WEITRON

http://www.weitron.com.tw

WT1117A

7/11

Rev-A 18-Oct-06

APPLICATION INFORMATION

Adjustable Operation

Stability Considerations

V

= V

X (

) + I

X R2

OUT

REF

Adj

R1 + R2

R1

V = I x ESR

R1

R2

Adj

WT1117A

Figure1. Resistor Divider Scheme

WT1117A linear regulators provide fixed and adjustable output voltages at currents up to 1.0A. These regulators

are protected against overcurrent conditions and include thermal shutdown protection. The WT1117A's have

a composite PNP-NPN output transistorand require an output capacitor for stability. A detailed procedure for

selecting this capacitor follows.

The WT1117A has an output voltage range of 1.25 V to 5.5 V. An external resistor divider sets the output voltage

as shown in Figure 1. The regulator maintains a fixed 1.25V (typical) reference between the output pin and the

adjust pin.

A resistor divider network R1 and R2 causes a fixed current to flow to ground. This current creates a voltage

across R2 that adds to the 1.25V across R1 and sets the overall output voltage. The adjust pin current

(typically 35μA) also flows through R2 and adds a small error that should be taken into account if precise

The output voltage is set according to the formula:

The term IAdj R2 represents the error added by the adjust pin current.

R1 is chosen so that the minimum load current is at least 2.0mA R1 and R2 should be the same type e.g.

metal film for best tracking over temperature. While not required, a bypass capacitor from the adjust pin to ground

will improve ripple rejection and transient response. A 0.1μF tantalum capacitor is recommended for first

cut design. Type and value may be varied to obtainoptimum performance vs. price.

The output compensation capacitor helps to determine three main characteristics of a linear regulator’s performance:

start-up delay,load transient response, and loop stability. The capacitor value and type is based on cost, availability,

size and temperature constrains, A tantalum or aluminum electrolytic capacitor is preferred, as a film or ceramic

but when the circuit operates at low temperatures, both the value and ESR of the capacitor will vary widelt.For optimum

performance over the full operating temperature range, a tantalum capacitor is best, A 22µF tantalum capacitor will work

fine in most applications, but with high current regulators such as the WT1117A higher capacitance values will improve

the transient response and stability. Most applications for the WT1117A’s involve large changes in load current, so the

output capacitor must supply instantaneous load current. The ESR of the output capacitor causes an immediate drop

in output voltage given by:

In microprocessor applications an output capacitor network of several tantalum and ceramic capacitors in parallel is

commonly used. This reduces overall ESR and minimizes the instantaneous output voltage drop under transient load

conditions. The output capacitor network should be placed as close to the load as possible for the best results.

Used with large output capacitance values and theinput voltage is instantaneously shorted to ground, damage can occur.

In this case, a diode connected as shown above in Figure1.