design ideas Dual Output High Efficiency Converter Produces 3.3V and 8.5V Outputs from a 9V to 60V Rail Victor Khasiev The LTC3890 dual output DC/DC controller brings a unique combination of high performance features to applications that require low voltage outputs from high voltage inputs. It can produce two output voltages ranging from 0.8V to 24V from an input voltage of 4V to 60V. It is also very efficient, with a no-load quiescent current of only 50µA. Many high-input-voltage step-down DC/DC converter designs use a transformerbased topology or external high side drivers to operate from up to 60VIN . Others use an intermediate bus converter requiring an additional power stage. However, the LTC3890 simplifies design, with its smaller solution size, reduced cost and shorter development time compared to other design alternatives. 1µF Si 7850DP TG1 0.1µF 4.7µH VIN INTVCC BOOST1 SW1 Si 7850DP BG1 + 1000pF 31.6k = DFLS1100 34.8k + Figure 2. Transient response of 3.3V channel 8µH Si 7850DP BG2 LTC3890 PGND SENSE2+ SENSE1– VFB1 ITH1 SENSE2– VFB2 ITH2 100k 470pF TRACK/SS1 SGND TRACK/SS2 34.8k 0.1µF 0.1µF + 10.5k VOUT2 8.5V 3A COUT2 150µF = 6TPE150 Figure 1. High efficiency dual 8.5V/3.3V output step-down converter FEATURE RICH The LTC3890 is a high performance synchronous buck DC/DC controller with integrated N-channel MOSFET drivers. It uses a current mode architecture and operates from a phase-lockable fixed frequency from 50kHz to 900kHz. The device features up to 99% duty cycle capability for low voltage dropout applications, adjustable soft-start or voltage tracking and selectable continuous, pulse-skipping or Burst Mode operation with a no-load quiescent current of only 50µA. These features, combined with a minimum on-time of just 95ns, make this controller an ideal choice for high stepdown ratio applications. Power loss and (continued on page 35) 95 100 94 98 EFFICIENCY (%) 92 91 90 87 10VIN 96 30VIN 94 92 60VIN 90 88 88 ILOAD1 = 1A TO 5A Si 7850DP 0.01Ω 100k COUT1 150µF VIN 9V TO 60V SW2 SENSE1+ 89 1ms/DIV 0.1µF 0.01Ω VOUT1 3.3V 5A EFFICIENCY (%) ILOAD1 2A/DIV TG2 BOOST2 93 VOUT1 50mV/DIV 22µF 50V VIN = 36V 0 2 1 LOAD CURRENT (A) 3 Figure 3. Efficiency of the converter in Figure 1 for the VOUT2 8.5V channel 86 1 2 3 4 LOAD CURRENT (A) 5 6 Figure 4. Efficiency of the LTC3890 configured as a 2-phase single output of 8.5V at up to 6A July 2010 : LT Journal of Analog Innovation | 33 product briefs The LTC4361 overvoltage and overcurrent protection controller utilizes a 2% accurate 5.8V overvoltage threshold to detect an overvoltage event and responds quickly within 1µs (max) to isolate the downstream components from the input. The LTC4361 controls a low cost external N-channel MOSFET so that under normal operation it provides a low loss path from the input to the load. Inrush current limiting is achieved by controlling the voltage slew rate of the gate. If the voltage at the input exceeds the overvoltage threshold of 5.8V, the GATE is pulled low within 1µs to protect the load. While the IC operates from supplies between 2.5V and 5.5V, the input pins can withstand 80V transients or DC overvoltages. The LTC4361 features a soft shutdown controlled by the ON pin and provides a gate drive output for an optional external P-channel MOSFET for reverse voltage protection. A power good output pin indicates gate turn-on. Following an overvoltage condition, the LTC4361 automatically restarts with a start-up delay. The LTC4361 is available in two options; the LTC4361-1 latches off after an overcurrent event, where as the LTC4361-2 performs an auto-retry following a 130ms delay. The new LTC4360 overvoltage protection controller is recommended for applications that do not require overcurrent protection. While offering many of the same features as the LTC4361, the two LTC4360 versions are differentiated by pin functions. The LTC4360-1 features soft shutdown control with low shutdown current of 1.5µA, while the LTC4360-2 can drive an optional external P-channel MOSFET for negative voltage protection. The LTC4361 is offered in 8-lead (2mm × 2mm) DFN and SOT-23 packages, and the LTC4360 is offered in a tiny 8-lead SC70 package. 180MHZ, 1mA POWER EFFICIENT RAIL-TO-RAIL I/O OP AMPS The LTC6246/LTC6247/LTC6248 are single/ dual/quad low power, high speed unity gain stable rail-to-rail input/output operational amplifiers. On only 1mA of supply current, they feature an impressive 180MHz gain-bandwidth product, 90V/µs slew rate and a low 4.2nV/√Hz of inputreferred noise. The combination of high bandwidth, high slew rate, low power consumption and low broadband noise makes these amplifiers unique among rail-to-rail input/output op amps with similar supply currents. They are ideal for lower supply voltage high speed signal conditioning systems. The LTC6246 family maintains high efficiency performance from supply voltage levels of 2.5V to 5.25V and is fully specified at supplies of 2.7V and 5.0V. For applications that require power-down, the LTC6246 and the LTC6247 in MS10 offer a shutdown pin, which disables the amplifier and reduces current consumption to 42µA. The LTC6246 family can be used as a plug-in replacement for many commercially available op amps. n (LTC3890 continued from page 33) supply noise are minimized by operating the two output stages out-of-phase. Less than 10% mismatch can be achieved, as shown in Figure 5. DUAL OUTPUT APPLICATION CONCLUSION Figure 1 shows the LTC3890 operating in an application that converts a 9V to 60V input into 3.5V/5A and 8.5V/3A outputs. The transient response for the 3.3V output with a 4A load step is less than 50mV (as shown in Figure 2). Although there are many choices in dual-output controllers, the LTC3890 brings a new level of performance with its high voltage operation, high efficiency conversion and ease of design. n Figure 3 shows the efficiency of the 8.5V channel with a 36V input voltage. SINGLE OUTPUT APPLICATION The LTC3890 can also be configured as a 2-phase single output converter by simply connecting the two channels together. For example, a 9V to 60V input can be converted to an 8.5V output at 6A. Figure 4 shows the efficiency of this configuration at input voltages of 10V, 30V and 60V. Current mode control provides good current balance between the phases. IL1, IL2 1A/DIV 0A 1µs/DIV Figure 5. The inductor current in a 2-phase single output converter. Currents in both inductors shown with a 24V Input and 8.5V at 6A output. July 2010 : LT Journal of Analog Innovation | 35