycle to zero. At this point all switching stops, the output current decays through M2 and the LTC1430 runs a partial soft-start cycle and restarts. If the short is still present the cycle will repeat. Peak currents can be quite high in this condition, but the average current is controlled and a properly designed circuit can withstand short circuits indefinitely with only moderate heat rise in the output FETs. In addition, the soft-start cycle repeat frequency can drop into the low kHz range, causing vibrations in the inductor which provide an audible alarm that something is wrong. 0.1F PVCC RIMAX IMAX IFB 1k W U UO + 12A VCC 12A SS CSS ILIM - LTC1430 LTC1430 * F10 Figure 10. Current Limit Operation LTC1430 APPLICATI S I FOR ATIO Oscillator Frequency The LTC1430 includes an onboard current controlled oscillator which will typically free-run at 200kHz. An internal 20A current is summed with any current in or out of the FREQSET pin (pin 11), setting the oscillator frequency to approximately 10kHz/A. FREQSET is internally servoed to the LTC1430 reference voltage (1.26V). With FREQSET floating, the oscillator is biased from the internal 20A source and runs at 200kHz. Connecting a 50k resistor from FREQSET to ground will sink an additional 25A from FREQSET, causing the internal oscillator to run at approximately 450kHz. Sourcing an external 10A current into FREQSET will cut the internal frequency to 100kHz. An internal clamp prevents the oscillator from running slower than about 50kHz. Tying FREQSET to VCC will cause it to run at this minimum speed. Shutdown The LTC1430 includes a low power shutdown mode, controlled by the logic at the SHDN pin. A high at SHDN allows the part to operate normally. A low level at SHDN stops all internal switching, pulls COMP and SS to ground internally and turns M1 and M2 off. In shutdown, the LTC1430 itself will drop below 1A quiescent current typically, although off-state leakage in the external MOSFETs may cause the total PVCC current to be somewhat higher, especially at elevated temperatures. When SHDN rises again, the LTC1430 will rerun a soft-start cycle and resume normal operation. Holding the LTC1430 in shutdown during PVCC power up removes any PVCC1 sequencing constraints. LAYOUT CONSIDERATIONS Grounding Proper grounding is critical for the LTC1430 to obtain specified output regulation. Extremely high peak currents (as high as several amps) can flow between the bypass capacitors and the PVCC1, PVCC2 and PGND pins. These currents can generate significant voltage differences between two points that are nominally both "ground." As a general rule, GND and PGND should be totally separated on the layout, and should be brought together at only one U point, right at the LTC1430 GND and PGND pins. This helps minimize internal ground disturbances in the LTC1430 by keeping PGND and GND at the same potential, while preventing excessive current flow from disrupting the operation of the circuits connected to GND. The PGND node should be as compact and low impedance as possible, with the negative terminals of the input and output capacitors, the source of M2, the LTC1430 PGND node, the output return and the input supply return all clustered at one point. Figure 11 is a modified schematic showing the common connections in a proper layout. Note that at 10A current levels or above, current density in the PC board itself can become a concern; traces carrying high currents should be as wide as possible. Output Voltage Sensing The LTC1430 provides three pins for sensing the output voltage: SENSE+, SENSE- and FB. SENSE+ and SENSE- connect to an internal resistor divider which is connected to FB. To set the output of the LTC1430 to 3.3V, connect SENSE+ to the output as near to the load as practical and connect SENSE- to the common GND/PGND point. Note that SENSE- is not a true differential input sense input; it is just the bottom of the internal divider string. Connecting SENSE- to the ground near the load will not improve load regulation. For any other output voltage, the SENSE+ and SENSE- pins should be floated and an external resistor string should be connected to FB (Figure 12). As before, connect the top resistor (R1) to the output as close to the load as practical and connect the bottom resistor (R2) to the common GND/PGND point. In both cases, connecting the top of the resistor divider (either SENSE+ or R1) close to the load can significantly improve load regulation by compensating for any drops in PC traces or hookup wires between the LTC1430 and the load. Power Component Hook-Up/Heat Sinking As current levels rise much above 1A, the power components supporting the LTC1430 start to become physically large (relative to the LTC1430, at least) and can require special mounting considerations. Input and output capacitors need to carry high peak currents and must have low ESR; this mandates that the leads be clipped as short as possible and PC traces be kept wide and short. The W U UO 13 LTC1430 APPLICATI S I FOR ATIO power inductor will generally be the most massive single component on the board; it can require a mechanical holddown in addition to the solder on its leads, especially if it