DC-DC Controller Offers Up/Down Output Voltage Tracking

Dec 1, 2003 12:00 PM
By Ashok Bindra, Editor, Power Electronics Technology

Linear Technology, Milpitas, Calif., has readied a general-purpose dual-output 2-phase synchronous step-down dc-dc controller with integrated up/down output voltage tracking function. In addition to on-chip tracking and sequencing, the biCMOS-based LTC3708 offers extremely fast transient response. According to Linear, the unique no-clock-latency operation of the LTC3708 is responsible for very fast load transient response. Other controllers must wait for the end-of-a-clock cycle before they can respond to a change in load. Furthermore, 2-phase operation lowers input filtering and capacitor requirements, reduces EMI, and enhances power conversion efficiency. By eliminating the need for a sense resistor and incorporating on-chip high-current MOSFET drivers, the LTC3708 also cuts external components — reducing the board space and the total solution cost of the dual output dc-dc converter. Over and above, it supports constant frequency operation. Although the LTC3708 employs constant-on-time, valley current-mode architecture, its nominal frequency per phase can be up to 1 MHz. In fact, the operating frequency of the 2-phase converter is selected by an external resistor and is compensated for variations in input supply voltage. Moreover, an internal phase-locked loop (PLL) permits the user to synchronize the chip to an external clock. However, under such conditions, the external clock determines the operating frequency of the controller.

For applications requiring more than two voltages to be tracked, the controller can be daisy-chained. Consequently, multiple LTC3708s can be interfaced to track four or six output voltages, thus eliminating the reliance on digital control, programming or other glue-logic circuits normally found in such solutions. Sequencing is accomplished using an external soft-start timing capacitor. Hence, it caters to the stringent power-up and power-down sequencing requirements found in multiple supply rail systems, where FPGAs, CPLDs, graphic processors, or data converters and CPUs must coexist, each with a specific start-up and shutdown order. As a result, the dc-dc controller ensures proper start-up and shutdown of multivoltage systems. By using the TRACK2 pin, users can program the second channel output ramp-up and ramp-down. Through this pin, the second channel output can be set up to either coincidently or ratiometrically track the channel 1 output, as shown in the figure. As per the part's data sheet, the ratiometric mode saves a pair of external resistors, while the coincident mode offers better output regulation.

The LTC3708 drives external n-channel MOSFETs synchronously to allow efficiencies as high as 96% while delivering 5 A at 2.5 V from a 5-V input supply. According to the supplier, the efficiency is maintained between 90% and 95% for 100-mA to 15-A output loads. The optional use of a sense resistor allows the user to either increase efficiency by sensing the voltage drop across R_DS(ON) of the MOSFET or improve current limit accuracy by adding a sense resistor to the circuit. Likewise, the unit's inductor current is set by the R_DS(ON) of the synchronous MOSFET or by the optional sense resistor, and the inductor value is determined by the desired amount of ripple current and operating frequency.

The controller operates over a wide input range from 5 V to 36 V. Concurrently, the adjustable output voltages provide regulation from 0.6 V to a voltage very close to the input supply (V_IN × maximum duty cycle), with each output capable of delivering up to 20 A. The output accuracy is ±1% over the temperature range. Other features include adjustable cycle-by-cycle current limit, instant output overvoltage protection, optional short-circuit shutdown timer and power good output with 10-ms masking. The regulator current limit is user-programmable. Housed in a 5-mm × 5-mm QFN package, the LTC3708 is rated for operation from -40°C to 85°C. In 1,000-piece quantities, pricing starts at $4.80.

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