LTC3626 can produce a voltage proportional to its own chip temperature

Introduction

LTC3626 Synchronous buck regulator with current and temperature monitoring function is the first device of the linear voltage regulator switcher+ product line. The device is a high-efficiency, monolithic synchronous buck type switching regulator, which can use 3.6V to 20V input voltage to provide 2.5A maximum output current (circuit shown in Figure 1). LTC3626 uses a unique, controlled conduction time/constant frequency and current mode architecture, which makes it ideal for low duty ratio applications and high-frequency operations, and can respond quickly to load transients (see Figure 2). In addition, this device also has the function of pattern setting, tracking and synchronization. The LTC3626 3mm x 4mm Package has a very low thermal impedance and can operate without an external heatsink even when the maximum power is delivered to the load.

Figure 2: Load step response of the circuit shown in Figure 1

In addition to its excellent voltage-stabilizing capability, LTC3626 's current and temperature monitoring functions are more prominent. They can provide monitoring and control capabilities with minimal additional components.

Output/input Current detection

The LTC3626 detects the output current through a synchronous switch in the conduction time of the switch, and produces a proportional current on the imonout pin (in proportion to 1/16000). In Fig. 3, the accuracy of the imonout output is shown by comparing the measured output of the imonout pin with the calculated value. In most of the output current range, the error is kept below 1% level.

Similarly, by combining this same detection current signal with the duty ratio of a step-down regulator, a current proportional to the input current is generated on the Imonin pin (also in proportion to 1/16000). A greater than 5% accuracy is achieved within a wide current range (see Figure 4).

Two current signals are connected to the 1.2V internal voltage amplifier, which can turn off the device when triggering. Therefore, the input and output current limits can be set by simply connecting a resistor to a imonin or imonout pin (as shown in Figure 1). The relationship between the current limiting value and the resistor is:

For example: A 10k resistor will set a current limit of about 2 a.

This simple scheme provides monitoring and active control of input and output current limits, which can be achieved through external control circuitry (e.g., a DAC and a small number of passive components).

Figure 3: Relationship between output current and current of output current monitor

Figure 4: Relationship between input current and current in input current monitor

Temperature test

LTC3626 can produce a voltage proportional to the temperature of its own chip, which can be used to set the maximum temperature limit. At room temperature, the voltage on the temperature monitor pin (Tmon) is usually 1.5V. If you want to compute the chip temperature TJ, you can multiply the Tmon voltage with the 200°k/v temperature monitor voltage to the temperature conversion factor and subtract the 273°c offset. In addition, the LTC3626 also has a temperature limit comparator, which is set by the temperature limit value of the pin Tset and Tmon pin for its feed signal. Thus, by applying a voltage to the tset pin, the maximum temperature limit can be set according to the following formula:

Select a 125°C maximum temperature limit equivalent to the Tset pin on the set value of about 2V, IC will be at chip temperature TJ reached the limit immediately shut off.

Conclusion

LTC3626 integrates current and temperature monitoring functions with a high-performance step-down regulator in a compact package. A microprocessor or other external control logic circuit can be monitored by an easy-to-use input and output current and a temperature monitor pin, and the device is able to turn off itself by setting a threshold voltage on the temperature setting threshold pin.