LM1085

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SNVS038G – JULY 1999 – REVISED MARCH 2013

APPLICATION NOTE

GENERAL

Figure 15 shows a basic functional diagram for the LM1085-Adj (excluding protection circuitry) . The topology is

basically that of the LM317 except for the pass transistor. Instead of a Darlingtion NPN with its two diode voltage

drop, the LM1085 uses a single NPN. This results in a lower dropout voltage. The structure of the pass transistor

is also known as a quasi LDO. The advantage a quasi LDO over a PNP LDO is its inherently lower quiescent

current. The LM1085 is ensured to provide a minimum dropout voltage 1.5V over temperature, at full load.

Figure 15. Basic Functional Diagram for the LM1085, excluding Protection circuitry

OUTPUT VOLTAGE

terminal. As shown in figure 2, this voltage is applied across resistor R1 to generate a constant current I1. This

constant current then flows through R2. The resulting voltage drop across R2 adds to the reference voltage to

sets the desired output voltage.

becomes negligible when R1 is in the 100

Ω range.

For fixed voltage devices, R1 and R2 are integrated inside the devices.

Figure 16. Basic Adjustable Regulator

STABILITY CONSIDERATION

Stability consideration primarily concern the phase response of the feedback loop. In order for stable operation,

the loop must maintain negative feedback. The LM1085 requires a certain amount series resistance with

capacitive loads. This series resistance introduces a zero within the loop to increase phase margin and thus

increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is

used to provide the appropriate zero (approximately 500 kHz).

The Aluminum electrolytic are less expensive than tantalums, but their ESR varies exponentially at cold

temperatures; therefore requiring close examination when choosing the desired transient response over

temperature. Tantalums are a convenient choice because their ESR varies less than 2:1 over temperature.

The recommended load/decoupling capacitance is a 10uF tantalum or a 50uF aluminum. These values will

assure stability for the majority of applications.

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