ADP7185 Data Sheet
Rev. 0 | Page 16 of 19
Figure 48 shows the change in capacitance vs. the dc bias voltage
characteristics of a 0805 case, 4.7 μF, 10 V, X5R capacitor. The
capacitor size and voltage ratings strongly influence 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.
0
024681012
CHANGE IN CAPACITANCE (µF)
DC BIAS VOLTAGE (V dc)
5.64
4.70
1.88
2.82
3.76
0.94
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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 over temperature
and tolerance at the chosen output voltage.
To guarantee the performance of the ADP7185, it is imperative
to evaluate the effects of dc bias, temperature, and tolerances on
the behavior of the capacitors 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 noises from VIN.
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0.05
0
–0.55
–1.60
VOUT (V)
VIN (V)
–1.74 –1.72 –1.70 –1.68 –1.66 –1.64 –1.62
–0.50
–0.45
–0.40
–0.35
–0.30
–0.25
–0.20
–0.15
–0.10
–0.05
Figure 49. Typical UVLO Behavior, VOUT = −0.5 V
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP7185 is protected against damage due to excessive
power dissipation by current-limit and thermal overload
protection circuits. The ADP7185 is designed to reach current
limit when the output load reaches −900 mA (typical). When
the output load exceeds −900 mA, 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 GND occurs.
At first, the ADP7185 reaches current limit so that only −900 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 zero. As the junction temperature cools and
drops below 135°C, the output turns on and conducts −900 mA
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 −900 mA and 0 A
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,
externally limit device power dissipation so that junction
temperatures do not exceed 125°C.