RT9176 Ver la hoja de datos (PDF) - Richtek Technology

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RT9176

1.5A Fixed and Adjustable Low Dropout Positive Voltage Regulators

Richtek Technology 

Application Information

Output voltage adjustment

Like most regulators, the RT9176 regulates the output by

comparing the output voltage to an internally generated

reference voltage. On the adjustable version as shown in

Figure 4, the VREF is available externally as 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:

R2

VOUT = VREF (1+ R1 ) + IADJ R2

On fixed versions of RT9176, the voltage divider is provided

internally.

VIN

10uF

RT9176

VIN

VOUT

ADJ

IADJ

65uA

VREF R1

CADJ

R2

VOUT

10uF

RT9176

Output Capacitor

RT9176 requires 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, 10μF

tantalum or 50μF aluminum electrolytic is sufficient and

please takes the notice of output capacitor ESR range

(Figure 5).

The output capacitor does not have a theoretical upper

limit and increasing its value will increase stability.

COUT = 100μF or more is typical for high current regulator

design.

Region of Stable COUT ESR vs. Load Current

10

COUT = 100uF

Instable

COUT = 10uF

1

Stable

Figure 4. Basic Adjustable Regulator

Input Bypass Capacitor

An input capacitor is recommended. A 10μF tantalum on

the input is a suitable input bypassing for almost all

applications.

Adjust Terminal Bypass Capacitor

The adjust terminal can be bypassed to ground with a

bypass capacitor (CADJ) to improve ripple rejection. This

bypass capacitor prevents ripple from being amplified as

the output voltage is increased. At any ripple frequency,

the impedance of the CADJ should be less than R1 to

prevent the ripple from being amplified:

(2p * fRIPPLE * CADJ) < R1

The R1 is the resistor between the output and the adjust

pin. Its value is normally in the range of 100-200Ω. For

example, with R1 = 124Ω and fRIPPLE = 120Hz, the CADJ

should be > 11μF.

0.1

Instable

0.01

0

0.2

0.4

0.6

0.8

1

Load Current (A)

Figure 5

Load Regulation

When the adjustable regulator is used (Figure 6), the best

load regulation is accomplished when the top of the resistor

divider (R1) is connected directly to the output pin of the

RT9176. When so connected, RP is not multiplied by the

divider ratio. For Fixed output version, the top of R1 is

internally connected to the output and ground pins can be

connected to low side of the load.

DS9176-11 March 2007

www.richtek.com

9

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