LM7332

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SNOSAV4B – APRIL 2008 – REVISED JANUARY 2016

Device Functional Modes (continued)

7.4.2 Output Voltage Swing Close to V−

The output stage design of the LM7332 allows voltage swings to within millivolts of either supply rail for

maximum flexibility and improved useful range. Because of this design architecture, with output approaching

either supply rail, the output transistor collector-base junction reverse bias decreases. With output less than a Vbe

from either rail, the corresponding output transistor operates near saturation. In this mode of operation, the

transistor exhibits higher junction capacitance and lower ft which reduces phase margin. With the Noise Gain

(NG = 1 + RF/RG, RF and RG are external gain setting resistors) of 2 or higher, there is sufficient phase margin

that this reduction in phase margin is of no consequence. However, with lower Noise Gain (<2) and with less

than 150 mV to the supply rail, if the output loading is light, the phase margin reduction could result in unwanted

oscillations.

In the case of the LM7332, due to inherent architectural specifics, the oscillation occurs only with respect to the

output transistor at V− when output swings to within 150 mV of V−. However, if this output transistor's collector

current is larger than its idle value of a few microamps, the phase margin loss becomes insignificant. In this case,

300

μA is the required collector current of the output transistor to remedy this situation. Therefore, when all the

aforementioned critical conditions are present at the same time (NG < 2, VOUT < 150 mV from supply rails and

output load is light) it is possible to ensure stability by adding a load resistor to the output to provide the output

transistor the necessary minimum collector current (300

μA).

For 12-V (or ±6-V) operation, for example, add a 39-k

Ω resistor from the output to V+ to cause 300-µA output

sinking current and ensure stability. This is equivalent to about 15% increase in total quiescent power dissipation.

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