ADP1755 Analog Devices, ADP1755 Datasheet - Page 15

• Manufacturer Part Number: ADP1755
• Description: 1.2A Low-Vin, Adjustable-Vout LDO Linear Regulator
• Manufacturer: Analog Devices

In this example, the worst-case temperature coefficient
(TEMPCO) over −40°C to +85°C is assumed to be 15% for an
X5R dielectric. The tolerance of the capacitor (TOL) is assumed
to be 10%, and C
Substituting these values in Equation 3 yields
Therefore, the capacitor chosen in this example meets the
minimum capacitance requirement of the LDO over temper-
ature and tolerance at the chosen output voltage.
To guarantee the performance of the ADP1754/ADP1755, it is
imperative that the effects of dc bias, temperature, and toler-
ances on the behavior of the capacitors be evaluated for each
application.
UNDERVOLTAGE LOCKOUT
The ADP1754/ADP1755 have an internal undervoltage lockout
circuit that disables all inputs and the output when the input
voltage is less than approximately 1.58 V. This ensures that the
ADP1755/ADP1755 inputs and the output behave in a predicta-
ble manner during power-up.
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP1754/ADP1755 are protected against damage due to
excessive power dissipation by current-limit and thermal
overload protection circuits. The ADP1754/ADP1755 are
designed to reach current limit when the output load reaches
2 A (typical). When the output load exceeds 2 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 150°C (typical). Under
extreme conditions (that is, high ambient temperature and
power dissipation) when the junction temperature begins to
rise above 150°C, the output is turned off, reducing the output
current to zero. When the junction temperature drops below
135°C (typical), the output is turned on again and the output
current is restored to its nominal value.
Consider the case where a hard short from VOUT to ground
occurs. At first, the ADP1754/ADP1755 reach current limit so
that only 2 A is conducted into the short. If self-heating of the
junction becomes great enough to cause its temperature to
rise above 150°C, thermal shutdown activates, turning off the
output and reducing the output current to zero. As the junction
temperature cools and drops below 135°C, the output turns on
and conducts 2 A into the short, again causing the junction
temperature to rise above 150°C. This thermal oscillation between
135°C and 150°C causes a current oscillation between 2A and
0 A that continues as long as the short remains at the output.
Current-limit and thermal overload protections are intended to
protect the device against accidental overload conditions. For
reliable operation, device power dissipation should be externally
limited so that junction temperatures do not exceed 125°C.
C
EFF
= 4.46 μF × (1 − 0.15) × (1 − 0.1) = 3.41 μF
OUT
= 4.46 μF at 1.8 V, as shown in Figure 35.
Rev. B | Page 15 of 20
THERMAL CONSIDERATIONS
To guarantee reliable operation, the junction temperature of the
ADP1754/ADP1755 must not exceed 125°C. To ensure that the
junction temperature stays below this maximum value, the user
needs to be aware of the parameters that contribute to junction
temperature changes. These parameters include ambient temp-
erature, power dissipation in the power device, and thermal
resistance between the junction and ambient air (θ
value is dependent on the package assembly compounds used
and the amount of copper to which the GND pin and the exposed
pad (EPAD) of the package are soldered on the PCB. Table 6 shows
typical θ
sizes. Table 7 shows typical Ψ
Table 6. Typical θ
Copper Size (mm
0
100
500
1000
6400
1
Table 7. Typical Ψ
Copper Size (mm
100
500
1000
The junction temperature of the ADP1754/ADP1755 can be
calculated from the following equation:
where:
T
P
where:
V
I
I
Power dissipation due to ground current is quite small and can
be ignored. Therefore, the junction temperature equation can
be simplified as follows:
As shown in Equation 6, for a given ambient temperature, input-
to-output voltage differential, and continuous load current, a
minimum copper size requirement exists for the PCB to ensure
that the junction temperature does not rise above 125°C. Figure 36
through Figure 41 show junction temperature calculations for
different ambient temperatures, load currents, V
differentials, and areas of PCB copper.
LOAD
GND
Device soldered to minimum size pin traces.
1
D
A
IN
is the power dissipation in the die, given by
is the ambient temperature.
and V
is the ground current.
T
P
T
is the load current.
J
D
J
= T
= T
= [(V
JA
OUT
values for the 16-lead LFCSP for various PCB copper
A
A
+ (P
+ {[(V
IN
are the input and output voltages, respectively.
− V
D
2
2
× θ
JA
)
)
IN
OUT
JB
Values
− V
Values
JA
) × I
)
OUT
LOAD
) × I
JB
] + (V
values for the 16-lead LFCSP.
LOAD
ADP1754/ADP1755
] × θ
Ψ
32.7
31.5
25.5
θ
130
80
69
54
42
IN
JA
JB
× I
(°C/W), LFCSP
(°C/W) @ 1 W
JA
GND
}
)
IN
JA
to V
). The θ
OUT
JA
(4)
(5)
(6)