ADP170AUJZ-2.8-R7 datasheet(14/20 Pages) AD

14 / 20 page

ADP170/ADP171

Rev. B | Page 14 of 20

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP170/ADP171 are protected against damage due to

excessive power dissipation by current and thermal overload

protection circuits. The ADP170/ADP171 are designed to limit

the current when the output load reaches 450 mA (typical).

When the output load exceeds 450 mA, the output voltage is

reduced to maintain a constant current limit.

Thermal overload protection is included, which limits the junction

temperature to a maximum of 150°C (typical). Under extreme

conditions (that is, high ambient temperature and power dissip-

ation), when the junction temperature starts to rise above 150°C,

the output is turned off, reducing the output current to 0. 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 ADP170/ADP171 will limit the current so that only

450 mA is conducted into the short. If self-heating of the junction

is great enough to cause its temperature to rise above 150°C,

thermal shutdown will activate, turning off the output and

reducing the output current to 0. As the junction temperature

cools and drops below 135°C, the output turns on and conducts

450 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 450 mA and 0 mA,

which continues as long as the short remains at the output.

Current and thermal limit protections are intended to protect

the device against accidental overload conditions.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP170/ADP171 must not exceed 125°C. To ensure the junction

temperature 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 θJA number is dependent

on the package assembly compounds used and the amount of

copper to which the GND pin of the package is 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

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 ADP170/ADP171 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:

ILOAD is the load current.

IGND is the ground current.

VIN and VOUT are input and output voltages, respectively.

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation

simplifies to the following:

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,

there exists a minimum copper size requirement for the PCB to

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

Figure 33 to Figure 38 show junction temperature calculations

for different ambient temperatures, load currents, VIN to VOUT

differentials, and areas of PCB copper.

140

120

100

0.5

1.0

1.5

2.0

2.5

3.0

VOUT – VIN (V)

ILOAD = 1mA

ILOAD = 10mA

ILOAD = 25mA

ILOAD = 100mA

ILOAD = 150mA

ILOAD = 300mA

TJ MAX

Figure 33. 500 mm2 of PCB Copper, TA = 25°C

140

120

100

VOUT – VIN (V)

ILOAD = 1mA

ILOAD = 10mA

ILOAD = 25mA

ILOAD = 100mA

ILOAD = 150mA

ILOAD = 300mA

TJ MAX

0.5

1.0

1.5

2.0

2.5

3.0

Figure 34. 100 mm2 of PCB Copper, TA = 25°C

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