DESIGN FEATURES 60V/3A Step-Down DC/DC Converter Maintains High Efficiency over a Wide by Mark W. Marosek Range of Input Voltages Introduction sients as high as 60V). It is designed to maintain excellent efficiencies at both high and low input-to-output voltage differentials over a wide input voltage range. Its current mode architecture adds flexible frequency compensation with no restriction on the use of a ceramic output capacitor—resulting in small solutions with extremely low output ripple voltage. The LT3430 is pin compatible with the LT1766 (60V, 1.5A, 200kHz) and LT1956 (60V transient, 1.5A, 500kHz) step down DC/DC converters.1 The LT3430 is a monolithic step-down DC/DC converter that features a 3A peak switch current limit and the ability to operate with up to 60V input. The LT3430 runs at a fixed frequency of 200kHz and is packaged in a small thermally enhanced 16-pin TSSOP package to save space and simplify thermal management. The 5.5V to 60V input range makes the LT3430 ideal for FireWire® Peripherals (typically 8V to 40V input), as well as automotive systems requiring 12V, 24V and 42V input voltages (with the ability to survive load dump tran- LT3430 Features ❏ Wide input range 5.5V to 60V ❏ 3A peak switch current ❏ Small thermally enhanced 16-pin TSSOP Package ❏ Constant 200kHz switching frequency ❏ 100mΩ saturating switch ❏ Current mode architecture ❏ Peak switch current maintained over full duty cycle range ❏ 30µA shutdown current ❏ 1.2V feedback reference ❏ Easily synchronizable VIN 3, 4 BIAS 10 RLIMIT 2.9V BIAS REGULATOR – + INTERNAL VCC CURRENT COMPARATOR Σ SLOPE COMP RSENSE SYNC 14 BOOST ANTI-SLOPE COMP 6 SHUTDOWN COMPARATOR 200kHz OSCILLATOR S RS FLIP-FLOP Q1 POWER SWITCH DRIVER CIRCUITRY – R + 0.4V 5.5µA SW + 2, 5 FREQUENCY FOLDBACK – LOCKOUT COMPARATOR ×1 2.38V Q2 FOLDBACK CURRENT LIMIT CLAMP Q3 11 VC ERROR AMPLIFIER gm = 2000µMho 12 FB + VC(MAX) CLAMP – SHDN 15 1.22V GND 1, 8, 9, 16 1766 F01 Figure 1. Simplified block diagram Linear Technology Magazine • August 2002 7 DESIGN FEATURES VIN 7.5V–60V D2 MMSD914T1 VIN BOOST SW LT3430EFE OFF ON R3 3.3k C5 0.022µF SHDN BIAS R1 15.4k VOUT + FB VC GND SYNC C4 220pF VOUT 5V AT 2A C2 L1 0.68µF 22µH D1 30BQ060 R2 4.99k C1 100µF 10V SOLID TANTALUM VIN = 12V 90 EFFICIENCY (%) C3 4.7µF 100V CER 100 80 VIN = 42V 70 60 VOUT = 5V DN302 F01 C1: AVX D CASE 100µF 10V TPSD107M010R0100 C2: AVX 0.68µF X7R 16V 0805YC684KAT1A C3: UNITED CHEMI-CON 4.7µF 100V TCCR70E2A475M C4: AVX 220pF X7R 50V 08055A221KAT C5: AVX .022µF X7R 16V 0805YC223KAT D1: INTERNATIONAL RECTIFIER 60V 3A SCHOTTKY 30BQ060 L1: SUMIDA 22µH CDRH104R (207) 282-5111 (847) 696-2000 (310) 322-3331 (847) 956-0667 Figure 2. Efficient 42V to 5V step-down converter Circuit Description The block diagram in Figure 1 shows all of the key functions of the LT3430 step-down DC/DC converter. Its current mode architecture uses two feedback loops to control the duty cycle of the internal power switch—a transconductance error amplifier monitors the error between output voltage (via the FB pin) and an internal 1.22V reference, and a current sense comparator monitors switch current on a cycle-by-cycle basis. The LT3430 runs at a fixed frequency of 200kHz or can be externally synchronized up to 700kHz using the SYNC pin. The LT3430 includes a shutdown pin with an accurate 2.38V threshold for undervoltage lockout, and a 0.4V threshold for micropower shutdown (IQ = 30µA). The BIAS pin provides power savings by allowing control circuitry to be supplied from the output. The LT3430 also uses frequency foldback and current limit foldback to control power dissipation in the IC, external catch diode and inductor in the event of an output short circuit to ground. Peak Switch Current over the Full Duty Cycle Range (Not Your Average Current Mode Converter) The LT3430 maintains peak switch current over the full duty cycle range (wide input voltage range). Although the LT3430 uses a current mode architecture—to allow small, low noise power supply solutions—its peak 8 switch current does not fall off at high duty cycles, unlike most current mode converters. This typical reduction of peak switch current is a result of the necessary slope compensation in the current sensing loop, which exists to prevent sub-harmonic oscillations for duty cycles above 50%. The LT3430 uses a patented process to cancel the effect of slope compensation on peak switch current without affecting frequency compensation. For applications that require high duty cycles, this offers significant advantages— including a lower inductor value, lower minimum VIN and/or higher output current capability—over typical current mode converters with similar peak switch current limits. Efficiency The LT3430 is designed to provide efficient solutions at both high and low input-to-output voltage differentials, over a wide input voltage range. A typical high input voltage application with a large input-to-output differential, a 42V to 5V converter, is shown in Figure 2. To obtain high efficiency at high input voltages requires fast output-switch edge rates, and minimal quiescent current drawn from the input at light loads. The BIAS pin allows power for the internal control circuitry to be supplied from the regulated output if it is greater than 3V. The peak efficiency for a 42V to 5V conversion is greater than 82% as shown in Figure 3. 