Data Sheet ADP1763
Rev. D | Page 15 of 19
Substituting these values in Equation 4 yields
CEFF = 10 F × (1 − 0.15) × (1 − 0.1) = 7.65 F
Therefore, the capacitor chosen in this example meets the
minimum capacitance requirement of the LDO over temperature
and tolerance at the chosen output voltage.
To guarantee the performance of the ADP1763, it is imperative
that the effects of dc bias, temperature, and tolerances on the
behavior of the capacitors be evaluated for each application.
UNDERVOLTAGE LOCKOUT
The ADP1763 has an internal undervoltage lockout circuit that
disables all inputs and the output when the input voltage is less
than approximately 1.06 V. The UVLO ensures that the ADP1763
inputs and the output behave in a predictable manner during
power-up.
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP1763 is protected against damage due to excessive power
dissipation by current-limit and thermal overload protection
circuits. The ADP1763 is designed to reach the current limit
when the output load reaches 4 A (typical). When the output
load exceeds 4 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 ADP1763 reaches the current limit so that only
4 A is conducted into the short circuit. 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 4 A into the short circuit, again causing the
junction temperature to rise above 150°C. This thermal oscillation
between 135°C and 150°C causes a current oscillation between
4 A and 0 A that continues as long as the short circuit remains
at the output.
Current-limit and thermal overload protections are intended to
protect the device against accidental overload conditions. For
reliable operation, limit device power dissipation externally so
that junction temperatures do not exceed 125°C.
PARALLELING ADP1763 FOR HIGH CURRENT
APPLICATIONS
In applications where high output current is required while
maintaining low noise and high PSRR performance, connect two
ADP1763 devices in parallel to handle loads up to 5 A.
When paralleling the ADP1763, the two outputs must be of the
same voltage setting to maintain stable current sharing between
the two LDO regulators. To improve current sharing accuracy, add
identical ballast resistors (RBALLAST) at the output of each regulator,
as shown in Figure 34. Note that large ballast resistors improve
current sharing accuracy but degrade the load regulation perform-
ance and increase the losses along the power line; therefore, it is
recommended to keep the ballast resistors at a minimum. In
addition, tie the VADJ, SS, and REFCAP pins of the LDO regu-
lators together to minimize error between the two outputs.
VIN
EN ENABLE
SS
VREG
VOUT
SENSE
C
OUT
10µF
PG
VADJ
GND
REFCAP
C
IN
10µF
R
BALLAST
= 5mΩ
R
BALLAST
= 5mΩ
V
OUT
= 1.2V/5A
ADP1763
V
IN
= 1.5V
VIN
EN
SS
VREG
VOUT
SENSE
C
OUT
10µF
PG
VADJ
GND
REFCAP
C
IN
10µF
ADP1763
C
REG
1µF
C
REF
1µF
R
ADJ
4.02kΩ
R
PULLUP
100kΩ
C
SS
1nF
C
REG
1µF
C
REF
1µF
12923-133
Figure 34. Two ADP1763 Devices Connected in Parallel to Achieve Higher Current Output