Controller Expands Power Management Capabilities

Jan 1, 2005 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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Voltage sequencing, tracking and margining are becoming popular features in dc-dc converter modules, but these functions are less commonly found in dc-dc controller chips. A synchronous step-down controller just introduced by Linear Technology incorporates all three of these power management functions. The new chip, the LTC3770, also is distinguished by its fast transient response, a PLL that allows synchronization to a system clock and a highly accurate reference. Capable of operating from a 4-V to 32-V input, the current-mode controller produces a stepped-down output at currents up to about 25 A.

The LTC3770 provides up and down tracking of its output voltage via its tracking and soft-start input. When the step-down controller is configured to be the master of two outputs, a capacitor to ground at this pin sets the ramp rate for the output voltage (see the figure). When the IC is configured to be the slave of two outputs, the V_FB voltage of the master IC is reproduced by a resistor divider and applied to this pin.

Meanwhile, margining is a resistor-programmable function. A resistor placed between the programmable margining input pin and ground sets the margining current. This current multiplied by the resistor between the VREFOUT and VREFIN pins determines the margining voltage offset. In addition, the MSB and LSB logic inputs for the margining function together determine whether the IC is in margin high, margin low or no margin state.

The LTC3770's constant on-time, valley current-mode architecture, combined with a very low minimum on-time (50 ns typ.), allows the control loop to respond quickly to load steps. The LTC3770 can perform R_DS(on) current sensing by sensing the voltage drop across the synchronous power MOSFET. For systems requiring more precise control of output current, a conventional sense resistor can be used in the source of the lower MOSFET. In either case, the current limit is user-programmable.

The controller can operate at very low duty cycles. In the extreme case, an output as low as 0.6 V can be supplied from input voltages as high as 32 V, even under no-load conditions. In the applications where such low output voltages are required, the precision of the chip's voltage reference becomes significant. The LTC3770 specifies an output voltage accuracy of ±0.5% at room temp and ±0.67% from 0°C to 85°C. Across the device's full operating temperature range (-40°C to 85°C), the total error on output voltage is still only ±1%.

The controller's operating frequency is selected by a single external resistor. For applications that require constant-frequency operation, a phase lock loop allows the LTC3770 to be synchronized to an external clock, which can reduce EMI.

The LTC3770 is considered a successor to Linear's LTC1778 step-down controller introduced a few years ago. There are a several differences between the two ICs, including faster transient response, and the addition of on-chip margining and tracking in the LTC3770. In addition, the new chip adds the PLL synchronization capability.

The LTC3770 is offered in a 5-mm × 5-mm QFN and in a 28-lead SSOP. In quantities of 1000, unit pricing starts at $3.55.

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