STCC02-E datasheet(9/13 Pages) STMICROELECTRONICS

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STCC02-ED5

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APPLICATION RECOMMENDATIONS

■ EVALUATION OF THE STCC02 DISSIPATION IN ITS APPLICATION

In order to define accurately at which maximum input supply voltage the STCC02 can work safely, the dis-

sipated power has to be evaluated. Indeed, the STCC02 device can withstand voltages up to 30V, as

specified in the table 1 "ABSOLUTE RATINGS" section.

However, when the VIN voltage is high, it will also increase the power dissipation PDIS and the junction

temperature TJ of the whole circuit.

For the evaluation of the maximum junction temperature, the following equation should be used to calcu-

late dissipated power:

PDIS = (VIN – VDD) × IDD + VIN . IQ + VON × (IM@FAN1 + IM@MAG2 + IM@BUZ3) +

Indeed, the power dissipation is mainly due to the regulator and to the currents sunk by the three driver

outputs FAN1, MAG2, and BUZ3.

Furthermore, the input voltage VIN is linked to the relays conduction in most applications. When the relay

coils are driven, the storage supply capacitor is discharged and VINis no longer equal to the peak voltage

of the transformer secondary winding. In this case, VIN should approach the average value of the second-

ary voltage. This value is then approximately 36% lower that in stand-by operation, as explained by the

following equation:

VIN (relays_on)

VIN (relays_off)

When the relays are off, the dissipation losses formula is:

PDIS = (VIN – VDD) × IDD × IQ +

For instance if VIN = 27V, VDD = 5V, IQ = 2.5mA, IDD = 20mA, RBUZ = 1.5kΩ, τ = duty cycle = 50%, the

dissipated power in the STCC02 is evaluated at 0.68W.

When the relays are on, the full formula of the dissipation losses is applied. For instance in the

same AC line conditions with the relays on, VIN drops down to 17.5V. Considering IM BUZ3 = 10mA,

IM FAN1 = IM MAG2 = 100mA and VON = 1.5V, the dissipated power in the STCC02 becomes 0.67W.

The maximum junction temperature is given by:

TJ max = TAMB max + Rth (j – |a) × PDIS

The maximum allowed input supply voltage is then chosen in order to keep the junction temperature below

its maximum operating value 150°C.

Since the maximumjunction temperature is 150°C, the maximum ambient temperature TAMB is 80°C in this

application, and the thermal resistance is 100°C/W, the maximum allowed dissipation becomes 0.70 W.

The two dissipation cases described above are compatible with the package dissipation capability.

Otherwise, the ambient temperature TAMB, the input voltage VIN or the load current IDD should limited by

design to meet the circuit thermal requirements.

–

1.2

BUZ

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≈

–

1.2

BUZ

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