ADM7155ACPZ-01-R7 datasheet(18/24 Pages) AD

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ADM7155

Data Sheet

Rev. C | Page 18 of 24

TIME (ms)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

ENABLE (VEN)

CBYP = 1µF

CBYP = 3.3µF

CBYP = 10µF

Figure 54. Typical Start-Up Behavior with CBYP = 1 μF to 10 μF

TIME (ms)

200

180

160

140

120

100

3.5

3.0

2.5

2.0

1.5

1.0

0.5

ENABLE (VEN)

CBYP = 10µF

CBYP = 33µF

CBYP = 100µF

CBYP = 330µF

Figure 55. Typical Start-Up Behavior with CBYP = 10 μF to 330 μF

REF, BYP, AND VREG PINS

REF, BYP, and VREG generate voltages internally (VREF, VBYP,

and VREG) that require external bypass capacitors for proper

operation. Do not, under any circumstances, connect any loads

to these pins, because doing so compromises the noise and

PSRR performance of the ADM7155. Using larger values of

CBYP, CREF, and CREG is acceptable but can increase the start-up

time, as described in the Start-Up Time section.

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADM7155 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADM7155 is designed to current limit when the

output load reaches 960 mA (typical). When the output load

exceeds 960 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. Under extreme

conditions (that is, high ambient temperature and/or high

power dissipation), when the junction temperature starts to rise

above 150°C, the output is turned off, reducing the output

current to zero. When the junction temperature drops below

135°C, the output is turned on again, and the output current is

restored to the operating value.

Consider the case where a hard short from VOUT to GND

occurs. At first, the ADM7155 current limits, so that only

960 mA is conducted into the short. If self heating of the

junction is great enough to cause the temperature to rise above

150°C, thermal shutdown activates, turning off the output and

reducing the output current to zero. As the junction tempera-

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

conducts 900 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

900 mA and 0 mA that continues for as long as the short

remains at the output.

Current-limit and thermal limit protections are intended to

protect the device against accidental overload conditions. For

reliable operation, device power dissipation must be externally

limited so that the junction temperature does not exceed 150°C.

THERMAL CONSIDERATIONS

In applications with a low input to output voltage differential,

the ADM7155 does not dissipate much heat. However, in

applications with high ambient temperature and/or high input

voltage, the heat dissipated in the package may become large

enough that it causes the junction temperature of the die to

exceed the maximum junction temperature of 150°C.

When the junction temperature exceeds 150°C, the converter

enters thermal shutdown. It recovers only after the junction

temperature decreases below 135°C to prevent any permanent

damage. Therefore, thermal analysis for the chosen application

is important to guarantee reliable performance over all

conditions. The junction temperature of the die is the sum of

the ambient temperature of the environment and the tempera-

ture rise of the package due to the power dissipation, as shown

in Equation 2.

To guarantee reliable operation, the junction temperature of the

ADM7155 must not exceed 150°C. To ensure that the junction

temperature stays below this maximum value, the user must 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 that are used

and the amount of copper used to solder the package GND pin

and exposed pad to the PCB.

Table 7 shows typical θJA values of the 8-lead SOIC and 8-lead

LFCSP packages for various PCB copper sizes.

Table 8 shows the typical ΨJB values of the 8-lead SOIC and

8-lead LFCSP.

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