How to get best use from step-down voltage regulator

Increases in digital IC integration, coupled with advances in printed circuit board layout and assembly techniques, continue to push system performance and power density higher. Many of these systems, powered from a 12V rail or battery stack, utilize point-of-load regulators to maximize power chain efficiency while maintaining a small form factor. The LTC3626 synchronous, monolithic step-down regulator is suited for these operating environments, given its ability to provide a flexible DC/DC conversion while occupying a very small footprint.

The LTC3626 can supply 2.5A of output current over an input voltage range of 3.6V to 20V from a 3mm × 4mm, 20-pin QFN package. Its patented controlled on-time architecture yields outstanding transient response and enables high step-down ratios at high switching frequencies, minimizing board footprint.

The LTC3626 integrates easy-to-use features that would normally require additional ICs and design time to implement. Specifically, with the addition of just a couple of passive components, the LTC3626 can be configured to provide accurate measures of its output current, input current, and on-die temperature. It can be just as easily programmed to limit each measured parameter.

These built-in features expand the designer’s insight into the performance of the system and increase the level of control with remarkably little extra design investment. Additionally, optional internal loop compensation is available to minimize the design effort.

The LTC3626 also includes user-selectable Burst Mode operation or forced-continuous mode, resistor-programmable switching frequencies from 500kHz to 3MHz, power good status output, output tracking capability, and external clock synchronization.

Current monitor and limit

One way to measure the overall performance of a system is to monitor the current at the output of the power supply. Supply current monitoring also informs designers if downstream ICs are operating as expected—useful in design and debug, and during normal operation.

The LTC3626 enables to monitor the supply current by producing a fraction of its average output current at its IMON_OUT pin; specifically, the current at the IMON_OUT pin is equal to the average output current divided by 16,000.

Figure 1 shows the typical performance of the output current measurement for an ambient temperature range of –40°C to 85°C. Figure 2 shows the error between the actual average output current and the average output current as measured by the LTC3626.



The current at the IMON_OUT pin can be measured directly or converted to a voltage by placing a resistor from the IMON_OUT pin to ground. Converting the output of the IMON_OUT pin to a voltage makes it easy to scale the output for digitization via a microcontroller or stand-alone ADC. Figure 3 shows the LTC3626 configured to run with the output current monitor activated while the LTC2460, 16-bit ADC, digitizes the result for digital processing.


The LTC3626 also features an easily programmed average output current limit. Specifically, the LTC3626 contains an on-chip current limit amplifier with a reference of approximately 1.2V. To program an average output current, simply size the resistor from IMON_OUT to ground such that the resultant voltage is 1.2V for the current at which the limit should be activated.

Similar to the average output current, the LTC3626 produces an estimate of the average input current at the IMON_IN pin. That is, the current at the IMON_IN pin is an estimate of the average input current divided by 16,000. Just like the average output current, the LTC3626 offers a simple mechanism to program a limit for the average input current. This feature is useful for applications that must limit the average current drawn from the input supply. Figure 4 shows the LTC3626 configured to limit the average input current to 475mA while producing an output voltage of 2.5V from a 5V input voltage.