AX1117-5 Ver la hoja de datos (PDF) - Unspecified

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AX1117-5

1.2A Low Dropout Positive Voltage Regulator

Unspecified 

Page No. : 4/7

Applications Description

• Output Voltage Adjustment

Like most regulators, AX1117 series regulate the output by

comparing the output voltage to an internally generated reference

voltage. On the adjustable version, the VREF is available externally as

V IN

1.25V between VOUT and ADJ. The voltage ratio formed by R1 and R2

should be set to conduct 10mA (minimum output load). The output

voltage is given by the following equation: VOUT =VREF (1+R2/R1) + IADJ

R2

On fixed versions of AX1117 series, the voltage divider is provided

internally.

• Thermal Protection

In

Out

ADJ

IADJ

10uA

V REF

V OUT

R1

R2

AX1117 series have thermal protection which limits junction

temperature to 150°C. However, device functionality is only

guaranteed to a maximum junction temperature of +125°C.

The power dissipation and junction temperature for AX1117 in all

packages given by

PD=(VIN - VOUT) IOUT, TJUNCTION=TAMBIENT+(PDxθJA), Note: TJUNCTION must

not exceed 125°C

VOUT =VREF (1+R2/R1) + IADJ R2

• Current Limit Protection

AX1117 series are protected against overload conditions. Current protection is triggered at typically 1.5A.

• Stability And Load Regulation

AX1117 series require a capacitor from VOUT to GND to provide

compensation feedback to the internal gain stage. This is to ensure

stability at the output terminal. Typically, a 10uF tantalum or 50uF

RP

Parasitic

aluminum electrolytic is sufficient.

Note : It is important that the ESR for this capacitor does not exceed

0.5Ω.

Line Resistance

In

Out

V IN

Adj

The output capacitor does not have a theoretical upper limit and

increasing its value will increase stability. COUT = 100 uF or more is

typical for high current regulator design.

AX1117 series load regulation are limited by the resistance of the

R1

Connect

R1 to Case

RL

R2

wire connecting it to the load(RP). For the adjustable version, the best

load regulation is accomplished when the top of the resistor divider(R1)

is connected directly to the output pin of the AX1117 series. When so

connected, RP is not multiplied by the divider ratio. For fixed output

versions, the top of R1 is internally connected to the output and

ground pin can be connected to low side of the load as a negative

Connect

R2 to Load

side sense if, so desired.

• Thermal Consideration

The AX1117 series contain thermal limiting circuitry designed to protect itself for over-temperature conditions. Even for normal load

conditions, maximum junction temperature ratings must not be exceeded. As mention in thermal protection section, we need to

consider all sources of thermal resistance between junction and ambient. It contains junction-to-case, case-to-heat-sink interface and

heat sink resistance itself. An additional heat sink is applied externally sometimes. It can increase the maximum power dissipation. For

example, the equivalent junction temperature of 300mA output current is 115°C without external heat sink. Under the same junction

temperature IC can operates 500mA with an adequate heat sink. Therefore, to attach an extra heat sink is recommended.

Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. The bonding

wires are appending paths. The former is the lowest resistance path. Proper mounting is required to ensure the best possible thermal

flow this area of the package to the heat sink. Thermal compound at the case-to-heat-sink interface is strongly recommended. The

case of all devices in this series is electrically connected to the output. Therefore, if the case of the device must be electrically isolated,

a thermally conductive spacer can be used, as long its thermal resistance is considered.

• Protection Diode

(The figure is shown as Regulator with Reverse Diode Protection in advanced applications)

In general operation, AX1117 series don’t need any protection diodes. From the cross-section structure of AX1117 sries, the output

pin is connected to P+ substrate, and the input pin is connected to N- well. There is a parasitic reverse diode between them. It can

handle microsecond surge currents of 5A to 10A. Even with large output capacitance, it is very difficult to get those values of surge

currents in normal operation. Only with high value output capacitors, such as 1000uF. And with the input pin instantaneously shorted to

ground. can damage occur. A crowbar circuit at the input of the AX1117 series can generate those kinds of currents, and a diode from

output to input is recommended. Normal power supply cycling or even plugging and unplugging in the system will not generate

currents large enough to do any damage.

AVANTICS - AX1117 Series

Specification No.: 050309-FE0005

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