DESIGN FEATURES LT1766: 1.5A Converter Runs Directly from 60V DC by Mark W. Marosek Introduction With a maximum input voltage capability of 60V and peak switch current of 1.5A, the LT1766 is targeted for high voltage, industrial and automotive applications. The 60V maximum operating input voltage makes the LT1766 ideal for 12V, 24V and (future) 42V automotive systems, which must survive load-dump input transients as high as 60V. At high input-tooutput voltage differentials, power dissipation is minimized by providing very fast switch edge rates and by using a BIAS pin connected to the regulated output to supply internal control circuitry. At low input-tooutput voltage dif ferentials, a VIN supply-boost capacitor is used to fully saturate an internal 200mΩ power switch to reduce DC switch losses. The LT1766 runs at a fixed frequency of 200kHz and can be externally synchronized. In addition, a pin is provided for programming undervoltage lockout and placing the part into micropower shutdown. The LT1766 maintains the 1.5A peak switch current over the full duty cycle range. This is achieved by the use of patented circuitry that cancels the effect of slope compensation on peak switch current without affecting frequency compensation. The LT1766 is packaged in a 16-pin SSOP (GN-16) package, which occupies the same space as an SO-8 package. The LT1766 provides a wide range of functions, such as undervoltage lockout and micropower shutdown, along with external synchronization capability, without sacrificing the power saving features of a BIAS or BOOST pin. Also, the GN-16 package, with its fused corner pins, improves thermal performance, reducing the θJA of the package from 120°C/W (typical SO-8) to 95°C/W. 4 RSENSE 2.9V BIAS REGULATOR INTERNAL VCC Σ + BIAS 10 – RLIMIT CURRENT COMPARATOR SLOPE COMP/ BOOST ANTI-SLOPE COMP 6 200kHz OSCILLATOR SYNC 14 S RS R FLIP-FLOP DRIVER CIRCUITRY Q1 POWER SWITCH SHUTDOWN COMPARATOR – + 2 SW FREQUENCY FOLDBACK 5.5µA 0.4V + SHDN 15 LOCKOUT COMPARATOR – ×1 Q2 12 FB + 2.38V Q3 – VC MAX CLAMP FOLDBACK CURRENT LIMIT CLAMP 11 VC ERROR AMPLIFIER gm = 2000µ 1.22V GND Ω 1, 8, 9, 16 Figure 1. LT1766 block diagram Linear Technology Magazine • May 2001 5 DESIGN FEATURES AVX TPSD107M010R0100 AVX X7R 0805YC334KAT1A MARCON TCCR70EA475M AVX X7R 08055C332KAT (207) 282-5111 (843) 946-0362 (847) 696-2000 (843) 946-0362 90 VOUT 5V/1.0A R1 15.4k D1 60V 1.5A C4 1nF, 50V C1: C2: C3: C4: C2 0.33µF, 16V + R2 4.99k C1 100µF, 10V SOLID TANTALUM 80 70 60 50 0.00 D1: IR 10MQ060N D2: ZETEX FMM914TA L1: COILTRONICS UP2-680 0.25 0.50 0.75 1.00 LOAD CURRENT (A) 1.25 Figure 2b. Efficiency vs load current for Figure 2a’s circuit (310) 322-3331 (516) 543-7100 (561) 241-7876 LT1766 Features Figure 2a. 42V to 5V step-down converter Circuit Description feature of the LT1766, referred to as “anti-slope compensation” in the block diagram, eliminates the effects of slope compensation on the peak switch current at high duty cycles without affecting the frequency compensation. For high duty cycle applications, this is a significant benefit over competing current mode converters with similar peak switch current limits. The LT1766 also includes an accurate 1.2V reference. This reference is scaled to provide an accurate 2.38V threshold on the SHDN pin, allowing programming of undervoltage lockout; a second, lower threshold (0.4V) allows shutdown with the input supply current reduced to 25µA. The part can be externally synchronized to frequencies up to 700kHz. The LT1766 is optimized to provide high efficiency for both high and low input-to-output voltage differentials in a buck-mode switching regulator topology. The block diagram in Figure 1 shows all of the key functions of the LT1766 step-down converter. A current mode architecture is used to provide fast transient response and good loop stability. Two feedback loops exist to control the duty cycle of the power switch using a transconductance error amplifier and a current sense comparator that monitors switch current on a cycle-by-cycle basis. Nonlinear slope compensation has been added to the current sense signal to prevent the subharmonic oscillation associated with current mode control when the regulator duty cycle is greater than 50%. An added EFFICIENCY (%) C3 4.7µF 100V CERAMIC 100 L1 68µH 2 SW BOOST 4 LT1766 VIN 10 15 BIAS SHDN 12 14 FB SYNC 11 1, 8, 9, 16 VC GND 6 VIN = 42V D2 D2 The LT1766 provides the following features: ❏ Wide input range: 5.5V to 60V ❏ Constant 200kHz switching frequency ❏ 1.5A peak switch current ❏ 0.2Ω saturating switch ❏ Peak switch current maintained over full duty cycle range ❏ 25µA shutdown current ❏ 1.2V feedback reference ❏ Easily synchronizable 42V to 5V Buck Converter The LT1766 was designed to address the need for high efficiency over a wide range of input voltages. A typical high input voltage application, a 42V to 5V converter, is shown in Figure 2a. To achieve high efficiency at high input voltages, fast output-switch edge rates are required; the LT1766 achieves edge rates of 1.2V/ns (rise) and 1.7V/ns (fall). In addition, light