AP7173 Datasheet, PDF (12/15 Pages) Diodes Incorporated – 1.5A LOW DROPOUT LINEAR REGULATOR WITH PROGRAMMABLE SOFT-START

AP7173
1.5A LOW DROPOUT LINEAR REGULATOR WITH
PROGRAMMABLE SOFT-START
Application Note (Continued)
ENABLE/SHUTDOWN
The EN pin can be used with standard digital signals or
relatively slow-ramping analog signals. Pulling the V_EN below
0.4V turns the regulator off, while driving the V_EN above 1.1V
turns the regulator on. Figure 30 shows an example where an
RC circuit is used to delay start the AP7173.
If not used, the EN pin can be connected to the VCC or IN pin
when the V_IN is greater than 1.1V, as long as good decoupling
measures are taken for the EN pin.
R3
VIN
IN
OUT
C1
VVCC
PG
FB
AP7173
VCC
SS
R4
C3
EN
GND
C5
VOUT
R1 C2
R2
CSS
Figure 30. Delayed Start Using an RC Circuit
to Enable AP7173
POWER-GOOD
The power-good (PG) pin is an open-drain output and can be
pulled up through a resistor of 10kΩ to1MΩ to V_IN, V_OUT or any
other rail that is 5.5V or lower. When the V_OUT ≥ V_PG,TH+V_PG,HYS,
the PG output is high-impedance; if the V_OUT drops to below
V_PG,TH, V_VCC ≤ 1.9V or the device is disabled, the PG pin is
pulled to low by an internal MOSFET.
OVER-CURRENT AND SHORT-CIRCUIT PROTECTION
The AP7173 features a factory-trimmed, temperature and
supply voltage compensated internal current limit and an
over-current protection circuitry to protect the device against
overload conditions. It limits the device current to a typical
value of 3A and reduces the V_OUT when the load tries to pull
more current.
For more effective protection against short-circuit failure, the
AP7173 also includes a short-circuit foldback mechanism that
lowers the current limit to a typical value of 1.0A when the V_FB
drops to below 0.2V.
THERMAL PROTECTION
Thermal shutdown limits the AP7173 junction temperature and
protects the device from damage as a result of overheating.
Thermal protection turns off the V_OUT when the AP7173’s
junction temperature rises to approximately +150°C, allowing it
to cool down. When the junction temperature drops to
approximately +130°C, the output is re-enabled. Therefore, the
thermal protection circuit may cycle on and off at a rate
dependent on the power dissipation, thermal resistance, and
ambient temperature.
POWER DISSIPATION
Thermal shutdown is intented to protect the AP7173 against
abnormal overheating. For normal operation, excessive power
dissipation should be avoided and good heatsinking should be
provided. Power dissipation in the device is the product of the
device dropout voltage and the load current,
P_D = (V_IN - V_OUT) x I_OUT
As can be seen, power dissipation can be minimized by using
the lowest input voltage necessary to achieve the required
output voltage regulation.
To ensure that the device junction temperature does not
exceed the specified limit of 125°C, an application should
provide heat conduction paths that have junction-to-ambient
thermal resistance lower than the calculated value here:
R_θJA = (125°C –T_A ) / P_D
For the DFN package with exposed pad, the primary
conduction path for heat is through the exposed pad to the
printed circuit board (PCB). The pad should be attached to an
appropriate amount of copper PCB area to ensure that the
device does not overheat.
AP7173 Rev. 4
12 of 15
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MARCH 2008
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