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Data Sheet

ADP7159

Rev. B | Page 17 of 23

START-UP TIME

The ADP7159 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

approximately 0.0012 × CBYP, where CBYP is in microfarads.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TIME (ms)

–2

1µF

4.7µF

10µF

Figure 53. Typical Start-Up Behavior with CBYP = 1 µF to 10 µF

0.5

1.0

1.5

2.0

2.5

3.0

3.5

100

120

140

160

TIME (ms)

10µF

47µF

100µF

–20

Figure 54. Typical Start-Up Behavior with CBYP = 10 µF to 100 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 ADP7159. 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 SHUTDOWN

The ADP7159 is protected against damage due to excessive power

dissipation by current and thermal overload protection circuits.

The ADP7159 is designed to current limit when the output load

reaches 3 A (typical). When the output load exceeds 3 A, the

output voltage is reduced to maintain a constant current limit.

When the ADP7159 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 ADP7159 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 the current limit and thermal shutdown.

THERMAL CONSIDERATIONS

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

ADP7159 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 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

ADP7159 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

the junction temperature and ambient air (θJA). The θJA number

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.

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