ILC7281CS-XXXX 查看數據表（PDF） - Impala Linear Corporation

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ILC7281CS-XXXX

Micropower Dual 150mA CMOS RF LDO™ Regulators with Power Supply Reset Monitor

Impala Linear Corporation 

Micropower Dual 150mA CMOS RF-LDO™ Regulators with Power Supply Reset Monitor

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APPLICATIONS INFORMATION - Regulators

Enable/Shutdown

Forcing ENA/B (enable/shutdown) high (>2V) enables the reg-

ulator. ENA/B is compatible with CMOS logic gates. If the

enable/shutdown feature is not required, connect ENA/B to

VINA/B (supply input).

Input Capacitor

A 1µF capacitor should be placed from VINA/B to GND if there is

more than 10 inches of wire between the input and the ac filter

capacitor or if a battery is used as the input.

Reference Bypass Capacitor

CNOISE (reference bypass) is connected to the internal volt-

age reference of regulator A and B. A 470pF capacitor

connected from CNOISE to GND decouples the reference,

providing a significant reduction in output noise. CNOISE

reduces the regulator phase margin; when using CNOISE,

output capacitors of 2.2µF or greater are generally

required to maintain stability. The start-up speed of the

ILC7281 is inversely proportional to the size of the refer-

ence bypass capacitor. Applications requiring a slow

ramp-up of output voltage should consider larger values of

CNOISE. Likewise, if rapid turn-on is necessary, consider

omitting CNOISE. If output noise is not a major concern,

omit CNOISE and leave the pin open.

Output Capacitor

An output capacitor is required between VOUTA/B and GND to

prevent oscillation. The minimum size of the output capacitor

is dependent upon whether a reference bypass capacitor is

used. 1.0µF minimum is recommended when CNOISE is not

used (see figure 2). 2.2µF minimum is recommended when

CNOISE is 470pF (see figure 1). Larger values slow the regu-

lator’s transient response. The output capacitor value may be

increased without limit. The output capacitor should have an

ESR (effective series resistance) of about 5 or less and a res-

onant frequency above 1MHz. Ultra low ESR capacitors may

be used and will not cause a low-amplitude oscillation and/or

underdamped transient response which shows up in some

LDO regulators. Most ceramic capacitors are adequate so

there is no need for more expensive tantalum capacitors.

At lower values of output current, less output capacitance is

required for output stability. The capacitor can be reduced to

0.47µF for current below 10mA or 0.33µF for currents below

1mA

Dual-Supply Operation

When used in dual supply systems where the regulator load is

returned to a negative supply, the output voltage must be

diode clamped to ground.

Thermal Considerations

Multilayer boards having a ground plane, wide traces near the

pads, and large supply bus lines provide better thermal con-

ductivity. The 8-lead MSOP has a thermal resistance of

200°C/W when mounted on an FR4 board with minimum trace

widths and no ground plane.

Thermal Evaluation Examples

For example, at 50°C ambient temperature, the maximum

package power dissipation is:

P D(max) = (125°C - 50°C)/200°C/W

P D(max) = 375mW

If the intent is to operate the part from a 4V supply at the full

150mA load for both outputs in a 50°C maximum ambient

temperature, make the following calculations:

P D(each regulator) = (VIN - VOUT) ∗ IOUT + (VIN ∗ IGND)

P D(each regulator) = (4V - 3V) ∗ 150mA + (4V ∗ 0.12mA)

P D(each regulator) = 150mW

P D(both regulators) = 2 regulators ∗ 150mW

P D(both regulators) = 300mW

The actual total power dissipation of 300mW is below the

375mW package maximum; therefore the regulator can be

used. Note that both regulators cannot always be used at their

maximum current rating. For example, in a 5V input to 3.0V

output application at 50°C, if one regulator supplies 150mA,

the other regulator is limited to a much lower current. The first

regulator dissipates:

P D = (5V - 3.0V) 150 + 0.12mA (5V)

P D = 267.5mW

Then, the load that the remaining regulator can dissipate must

not exceed: 375mW - 267.5mW = 107.5mW. This means,

using the same 5V input and 3.3V output voltage, the second

regulator is limited to about 60mA

Taking advantage of the extremely low dropout voltage char-

acteristics of the ILC7281, power dissipation can be reduced

by using the lowest possible input voltage to minimize the

input to output voltage drop.

No-Load Stability

The ILC7281 will remain stable and in regulation with no load

unlike many other voltage regulators. This is expecially impor-

tant in CMOS RAM KeepAlive applications.

Impala Linear Corporation

ILC7281 1.0

(408) 574-3939 www.impalalinear.com

Nov 2000 7

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