ADP1720

Rev. A | Page 11 of 16

APPLICATIONS INFORMATION

CAPACITOR SELECTION

Output Capacitor

The ADP1720 is designed for operation with small, space-saving

ceramic capacitors, but it functionswith most commonly used

capacitors as long as care is taken about the effective series

resistance (ESR) value. The ESR of the output capacitor affects

stability of the LDO control loop. A minimum of 1 μF capacitance

with an ESR of 500 mΩ or less is recommended to ensure sta-

bility of the ADP1720. Transient response to changes in load

current is also affected by output capacitance. Using a larger

value of output capacitance improves the transient response of

the ADP1720 to large changes in load current. Figure 20 and

Figure 21 show the transient responses for output capacitance

values of 1 μF and 10 μF, respectively.

TIME (2µs/DIV)

1

LOAD STEP FROM 2.5mA TO 47.5mA

Figure 20. Output Transient Response, 1 μF

TIME (2µs/DIV)

1

LOAD STEP FROM 2.5mA TO 47.5mA

Figure 21. Output Transient Response, 10 μF

Input Bypass Capacitor

Connecting a 1 μF capacitor from IN to GND reduces the cir-

cuit sensitivity to printed circuit board (PCB) layout, especially

when encountering long input traces or high source impedance.

If greater than 1 μF of output capacitance is required, it is

recommended that the input capacitor be increased to match it.

Input and Output Capacitor Properties

Any good quality ceramic capacitors can be used with the

ADP1720, as long as they meet the minimum capacitance and

maximum ESR requirements. Ceramic capacitors are manufac-

tured with a variety of dielectrics, each with different behavior

over temperature and applied voltage. Capacitors must have a

dielectric adequate to ensure the minimum capacitance over

the necessary temperature range and dc bias conditions. X5R

or X7R dielectrics with a voltage rating of 6.3 V or 10 V are

recommended for the output capacitor. X5R or X7R dielectrics

with a voltage rating of 50 V or higher are recommended for the

input capacitor.

Y5V and Z5U dielectrics are not recommended, due to their

poor temperature and dc bias characteristics.

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

Current limit and thermal overload protection circuits on the

ADP1720 protect the part from damage caused by excessive power

dissipation. The ADP1720 is designed to current limit when

the output load reaches 90 mA (typical). When the output

load exceeds 90 mA, the output voltage is reduced to maintain

a constant current limit.

Thermal overload protection is included, which limits the junction

temperature to a maximum of 150°C (typical). Under extreme

conditions (that is, high ambient temperature and power dissipa-

tion), when the junction temperature starts to rise above 150°C,

the output is turned off, reducing the output current to zero.

When the junction temperature drops below 135°C, the output is

turned on again, and output current is restored to its nominal value.

Consider the case where a hard short from OUT to GND occurs.

At first, the ADP1720 current limits so that only 90 mA is

conducted into the short. If self-heating of the junction is

great enough to cause its temperature to rise above 150°C,

thermal shutdown activates, turning off the output and

reducing the output current to zero. As the junction

temperature cools and drops below 135°C, the output turns on

and conducts 90 mA into the short, again causing the junction

temperature to rise above 150°C. This thermal oscillation

between 135°C and 150°C causes a current oscillation between

90 mA and 0 mA, which continues as long as the short

remains at the output.

Current and thermal limit protections are intended to protect

the device against accidental overload conditions. For reliable

operation, device power dissipation must be externally limited

so that junction temperatures do not exceed 125°C.