© 2014 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN25800

• Rev. 1.5

7

Circuit Description

The FAN25800 is a linear low-dropout (LDO) regulator

that has high PSRR and low output noise. The enable

control pin can be used to shut down the device and

disconnect the output load from the input. During

shutdown, the supply current drops below 1 µA. The

LDO is designed to be stable with space-saving ceramic

capacitors as small as 0201 case size.

Enable and Soft-Start

When EN is LOW, all circuits are off and the IC draws

<550 nA of current. The EN pin does not have an

internal pull-down resistor and must not be left floating.

threshold, the regulator begins a soft-start cycle for the

output. The soft-start cycle controls inrush current,

Short-Circuit and Thermal Protection

The output current is short-circuit protected. When an

output fault occurs, the output current is automatically

Short-circuit fault or output overload may cause the die

temperature to increase and exceed the maximum

rating

due

to

power

dissipation.

In

such

cases

die), the device may enter thermal shutdown.

If the die temperature exceeds the thermal shutdown

temperature threshold, the onboard thermal protection

disables the output until the temperature drops below its

hysteresis value. At that point, the output is re-enabled

and a new soft-start sequence occurs.

Thermal Considerations

For best performance, the die temperature and the

power dissipated should be kept at moderate values.

The maximum power dissipated can be evaluated

based on the following relationship:



















JA

A

J

D

T

T

P

(max)

(max)

(1)

surrounding PCB layout and can be improved by

providing a heat sink of surrounding copper ground.

The addition of backside copper with through-holes,

stiffeners, and other enhancements can help red

The heat contributed by the dissipation of devices

nearby must be included in design considerations.

Capacitor Selection

An output capacitor with an effective capacitance

between 400 nF and 15 µF is required for loop stability.

The ESR value should be within 3 to 100 m

. DC bias

characteristics of the capacitors must be considered

when selecting the voltage rating and the case size of

the capacitor. Figure 16 is a typical derating curve for a

0201 case size, 1.5 µF, 6.3 V, X5R capacitor.

Figure 16. Capacitor DC Bias Characteristics

Typical Application for Post Regulation

Due to its high PSRR and low output noise, the

FAN25800 can be used as a post-DC-DC regulator to

reduce output ripple and output noise at high efficiency

for noise-sensitive applications. Figure 17 shows a post-

DC-DC regulation of the LDO with a buck converter.

The capacitor on the output of the buck converter can

be shared by the LDO as its input capacitor.

DC-DC

Converter

FAN25800

EN

EN

L

C2

C1

C3

VIN

VOUT

3.6 V

3.3 V

5.0 V

VIN

SW

GND

GND

Figure 17. LDO as Post DC-DC Regulator

PCB Layout Recommendations

Capacitors should be placed as close to the IC as

possible. All power and ground pins should be routed to

their capacitors using top copper. The copper area

connecting to the IC should be maximized to improve

thermal performance.

Figure 18. Recommended Layout