Tom Gross, Linear Technology
Design Note 511
Introduction
The LTC3626 synchronous buck regulator with current and temperature monitoring is the first of Linear’s SWITCHER+™ line of monolithic regulators. It is a high efficiency, monolithic synchronous step-down switching regulator capable of delivering a maximum output current of 2.5 A from an input voltage ranging from 3.6 V to 20 V (circuit shown in Figure 1). The LTC3626 employs a unique controlled on-time/constant-frequency, current-mode architecture, making it ideal for low duty cycle applications and high frequency operation, while yielding fast response to load transients (see Figure 2). It also features mode setting, tracking and synchronization capabilities. The LTC3626’s 3 mm × 4 mm package has such low thermal impedance that it can operate without an external heat sink even while delivering maximum power to the load.
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| Figure 1. | 20 V Maximum Input, 2.5 A, 2 MHz Buck Regulator with Current and Temperature Monitoring. |
Beyond its impressive regulator capabilities, the LTC3626’s current and temperature monitoring functions stand out. They offer both monitoring and control capabilities with minimal additional components.
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| Figure 2. | Load Step Response for Figure 1 Circuit. |
Output/Input Current Sensing
The LTC3626 senses the output current through the synchronous switch during the switch’s on-time and generates a proportional current (scaled to 1/16000) at the IMONOUT pin. Figure 3 shows the accuracy of the IMONOUT output by comparing the measured output of the IMONOUT pin with calculated values. Error remains less than 1% over most of the output current range.
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| Figure 3. | Output Current vs Output Current Monitor |
Likewise, this same sense current signal is combined with the buck regulator’s duty cycle to produce a current proportional to the input current – again by 1/16000 – at the IMONIN pin. A precision of better than 5% is achieved over a wide current range (see Figure 4).
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| Figure 4. | Input Current vs Input Current Monitor |
Both current signals are connected to internal voltage amplifiers, referenced to 1.2 V, that can shut down the part when tripped. So the input and output current limits are set by simply connecting a resistor to the IMONIN or IMONOUT pins, respectively, as shown in Figure 1. The relationship between the current limit and the resistor is:
For example, a 10 k resistor sets a current limit of approximately 2 A.
This simple scheme allows both monitoring and active control of the input and output current limits – the latter can be implemented via external control circuitry, such as a DAC with a few passive components.
Temperature Sensing
The LTC3626 generates a voltage proportional to its own die temperature, which can be used to set a maximum temperature limit. The voltage at the temperature monitor pin (TMON) is typically 1.5 V at room temperature. To calculate the die temperature, TJ, multiply the TMON voltage by the temperature monitor voltage-to-temperature conversion factor of 200 °K/V, and subtract the 273 °C offset. The LTC3626 also has a temperature limit comparator fed by the temperature limit set pin, TSET, and the TMON pin. Hence, by applying a voltage to the TSET pin, a maximum temperature limit can be set according to the following:
Choosing a maximum temperature limit of 125 °C equates to an approximate 2 V setting on the TSET pin – the IC will shut down once the die temperature TJ reaches this limit.
Conclusion
The LTC3626 combines current and temperature monitoring capabilities with a high performance buck regulator in a compact package. A microprocessor or other external control logic can supervise conditions via easy-to-use input and output current and temperature monitor pins, and it can shut itself down by setting a threshold voltage on the temperature set limit pin.
