3A Low-Voltage Low-Dropout Regulator

LM39300/39301/39302

HTC

7

APPLICATION INFORMATION

The LM39300/1 is a high-performance low-dropout voltage regulator suitable for moderate to high-current voltage

regu-lator applications. Its 500mV dropout voltage at full load makes it especially valuable in battery-powered

systems and as a high-efficiency noise filter in post-regulator applications. Unlike older NPN-pass transistor

designs, where the mini-mum dropout voltage is limited by the base-to-emitter voltage drop and collector-to-

saturation voltage.A trade-off for the low dropout voltage is a varying base drive requirement. The LM39300/1/2

regulator is fully protected from damage due to fault conditions. Current limiting is provided. This limiting is linear

output current during overload conditions is constant. Thermal shutdown disables the device when the die

temperature exceeds the maximum safe operating tem-perature. Transient protection allows device (and load) sur-

vival even when the input voltage spikes above and below nominal. The output structure of these regulators allows

voltages in excess of the desired output voltage to be applied without reverse current flow.

Thermal Design

Linear regulators are simple to use. The most complicated design parameters to consider are thermal

characteristics.Thermal design requires four application-specific param-eters:

Calculate the power dissipation of the regulator from these numbers and the device parameters from this

datasheet,where the ground current is taken from the data sheet.

The heat sink thermal resistance is determined by:

where TJ (max) 125 ℃ and

The heat sink may be significantly reduced in applications where the minimum input voltage is known and is large

compared with the dropout voltage. Use a series input resistor to drop excessive voltage and distribute the heat

between this resistor and the regulator. The low dropout properties of Taejin regulators allow signifi-cant reductions

in regulator power dissipation and the asso-ciated heat sink without compromising performance. When this

technique is employed, a capacitor of at least 1.0F is needed directly between the input and regulator ground.Refer

to Application Note 9 for further details and examples on thermal design and heat sink specification.

Output Capacitor

The LM39300/1/2 requires an output capacitor to maintain stability and improve transient response. Proper capacitor

selection is important to ensure proper operation. The LM39300/1/2 output capacitor selection is dependent upon

the ESR (equivalent series resistance) of the output capacitor to maintain stability. When the output capacitor is 47F

or greater, the output capacitor should have less than 1 of ESR. This will improve transient response as well as

promote stability. Ultralow ESR capacitors, such as ceramic chip capacitors may promote instability. These very low

ESR levels may cause an oscillation and/or underdamped tran-sient response. A low-ESR solid tantalum capacitor

works extremely well and provides good transient response and stability over temperature. Aluminum electrolytics can

also be used, as long as the ESR of the capacitor is < 1.The value of the output capacitor can be increased without

limit. Higher capacitance values help to improve transient response and ripple rejection and reduce output noise.

Input Capacitor

An input capacitor of 1F or greater is recommended when the device is more than 4 inches away from the bulk ac

supply capacitance, or when the supply is a battery. Small, surface-mount, ceramic chip capacitors can be used for

the bypass-ing. Larger values will help to improve ripple rejection by bypassing the input to the regulator, further

improving the integrity of the output voltage.Transient Response and 3.3V.

Fig 1. Capacitor Requirements