50 0 0.5 1 1.5 LOAD CURRENT (A) 2 2.5 Figure 3. Efficiency of the circuit shown in Figure 2 The LT3430 is also capable of excellent efficiencies at lower input voltages. The peak efficiency for a 12V to 5V converter is greater than 90% as is also shown in Figure 3. One important factor in achieving high efficiency for low input-to-output voltage conversions is to use a low resistance saturating switch. A prebiased capacitor, connected between the BOOST and SW pins, generates a boost voltage above the input supply during switching. Driving the switch from this boost voltage allows the 100mΩ power switch to fully saturate. Any output voltage of at least 3.3V is enough to generate the required boost supply. Space Saving and Low Output Ripple Voltage Solutions The high switching frequency and current mode architecture of the LT3430 combine to make it possible to design space-saving solutions with low output ripple voltage. The 200kHz switching frequency of the LT3430 reduces the inductor value required to achieve low inductor ripple current, allowing for the use of a physically smaller inductor. The current mode architecture of the LT3430 allows for flexible frequency compensation to accommodate various output voltages, load currents and output capacitor types. This flexibility allows for a small, low ESR ceramic capacitor to be used at the output—making for an extremely low output ripple voltage solution in a small space. continued on page 19 Linear Technology Magazine • August 2002 DESIGN FEATURES measure, such as one supply failing or one fuse damaged. The supply measurement is also more accurate, since the voltage drop across the fuses or diodes does not affect it. Resistors R9 and R10 pull up the fuse pins so that damaged fuses can be detected. The status signals may be wired off the card, with optoisolators, to an isolated microprocessor or microcontroller that controls system performance and warning functions. This allows an automated system supervisor to issue a warning or record the event, despite operating from an isolated supply. The L T4250L switches the –48V supply via Q1 during hot swapping and low supply conditions, and monitors the supply voltage provided to the load. The PWRGD output of the LT4250 drives an optoisolator, providing a supply status signal to the DC/DC converter. This signal may also be used to monitor the condition of the ORing diodes by comparing it to the supply status signals from the LTC1921. Conclusion Reliability is top priority for the designers of modern telephone and communication equipment. Designers take extra care to protect circuitry from failure-causing temperature and voltage changes, employing redundancy whenever possible, especially for power supplies. They monitor supplies for early warnings of impending failure, often using complicated circuitry that can include a voltage reference, comparators, an LDO and several precision resistor dividers. Designers may also use discrete components to indicate the state of power supply fuses. The resulting circuits can be expensive in terms of component cost, board space and engineering time. The LTC1921 replaces this complicated monitoring circuitry with a simple integrated precision monitoring system contained entirely in an MSOP-8 or SO-8 package. LT3430, continued from page 8 VIN 8V TO 40V C3 4.7µF CER 50V OFF ON R1 3.3k C2 0.022µF D2 MMSD914T1 VIN BOOST SYNC SW LT3430EFE BIAS SHDN VC FB C1 220pF VOUT 5V AT 2A C4 L1 0.68µF 22µH IL1 1A/DIV R2 15.4k VOUT GND D1 30BQ060 R3 4.99k C5 100µF CER OUTPUT RIPPLE VOLTAGE 20mV/DIV DN302 F03 C3: TDK C5750X7R1H475K C5: TDK C4532X5R0J107M L1: SUMIDA CEI-122 220 (408) 392-1400 VIN = 24V VOUT = 5V IOUT = 2A (847) 956-0667 Figure 4. Low profile (max height of 3.0mm) FireWire peripheral supply with low output ripple voltage Figure 4 shows a 5V/2A solution for FireWire peripherals which takes advantage of the LT3430 current mode architecture by using a low ESR ceramic capacitor at the output. The circuit provides a low profile (all components less than 3.0mm height), low output ripple voltage solution. Output ripple voltage is only 26mVP–P, as shown in Figure 5, using a 22µH inductor, with VIN = 24V and VOUT = 5V at 2A. 2µs/DIV Figure 5. Output ripple voltage for the circuit shown in Figure 4 Conclusion The LT3430 features a 3A peak switch current limit, 100mΩ internal power switch and a 5.5V to 60V operating range, making it well suited to automotive, industrial and FireWire peripheral applications. It is highly efficient over the entire operating range, and it includes important features to save space and reduce output ripple—including a 200kHz fixed operating frequency, a current mode architecture and availability in a small thermally enhanced 16-pin TSSOP package. Notes 1 The ‘no connect’ pins 3 and 5 of the LT1766 and LT1956 must be connected for the LT3430 to handle the increased current in the SW output (pins 2 and 5) and the VIN input (pins 3 and 4). For more information on parts featured in this issue, see http://www.linear.com/go/ltmag Linear Technology Magazine • August 2002 19