ADM7155

Data Sheet

Rev. C | Page 16 of 24

FREQUENCY (Hz)

1M

0.1

1

10

100

1k

10k

100k

0.1

1

10

100

10k

1k

NOISE FLOOR

1.0µF

3.3µF

10µF

33µF

100µF

330µF

1000µF

Figure 47. Noise Spectral Density vs. Frequency for

Capacitor Properties

Any good quality ceramic capacitors can be used with the

ADM7155 if they meet the minimum capacitance and

maximum ESR requirements. Ceramic capacitors are

manufactured with a variety of dielectrics, each with different

behavior over temperature and applied voltage. Capacitors must

have a dielectric adequate to ensure the minimum capacitance

over the necessary temperature range and dc bias conditions.

X5R or X7R dielectrics with a voltage rating of 6.3 V to 50 V

are recommended. However, Y5V and Z5U dielectrics are

not recommended because of their poor temperature and dc

bias characteristics.

Figure 48 depicts the capacitance vs. dc bias voltage of a 1206,

10 μF, 10 V, X5R capacitor. The voltage stability of a capacitor is

strongly influenced by the capacitor size and voltage rating. In

general, a capacitor in a larger package or higher voltage rating

exhibits better stability. The temperature variation of the X5R

dielectric is ~±15% over the −40°C to +85°C temperature range

and is not a function of package or voltage rating.

DC BIAS VOLTAGE (V)

10

04

8

26

0

12

10

8

6

4

2

Figure 48. Capacitance vs. DC Bias Voltage

Use Equation 1 to determine the worst case capacitance

accounting for capacitor variation over temperature,

component tolerance, and voltage.

(1)

where:

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 −40°C to +85°C is assumed to be 15% for an

X5R dielectric. The tolerance of the capacitor (TOL) is assumed

Substituting these values in Equation 1 yields

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 ADM7155, 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 ADM7155 also incorporates an internal UVLO circuit to

disable the output voltage when the input voltage is less than the

minimum input voltage rating of the regulator. The upper and

lower thresholds are internally fixed with about 200 mV of

hysteresis.

INPUT VOLTAGE (V)

2.30

2.25

2.20

2.15

2.10

2.05

2.00

1.95

1.90

0

2.5

2.0

1.5

1.0

0.5

–40°C

+25°C

+125°C

Figure 49 shows the typical hysteresis of the UVLO function.

This hysteresis prevents on/off oscillations that can occur when

caused by noise on the input voltage as it passes through the

threshold points.