ML4870CS-3 datasheet(6/8 Pages) MICRO-LINEAR

6 / 8 page

ML4870

OUTPUT CAPACITOR

The output capacitor filters the pulses of current from the

switching regulator. Since the switching frequency will

vary with inductance, the minimum output capacitance

required to reduce the output ripple to an acceptable

level will be a function of the inductor used. Therefore, to

maintain an output voltage with less than 100mV of ripple

at full load current, use the following equation:

OUT

(3)

The output capacitor’s Equivalent Series Resistance (ESR)

and Equivalent Series Inductance (ESL), also contribute to

the ripple. Just after the Q1 turns off, the current in the

DEISGN CONSIDERATIONS (Continued)

Figure 4. IOUT vs. VIN Using the Circuit of Figure 8

Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8

1000

800

600

400

200

VIN (V)

1.0

2.0

3.0

5.0

4.0

VOUT = 3.3V

VOUT = 5V

IOUT (mA)

100

1000

VOUT = 3.3V

VOUT = 5V

VIN = 2.4V

350

300

250

200

150

100

VIN (V)

1.0

3.0

5.0

7.0

Figure 6. Input Leakage vs. VIN in Shutdown

Figure 7. No Load Input Current vs. VIN

output capacitor ramps quickly to between 0.5A and

1.3A. This fast change in current through the capacitor’s

ESL causes a high frequency (5ns) spike to appear on the

output. After the ESL spike settles, the output still has a

ripple component equal to the inductor discharge current

times the ESR. To minimize these effects, choose an

output capacitor with less than 10nH of ESL and less than

100m

W of ESR.

Suitable tantalum capacitors can be obtained from the

following vendors:

AVX

(207) 282-5111

Kemet

(846) 963-6300

Sprague

(207) 324-4140

VIN (V)

1.0

2.0

3.0

5.0

4.0

VOUT = 3.3V

VOUT = 5V

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