THERMAL MANAGEMENT
Pushing system performance
Keith Bassett talks us through the current and temperature monitoring available in the new 20V, 2.5A synchronous monolithic buck regulator from Linear Technology
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, utilise point-of-load regulators to maximise power chain efficiency while maintaining a small form factor. The LTC3626 is a synchronous, monolithic step-down regulator that has been designed for these operating environments, and is able to provide a flexible, highly efficient DC/DC conversion while occupying a very small footprint. The device is capable of supplying 2.5A of output current over an input voltage range of 3.6V to 20V from a tiny, 3mm ◊ 4mm, 20-pin QFN package. Its patented controlled on-time architecture yields transient response and enables high step-down ratios at high switching frequencies, so minimising board footprint.
increase the level of control with remarkably little extra design investment. Additionally, optional internal loop compensation is available to minimise 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 synchronisation.
Current monitor 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 makes it possible to monitor the
The current at the IMONOUT pin can be
measured directly or converted to a voltage by placing a resistor from the IMONOUT
pin to a voltage makes it easy to scale the output for digitisation 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, digitises the result for digital processing. The LTC3626 also features an easily
ground. Converting the output of the IMONOUT
programmed average output current limit. Specifically, it 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 IMONOUT
resultant voltage is 1.2V for the current at which the limit should be activated.
pin to to ground such that the
Figure 1. Output current monitor vs output current
The LTC3626 integrates a number of easy-to- use, but powerful, features that would normally require additional ICs and design time to implement. Specifically, with the addition of just a couple of passive components. It 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
Figure 2. Output current monitor error vs output current
supply current by producing a fraction of its average output current at its IMONOUT specifically, the current at the IMONOUT
pin; 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 –40ºC to 85ºC. Figure 2 shows the error between the actual average output current and the average output current as measured by the LTC3626.
6 CIE Power Supplement May 2013
LTC3626 produces an estimate of the average input current at the IMONIN current at the IMONIN
Similar to the average output current, the pin. That is, the
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 device configured to limit the average input
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