Considerations for the thermal resistance of ldos

In general design, the consideration of the thermal resistance parameter is often ignored. However, this parameter has a great impact on the power supply system, because if you choose this parameter improperly, it will easily lead to a high power consumption and overheating of the chip and entering the hot protection status, power failure.

Recently, in the selection of a system's power chip (the input of the system is 3.3 V and the voltage of 1. 8 V is required. The maximum current required for this 1. 8 V is about 250mA ), to reduce the effect of ripple on the system performance, a sipex (SP6205EM5-ADJ, Sot-23-5).

The main parameters of the chip are: 2.7v ~ 5 V voltage input range; 500mA current output; adjustable output voltage; with throttling and thermal protection; etc.

At first glance, the power supply chip can meet the system requirements, 250mA of the required current with 500mA is enough balance. But he discussed with his colleagues that the chip could not reach 500mA of the output current because of its high thermal resistance. So I began to read the datasheet chip in detail and found that the problem exists.

 

The following is an example of the Sot-23-5 Package of the chip.

Thermal Resistance:Sot-23-5(Qja):191℃/W

DFN-8 (qja): 59 ℃/W

Maximum power consumption:Pd (max) = (TJ (max)-Ta)/θ ja, TJ indicates the node temperature, TA indicates the environment temperature, and θ ja indicates the thermal resistance.

When the chip exceeds the maximum allowable power consumption, the node temperature will be too high, so that the chip enters the hot protection mode.

 

The maximum power consumption of SP6205-ADJ (SOT-23-5 Package) is:

Pd (max) = (125 ℃-25 ℃)/(191 ℃/W) = 523 mW

In actual operation, the chip power consumption is:

Pd = (VIN-Vout) * Iout+ Vin * ignd (1)

Generally, ignd is UA level (for example, ignd = 500mA when sp6205 is output at 35mA). If the requirement is not particularly accurate, the effects of VIN * ignd can be ignored.

 

Therefore, the actual power consumption of the chip must be limited to the maximum allowable power consumption range. If the power consumption exceeds the PD (max), the chip enters the hot protection mode.

According to formula (1), we can calculate the following parameters:

(A) obtain the maximum output current when the input/output pressure difference is known.

For example, if the input voltage is 5 V and the output voltage is 3 V, then:

523 Mw = (5 V-3 V) * I (load (max) + 5 V * 35mA,

Then I (load (max) = 260. 6mA.

 

(B) When the load current is known, the maximum pressure difference allowed by the input and output is obtained.

For example, if the output voltage is 3 V and the load current is 400mA, then:

523 Mw = (VIN-3 V) * 400mA + Vin * 35mA,

Then VIN (max) = 4.3 V, △vmax = 1.3 V.

 

Therefore, the thermal resistance parameter should be carefully considered based on the actual needs during the selection of the driver.

Back to the system requirements mentioned at the beginning of this article, the actual maximum output current calculated is: I (load (max) = 347mA. Compared with the output current of 500mA on the chip, the current margin is greatly reduced. Moreover, the maximum current is calculated at an ideal ambient temperature of 25 ℃. As the operating time goes on, the temperature of the chip will gradually increase, the maximum output current is further reduced. The output current of 500mA is available only when the input/output pressure is less than 1 V and the room temperature is 25 ℃.

From the above analysis, it can be seen that the thermal resistance parameter of the ldos has a great impact on other parameters (such as output current and input/output pressure difference) and must be considered. When selecting the type, we should try to choose a chip or encapsulation with low thermal resistance, especially different packages of the same type of chip, the thermal resistance will be greatly different, such as the above DFN-8 encapsulation of the power chip sp6205, the thermal resistance is only 59 ℃/W, which is much smaller than the 191 ℃/W package of Sot-23-5.