continued on page 17 C2 0.33µF, 16V VIN = 12V (TRANSIENTS TO 60V) C3 4.7µF 100V CERAMIC C4 1nF, 50V 100 L1 33µH VOUT 5V/1.0A 90 R1 15.4k D1 60V 1.5A R2 4.99k + C1 100µF, 10V SOLID TANTALUM EFFICIENCY (%) 2 SW BOOST 4 LT1766 VIN 10 15 BIAS SHDN 12 14 FB SYNC 11 1, 8, 9, 16 VC GND 6 80 70 60 C1: C2: C3: C4: AVX TPSD107M010R0100 AVX X7R 0805YC334KAT1A MARCON TCCR70EA475M AVX X7R 08055C332KAT (207) 282-5111 (843) 946-0362 (847) 696-2000 (843) 946-0362 D1: IR 10MQ060N D2: ZETEX FMM914TA L1: SUMIDA CDHR6D38-330M Figure 3a. 12V to 5V step-down converter 6 (310) 322-3331 (516) 543-7100 (847) 956-0667 50 0.00 0.25 0.50 0.75 1.00 LOAD CURRENT (A) 1.25 Figure 3b. Efficiency vs load current for Figure 3a’s circuit Linear Technology Magazine • May 2001 DESIGN FEATURES TRACE A VPROG = 4.5V ILAMP = 9mARMS TRACE A VPROG = 4.5V ILAMP = 9mARMS TRACE B VPROG = 1.125V ILAMP = 1mARMS TRACE B VPROG = 1.125V ILAMP = 1mARMS 1ms/DIV 100µs/DIV Figure 3a. CCFL current for Figure 1’s circuit Figure 3b. CCFL current for Figure 1’s circuit (expanded time scale) be set so that the LT1768 runs in linear mode over the most widely used operating range. In the example in Figure 1, PWM mode runs from VPROG = 1V to VPROG = 3.0V with CCFL current modulated between 0mA and 6mA. The PWM modulation frequency is set to 220Hz by capacitor C3. When combined, these five modes of operation allow the creation of a DC-controlled CCFL current profile that can be tailored to each particular display. With linear mode, CCFL current control over the most widely used current range and with PWM mode at the low end, the LT1768 enables wide dimming ratios while maximizing CCFL lifetimes. LT1768 Fault Modes Additional Features The LT1768 also has fault detection to ensure that lamp current and Royer transformer ratings are not exceeded under fault conditions. If the current in either CCFL is less than 125µA for a minimum of one PWM clock cycle, the FAULT pin will be activated and ICCFLMAX will be halved. This function ensures that the maximum CCFL current set by RMAX will not be exceeded even under fault conditions. If current in both CCFLs is less than 125µA, and the voltage on the VC pin reaches its clamp value (indicating an open-load condition) for a minimum of one PWM cycle, the gate drive will be latched off. The latch can be cleared by setting VPROG to 0V or placing the LT1768 in shutdown mode. The LT1768 also contains a temperature-compensated 5V reference, an undervoltage lockout feature for VIN < 8V, thermal shutdown and a logiccompatible shutdown pin that reduces supply current to 50µA when activated. The LT1768 is available in a GN16 package. motive electronics. A typical 12V to 5V converter is shown in Figure 3a. The key to achieving high efficiency for low input-to-output voltage conversions is to use a saturating switch design. A prebiased capacitor, connected between the BOOST and SW pins, generates a boost voltage above the input supply during switching. Prebias is derived via a diode, D2, from the regulated output. Driving the switch from this boost voltage allows the 200mΩ power switch to fully saturate. The worst-case minimum BOOST voltage required to fully saturate the internal power switch is 3V above input supply. This means that an output voltage as low as 3.3V is ideal for prebiasing of the boost supply capacitor via a simple diode. The efficiency for a 12V to 5V conversion, which peaks at 90%, is shown in Figure 3b. Authors can be contacted at (408) 432-1900 LT1766, continued from page 6 loads at high input voltages require minimal quiescent current to be drawn from the input. A BIAS pin allows the internal control circuitry to be supplied from the regulated output if greater than 3V. At a duty cycle of 12%, for example, this technique reduces input quiescent current from 4.5mA to approximately 1.8mA. The efficiency for a 42V to 5V conversion (>80%) is shown in Figure 2b. 12V to 5V Converter The LT1766 is capable of excellent efficiencies at lower input voltages; combined with its ability to withstand voltage transients up to 60V, this makes it ideal for power conversion in harsh environments such as autoLinear Technology Magazine • May 2001 Conclusion The LT1768 high power CCFL controller, with its unique patented dimming control scheme, accurate minimum and maximum lamp currents and lamp fault protection enables wide dimming ratios and maximizes CCFL lifetime in single- or multiple-lamp LCD displays. References 1. Williams, Jim. November 1995. A Fourth Generation of LCD Backlight Technology. Linear Technology Corporation, Application Note 65. Conclusion Having a peak switch current of 1.5A and a maximum input voltage capability of 60V, the LT1766 provides an ideal solution for 12V, 24V or (future) 42V automotive electronics in addition to 48V nonisolated telecom applications, where input voltages as high as 60V must be accommodated. Packaged in a 16-pin SSOP (GN16), which occupies the same space as an SO-8, and running at a fixed frequency of 200kHz, the LT1766 provides a compact solution for high voltage step-down power conversions. 17