IL1084-xx
Application Note
GENERAL
Figure 1 shows a basic functional diagram for the IL1084-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 IL1084
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 IL1084 is guaranteed to
provide a minimum dropout voltage 1.5V over temperature, at full load.
FIGURE 1. Basic Functional Diagram for the IL1084,
excluding Protection circuitry
OUTPUT VOLTAGE
The IL1084 adjustable version develops at 1.25V reference voltage, (VREF), between the output and the adjust 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. The current IADJ from the adjustment terminal introduces an output error . But since it is small (120uA max), it
becomes negligible when R1 is in the 100Ω range. For fixed voltage devices, R1 and R2 are integrated inside the
devices.
FIGURE 2. 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 IL1084 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.
The adjustable versions allows an additional capacitor to be used at the ADJ pin to increase ripple rejection. If this is
done the output capacitor should be increased to 22uF for tantalums or to 150uF for aluminum.
Capacitors other than tantalum or aluminum can be used at the adjust pin and the input pin. A 10uF capacitor is a
reasonable value at the input. See Ripple Rejection section regarding the value for the adjust pin capacitor.
It is desirable to have large output capacitance for applications that entail large changes in load current (microprocessors
for example). The higher the capacitance, the larger the available charge per demand. It is also desirable to provide low
ESR to reduce the change in output voltage:
ꢀV = ꢀI x ESR
Rev. 04
IKSEMICON [ IK SEMICON CO., LTD ]