is a surface mount type. The power MOSFETs used require some care to ensure proper operation and reliability. Depending on the current levels and required efficiency, the MOSFETs chosen may be as large as TO-220s or as small as SO-8s. High efficiency circuits may be able to avoid heat sinking the power devices, especially with TO-220 type MOSFETs. As an example, a 90% efficient converter working at a steady 3.3V/10A output will dissipate only (33W/90%) * 10% = 3.7W. The power MOSFETs generally account for the SENSE + LTC1430 FB R2 SENSE - LTC1430 * F12 Figure 12. Using External Resistors to Set Output Voltages 100 4.7F 35V + 0.1F GND LTC1430 NC NC IMAX FREQSET IFB SENSE SHDN COMP + VCC PVCC2 PVCC1 G1 G2 FB SENSE - PGND NC C1 220pF CC 4700pF RC 7.5k CSS 0.01F GND SS GND Figure 11. Typical Schematic Showing Layout Considerations 14 U majority of the power lost in the converter; even assuming that they consume 100% of the power used by the converter, that's only 3.7W spread over two or three devices. A typical SO-8 MOSFET with a RON suitable to provide 90% efficiency in this design can commonly dissipate 2W when soldered to an appropriately sized piece of copper trace on a PC board. Slightly less efficient or higher output current designs can often get by with standing a TO-220 MOSFET straight up in an area with some airflow; such an arrangement can dissipate as much as 3W without a heat sink. Designs which must work in high ambient temperatures or which will be routinely overloaded will generally fare best with a heat sink. NC VOUT R1 NC 5V 1F 1N4148 PGND M1A* 0.1F 2.7H/15A 3.3V M1B* W U UO + TOTAL 880F (220F 10V x 4) M2* + TOTAL 1980F (330F 6.3V x 6) PGND * MOTOROLA MTD20N03HL LTC1430 * F11 LTC1430 PACKAGE DESCRIPTIO 0.300 - 0.325 (7.620 - 8.255) 0.009 - 0.015 (0.229 - 0.381) 0.065 (1.651) TYP 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 0.003 (0.457 0.076) ( +0.035 0.325 -0.015 +0.889 8.255 -0.381 ) *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm) 0.300 - 0.325 (7.620 - 8.255) 0.130 0.005 (3.302 0.127) 0.020 (0.508) MIN 0.009 - 0.015 (0.229 - 0.381) ( +0.035 0.325 -0.015 +0.889 8.255 -0.381 ) 0.125 (3.175) MIN *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm) Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. U Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.400* (10.160) MAX 8 7 6 5 0.045 - 0.065 (1.143 - 1.651) 0.130 0.005 (3.302 0.127) 0.255 0.015* (6.477 0.381) 1 2 3 4 0.100 (2.54) BSC N8 1098 N Package 14-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.770* (19.558) MAX 16 15 14 13 12 11 10 9 0.045 - 0.065 (1.143 - 1.651) 0.065 (1.651) TYP 0.018 0.003 (0.457 0.076) 0.255 0.015* (6.477 0.381) 1 2 3 4 5 6 7 8 0.100 (2.54) BSC N16 1098 15 LTC1430 PACKAGE DESCRIPTIO 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP 0.014 - 0.019 (0.355 - 0.483) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 0.016 - 0.050 (0.406 - 1.270) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0.053 - 0.069 (1.346 - 1.752) 0 - 8 TYP 0.016 - 0.050 (0.406 - 1.270) 0.014 - 0.019 (0.355 - 0.483) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE RELATED PARTS PART NUMBER LTC1530 LTC1625 LTC1628 LTC1703 LTC1706-81 LTC1709 LTC1736 LTC1753 LTC1873 LTC1929 DESCRIPTION High Power Synchronous Step-Down Controller No RSENSE Current Mode Synchronous Step-Down Controller Dual High Efficiency 2-Phase Synchronous Step-Down Controller TM Dual 550kHz Synchronous 2-Phase Switching Regulator Controller LTC1702 with 5-Bit Mobile VID for Mobile Pentium(R) Processor with Mobile VID Systems VID Voltage Programmer 2-Phase, 5-Bit Desktop VID Synchronous Step-Down Controller Synchronous Step-Down Controller with 5-Bit Mobile VID Control 5-Bit Desktop VID Programmable Synchronous Switching Regulator Dual Synchronous Switching Regulator with 5-Bit Desktop VID 2-Phase, Synchronous High Efficiency Converter Adds 5-Bit Mobile VID to 0.8V Referenced Switching Regulators Current Mode, VIN to 36V, IOUT Up to 42A Fault Protection, PowerGood, 3.5V to 36V Input, Current Mode 1.3V to 3.5V Programmable Output Using Internal 5-Bit DAC 1.3V to 3.5V Programmable Core Output Plus I/O Output Current Mode Ensures Accurate Current Sensing, VIN Up to 36V, IOUT Up to 40A No RSENSE is a trademark of Linear Technology Corporation. Pentium is a registered trademark of Intel Corporation. 16 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com U Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.189 - 0.197* (4.801 - 5.004) 0.053 - 0.069 (1.346 - 1.752) 8 0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 7 6 5 0.050 (1.270) BSC 1 2 3 4 SO8 1298 S Package 16-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.228 - 0.244 (5.791 - 6.197) 0.004 - 0.010 (0.101 - 0.254) 0.386 - 0.394* (9.804 - 10.008) 16 15 14 13 12 11 10 9 0.050 (1.270) BSC 0.150 - 0.157** (3.810 - 3.988) 1 2 3 4 5 6 7 8 S16 1098 COMMENTS SO-8 with Current Limit. No RSENSE Required Above 95% Efficiency, Needs No RSENSE, 16-Lead SSOP Package Fits SO-8 Footprint Constant Frequency, Standby 5V and 3.3V LDOs, 3.5V VIN 36V 1430fa LT/TP 0500 2K REV A * PRINTED IN USA (c) LINEAR TECHNOLOGY CORPORATION 1995 |
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