IL1084-1.8BT2 View Datasheet(PDF) - IK Semicon Co., Ltd

Part Name

Description

Manufacturer

IL1084-1.8BT2

5A LOW DROPOUT POSITIVE REGULATOR

IK Semicon Co., Ltd 

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.

OUTPUT VOLTAGE

FIGURE 1. Basic Functional Diagram for the IL1084,

excluding Protection circuitry

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.

STABILITY CONSIDERATION

FIGURE 2. Basic Adjustable Regulator

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

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