ADP7156ARDZ-1.2-R7 datasheet(18/23 Pages) AD

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Data Sheet

ADP7156

Rev. A | Page 17 of 22

START-UP TIME

The ADP7156 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 measured in microfarads.

–2

02468

VEN

1µF

4.7µF

10µF

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TIME (ms)

Figure 52. Typical Start-Up Behavior with CBYP = 1 μF to 10 μF

–20

100

120

140

160

VEN

10µF

47µF

100µF

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TIME (ms)

Figure 53. 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 ADP7156. 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 ADP7156 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP7156 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 ADP7156 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 ADP7156 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. For

example, a hard short from VOUT to ground or an extremely

long soft start timer usually causes thermal oscillations between

the current limit and thermal shutdown.

THERMAL CONSIDERATIONS

In applications with a low input to output voltage differential,

the ADP7156 does not dissipate much heat. However, in applica-

tions 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

ADP7156 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 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 exposed pad (ground) to

the PCB.

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