Data Sheet

ADP7157

Rev. A | Page 17 of 23

START-UP TIME

The ADP7157 uses an internal soft start to limit the inrush

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

3.3 V output is approximately 1.2 ms from the time the EN

active threshold is crossed to when the output reaches 90% of

its final value.

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

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2

4

6

8

10

12

14

16

18

20

TIME (ms)

–2

0

EN

1µF

4.7µF

10µF

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

20

40

60

80

100

120

140

160

TIME (ms)

EN

10µF

47µF

100µF

–20

0

REF, BYP, AND VREG PINS

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

to these pins, because doing so compromises the noise and

as described in the Start-Up Time section.

CURRENT-LIMIT AND THERMAL SHUTDOWN

The ADP7157 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP7157 is designed to current limit when the

output load reaches 1.8 A (typical). When the output load

exceeds 1.8 A, the output voltage is reduced to maintain a

constant current limit.

When the ADP7157 junction temperature exceeds 150°C, the

thermal shutdown circuit turns off the output voltage, reducing

the output current to zero. Extreme junction temperature can

be the result of high current operation, poor circuit board

design, or high ambient temperature. A 15°C hysteresis is

included so that the ADP7157 does not return to operation after

thermal shutdown until the on-chip temperature falls below

135°C. When the device exits thermal shutdown, a soft start is

initiated to reduce the inrush current.

Current limit and thermal shutdown protections are intended to

protect the device against accidental overload conditions. Cases

with a hard short from VOUT to ground or an extremely long

soft-start timer typically cause the device to experience thermal

oscillations between current limit and thermal shutdown.

THERMAL CONSIDERATIONS

In applications with a low input to output voltage differential, the

ADP7157 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 125°C.

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

ADP7157 must not exceed 125°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

is dependent on the package assembly compounds used, as well as

the amount of copper used to solder the package ground and the

exposed pad to the PCB.