9.1 Thermal Regulation

When power is dissipated in an IC, a temperature gradient occurs across the IC chip affecting the individual IC circuit components. With an IC regulator, this gradient can be especially severe since power dissipation is large. Thermal regulation is the effect of these temperature gradients on output voltage (in percentage output change) per Watt of power change in a specified time. Thermal regulation error is independent of electrical regulation or temperature coefficient, and occurs within 5 ms to 50 ms after a change in power dissipation. Thermal regulation depends on IC layout as well as electrical design. The thermal regulation of a voltage regulator is defined as the percentage change of V_OUT, per Watt, within the first 10 ms after a step of power is applied. The LM137's specification is 0.02%/W, max.

	LM137, VOUT = −10V	VIN − VOUT = −40V
	Horizontal sensitivity,5 ms/div	IL = 0A → 0.25A → 0A

Figure 21.

In Figure 21, a typical LM137's output drifts only 3 mV (or 0.03% of V_OUT = −10V) when a 10W pulse is applied for 10 ms. This performance is thus well inside the specification limit of 0.02%/W × 10W = 0.2% max. When the 10W pulse is ended, the thermal regulation again shows a 3 mV step as the LM137 chip cools off. Note that the load regulation error of about 8 mV (0.08%) is additional to the thermal regulation error. In Figure 22, when the 10W pulse is applied for 100 ms, the output drifts only slightly beyond the drift in the first 10 ms, and the thermal error stays well within 0.1% (10 mV).

	LM137, VOUT = −10V	VIN − VOUT = −40V
	Horizontal sensitivity, 20 ms/div	IL = 0A → 0.25A → 0A

Figure 22.