LM2771

SNVS435 – MARCH 2006

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THERMAL SHUTDOWN

Protection from damage related to overheating is achieved with a thermal shutdown feature. When the junction

temperature rises to 150ºC (typ.), the part switches into shutdown mode. The LM2771 disengages thermal

shutdown when the junction temperature of the part is reduced to 140ºC (typ.). Due to the high efficiency of the

LM2771, thermal shutdown and/or thermal cycling should not be encountered when the part is operated within

specified input voltage, output current, and ambient temperature operating ratings. If thermal cycling is seen

under these conditions, the most likely cause is an inadequate PCB layout that does not allow heat to be

sufficiently dissipated out of the WSON package.

CURRENT LIMIT PROTECTION

conditions where excessive current is drawn. Output current is limited to 500mA (typ).

Application Information

RECOMMENDED CAPACITOR TYPES

The LM2771 requires 3 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors

are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance

(ESR,

≤ 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally

are not recommended for use with the LM2771 due to their high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use

with the LM2771. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over -55ºC to 125ºC; X5R: ±15% over -55ºC to 85ºC).

Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the

LM2771. These types of capacitors typically have wide capacitance tolerance (+80%, -20%) and vary

significantly over temperature (Y5V: +22%, -82% over -30ºC to +85ºC range; Z5U: +22%, -56% over +10ºC to

+85ºC range). Under some conditions, a 1µF-rated Y5V or Z5U capacitor could have a capacitance as low as

0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum

capacitance requirements of the LM2771.

Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower

capacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using

capacitors at DC bias voltages significantly below the capacitor voltage rating will usually minimize DC bias

effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics.

Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and

capacitor manufacturers. It is strongly recommended that the LM2771 circuit be thoroughly evaluated early in the

design-in process with the mass-production capacitors of choice. This will help ensure that any such variability in

capacitance does not negatively impact circuit performance.

The table below lists some leading ceramic capacitor manufacturers.

Manufacturer

Contact Information

AVX

www.avx.com

Murata

www.murata.com

Taiyo-Yuden

www.t-yuden.com

TDK

www.component.tdk.com

Vishay-Vitramon

www.vishay.com

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