
ADP7183 Data Sheet
Rev. B | Page 16 of 19
Figure 48 shows the capacitance change vs. the bias voltage
characteristics of a 0805 case, 4.7 µF, 1 0 V, X5R capacitor. The
capacitor size and voltage rating strongly influences the voltage
stability of a capacitor. In general, a capacitor in a larger package
or with a higher voltage rating exhibits improved stability. The
temperature variation of the X5R dielectric is about ±15% over
the −55°C to +85°C temperature range and is not a function of
package size or voltage rating.
00246810 12
CHANGE IN CAPACIT ANCE ( µ F)
DC BIAS V OL TAGE ( V dc)
5.64
4.70
1.88
2.82
3.76
0.94
12897-053
Figure 48. Change in Capacitance vs. DC Bias Voltage
Use Equation 4 to determine the worst-case capacitance,
accounting for capacitor variation over temperature, component
tolerance, and voltage.
CEFF = COUT × (1 − Tempco) × (1 − TOL) (4)
where:
CEFF is the effective capacitance at the operating voltage.
COUT is the output capacitor.
Tempco is the worst case capacitor temperature coefficient.
TOL is the worst case component tolerance.
In this example, the worst-case temperature coefficient (Tempco)
over −55°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
COUT = 4.7 µF at 1.0 V.
Substituting these values in Equation 4 yields
CEFF = 4.7 µF × (1 − 0.15) × (1 − 0.1) = 3.6 µF
Therefore, the capacitor chosen in this example meets the
minimum capacitance requirement of the LDO regulator over
temperature and tolerance at the chosen output voltage.
To guarantee the performance of the ADP7183, 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 (UVLO)
The UVLO circuitry protects the system from power supply
brownouts. If the input voltage on VIN is more positive than
the minimum −1.58 V UVLO falling threshold, the LDO output
shuts down. The LDO enables again when the voltage to VIN is
more negative than the maximum −1.77 V UVLO rising threshold.
A typical hysteresis of 90 mV within the UVLO circuitry prevents
the device from oscillating due to the noise from VIN.
–0.55
–0.50
–0.45
–0.40
–0.35
–0.30
–0.25
–0.20
–0.15
–0.10
–0.05
0
0.05
–1.75 –1.73 –1.71 –1.69 –1.67 –1.65 –1.63 –1.61
V
OUT
(V)
V
IN
(V)
12897-349
Figure 49. Typical UVLO Behavior, VOUT = −0.5 V
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP7183 is protected against damage due to excessive
power dissipation by current-limit and thermal overload
protection circuits. The ADP7183 is designed to reach the
current limit when the output load reaches −600 mA (typical).
When the output load exceeds −600 mA, the output voltage
reduces to maintain a constant current limit.
Thermal overload protection is included, which limits the
junction temperature to a threshold 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 turns off, reducing the output
current to zero. When the junction temperature drops below
135°C (typical), the output turns on again, and the output
current is restored to its nominal value.
Consider the case where a hard short from VOUT to GND occurs.
At first, the ADP7183 reaches the current limit so that only
−600 mA 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 0 mA. As the junction temperature cools
and drops below 135°C, the output turns on and conducts
−600 mA 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 −600 mA
and 0 mA that continues as long as the short remains at the output.
Current-limit and thermal overload protections protect the device
against accidental overload conditions. For reliable operation,
device power dissipation must be externally limited so that
junction temperatures do not exceed 125°C.