LM3875 datasheet(12/20 Pages) NSC

12 / 20 page

Typical Performance Characteristics (Continued)

Common-Mode Rejection

Ratio

Large Signal Response

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Pulse Response

Open Loop

Frequency Response

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Application Information

GENERAL FEATURES

Under-Voltage Protection: Upon system power-up the

under-voltage Protection Circuitry allows the power supplies

and their corresponding caps to come up close to their full

values before turning on the LM3875 such that no DC output

spikes occur. Upon turn-off, the output of the LM3875 is

brought to ground before the power supplies such that no

transients occur at power-down.

Over-Voltage Protection: The LM3875 contains overvolt-

age protection circuitry that limits the output current to ap-

proximately 4Apeak while also providing voltage clamping,

though not through internal clamping diodes. The clamping

effect is quite the same, however, the output transistors are

designed to work alternately by sinking large current spikes.

SPiKe Protection: The LM3875 is protected from instanta-

neous peak-temperature stressing by the power transistor

array. The Safe Operating Area graph in the Typical Perfor-

mance Characteristics section shows the area of device

operation where the SPiKe Protection Circuitry is not en-

abled. The waveform to the right of the SOA graph exempli-

fies how the dynamic protection will cause waveform distor-

tion when enabled.

Thermal Protection: The LM3875 has a sophisticated ther-

mal protection scheme to prevent long-term thermal stress

to the device. When the temperature on the die reaches

165˚C, the LM3875 shuts down. It starts operating again

when the die temperature drops to about 155˚C, but if the

temperature again begins to rise, shutdown will occur again

at 165˚C. Therefore the device is allowed to heat up to a

relatively high temperature if the fault condition is temporary,

but a sustained fault will cause the device to cycle in a

Schmitt Trigger fashion between the thermal shutdown tem-

perature limits of 165˚C and 155˚C. This greatly reduces the

stress imposed on the IC by thermal cycling, which in turn

improves its reliability under sustained fault conditions.

Since the die temperature is directly dependent upon the

heat sink, the heat sink should be chosen as discussed in

the Thermal Considerations section, such that thermal

shutdown will not be reached during normal operation. Using

the best heat sink possible within the cost and space con-

straints of the system will improve the long-term reliability of

any power semiconductor device.

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