ADP130AUJZ-2.5-R7 datasheet(15/20 Pages) AD

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ADP130

Rev. 0 | Page 15 of 20

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP130 is protected against damage due to excessive power

dissipation by current limit and thermal overload protection

circuits. The ADP130 is designed to current limit when the

output load reaches 550 mA (typical). When the output load

exceeds 550 mA, the output voltage is reduced to maintain

a constant current limit.

Thermal overload protection limits the junction temperature to

a maximum of 150°C typical. Under extreme conditions (that is,

high ambient temperature and power dissipation) when the

junction temperature starts to rise above 150°C, the output is

turned off, reducing output current to zero. When the junction

temperature drops below 135°C, the output is turned on again and

output current is restored to its nominal value.

Consider the case where a hard short from VOUT to GND occurs.

At first, the ADP130 current limits so that only 550 mA is con-

ducted into the short. If self-heating of the junction is great enough

to cause its temperature to rise above 150°C, thermal shutdown

activates, turning off the output and reducing the output current to

zero. As the junction temperature cools and drops below 135°C,

the output turns on and conducts 550 mA into the short, again

causing the junction temperature to rise above 150°C. This

thermal oscillation between 135°C and 150°C causes a current

oscillation between 550 mA and 0 mA that continues as long

as the short remains at the output.

Current limit and thermal overload protections protect the device

against accidental overload conditions. For reliable operation,

device power dissipation must be externally limited so that

junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP130 must not exceed 125°C. To ensure that the junction tem-

perature stays below this maximum value, the user needs to be

aware of the parameters that contribute to junction temperature

changes. These parameters include ambient temperature, power

dissipation in the power device, and thermal resistances between

the junction and ambient air (θJA). The value of θJA is dependent on

the package assembly compounds used and the amount of copper

to which the GND pins of the package are soldered on the PCB.

Table 6 shows typical θJA values of the 5-lead TSOT package for

various PCB copper sizes.

Table 6. Typical θJA Values for Specified PCB Copper Sizes

Copper Size (mm2)

θJA (°C/W)

170

152

100

146

300

134

500

131

1 Device soldered to minimum size pin traces.

The junction temperature of the ADP130 can be calculated from

the following equation:

TJ = TA + (PD × θJA)

(2)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

PD = [(VIN − VOUT) × ILOAD] + (VIN × IGND)

(3)

where:

VIN and VOUT are the input and output voltages, respectively.

ILOAD is the load current.

IGND is the ground current.

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation can

be simplified as follows:

TJ = TA + {[(VIN − VOUT) × ILOAD] × θJA}

(4)

As shown in Equation 4, for a given ambient temperature, input-

to-output voltage differential, and continuous load current,

a minimum copper size requirement exists for the PCB to ensure

that the junction temperature does not rise above 125°C. Figure 40

through Figure 46 show junction temperature calculations for

different ambient temperatures, load currents, VIN to VOUT

differentials, and areas of PCB copper.

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