FAN2560 Просмотр технического описания (PDF) - Fairchild Semiconductor

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FAN2560

350mA Low-VIN LDO with Fast Transient Response

Fairchild Semiconductor 

Application Information

ENABLE Latch

A pull-down resistor latches the LOW state of the EN

input after this input is externally forced LOW. A low-

side switch turns ON a 370 kΩ pull-down resistance to

keep the EN in LOW state, even if the EN input is

subsequently left floating.

Soft-Start

A soft-start function prevents an excessive input current

flow during start-up. When the LDO is enabled, the soft-

start circuit limits the peak inrush current below the

specified maximum value, which increases when COUT

increases. To further reduce the peak inrush current,

the output capacitance may be lowered to 1µF, taking

advantage of FAN2560 stability over a wide range of

COUT capacitance.

Short-Circuit and Thermal Protection

The FAN2560 output current voltage characteristic has

a fold-back shape that indicates a short-circuit current

limit lower than the maximum load current. Although the

short-circuit current is limited to below 500mA, the

device can supply high peak output currents of up to 1A

for brief periods. However, this output overload may

cause the die temperature to increase and exceed

maximum ratings due to power dissipation. In such

cases, depending upon the ambient temperature, VIN,

load current, and the junction-to-air thermal resistance

(θJA) of the die, the device may enter thermal shutdown.

During output overload conditions, when the die

temperature exceeds the shutdown limit temperature of

150°C, the onboard thermal protection disables the

output until the temperature drops below this limit, at

which point the output is re-enabled.

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:

PD(max)

=

⎧

⎨

⎩

TJ(max) −

Θ JA

TA

⎫

⎬

⎭

(1)

where TJ(max) is the maximum allowable junction

temperature of the die, which is 125°C, and TA is the

ambient operating temperature. θJA is dependent on the

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 also aid in

reducing θJA. The heat contributed by the dissipation of

other devices located nearby must be included in

design considerations.

Reverse Current Path

During normal operation, VIN is higher than VOUT and

the parasitic diode for the series power FET is reverse

biased. If the output voltage is externally forced above

the input voltage, the parasitic diode gets forward

biased and starts to conduct. In this case, it is

necessary to limit the reverse current to maximum

100mA to avoid adversely affecting reliability.

Capacitors Selection

The FAN2560 is stable with a wide range of ceramic

output capacitors. An output capacitor of at least 0.7µF

effective capacitance and the minimum ESR over the

frequency range of 3 to 300mΩ is required to ensure

stability over the full range of supply voltages and load

currents. High-ESR tantalum or electrolytic capacitors

may be used, but a low ESR ceramic capacitor has to

be connected in parallel at the output, at a distance no

more than 1-inch from the VOUT pin. The MLCC

capacitors indicated in Table 1 have been successfully

tested with the FAN2560.

Table 1. Recommended Capacitors

Capacitance

1μF

2.2μF

2.2μF

Size

0603

0603

0402

Vendor

MURATA

MURATA

MURATA

Part number

GRM188R71C105KA120

GRM188R61A225KF340

GRM155R60J225ME15

© 2006 Fairchild Semiconductor Corporation

FAN2560 • Rev. 1.0.1

8

www.fairchildsemi.com

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