Data Sheet

ADM7151

Rev. B | Page 19 of 24

START-UP TIME

The ADM7151 uses an internal soft start to limit the inrush current

when the output is enabled. The start-up time for a 5 V output is

approximately 3 ms from the time the EN active threshold is

crossed to when the output reaches 90% of its final value.

The rise time of the output voltage (10% to 90%) is approximately

TIME (Seconds)

0.020

0.016

0

0.008

0.012

0.004

0.018

0.014

0.006

0.010

0.002

0

6

5

4

3

2

1

ENABLE

TIME (Seconds)

0.20

0.16

0

0.08

0.12

0.04

0.18

0.14

0.06

0.10

0.02

0

6

5

4

3

2

1

ENABLE

REF, BYP, AND VREG PINS

REF, BYP, and VREG are internally generated voltages 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

acceptable but can increase the start-up time, as described in

the Start-Up Time section.

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADM7151 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADM7151 is designed to current limit when the

output load reaches 1.3 A (typical). When the output load

exceeds 1.3 A, 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 155°C (typical). Under

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

high power dissipation) when the junction temperature starts to

rise above 155°C, the output is turned off, reducing the output

current to zero. When the junction temperature drops below

140°C, the output is turned on again, and output current is

restored to its operating value.

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

At first, the ADM7151 current limits, so that only 1.3 A is

conducted into the short. If self heating of the junction is great

enough to cause its temperature to rise above 155°C, thermal

shutdown activates, turning off the output and reducing the

output current to zero. As the junction temperature cools and

drops below 140°C, the output turns on and conducts 1.3 A into

the short, again causing the junction temperature to rise above

155°C. This thermal oscillation between 140°C and 155°C

causes a current oscillation between 1.3 A and 0 mA that

continues 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 low input to output voltage differential, the

ADM7151 does not dissipate much heat. However, in applications

with high ambient temperature and/or high input voltage, the

heat dissipated in the package can 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 155°C, the converter

enters thermal shutdown. It recovers only after the junction

temperature decreases below 140°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 temperature rise of the

package due to the power dissipation, as shown in Equation 2.

To guarantee reliable operation, the junction temperature of the

ADM7151 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

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.