WT1117A
APPLICATION INFORMATION
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.
Adjustable Operation
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
adjustment of V
is necessary.
OUT
The output voltage is set according to the formula:
R1 + R2
V
= V
X (
) + I
X R2
OUT
REF
Adj
R1
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.
WT1117A
V
V
V
V
IN
OUT
OUT
IN
V
Adj
C
C
2
REF
R1
R2
1
I
Adj
C
Adj
Figure1. Resistor Divider Scheme
Stability Considerations
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
capacitor with almost zero ESR can cause Instability. An aluminum electrolytic capacitor is the least expensive type.
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:
V = I x ESR
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.
WEITRON
http://www.weitron.com.tw
Rev-A 18-Oct-06
7/11
