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Data Sheet

ADP5076

Rev. A | Page 17 of 23

OUTPUT CAPACITORS

Higher output capacitor values reduce the output voltage ripple

and improve load transient response. When choosing this value,

it is also important to account for the loss of capacitance due to

the output voltage dc bias.

Ceramic capacitors are manufactured with a variety of dielectrics,

each with a 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

25 V or 50 V (depending on output) are recommended for best

performance. Y5V and Z5U dielectrics are not recommended

for use with any dc-to-dc converter because of their poor

temperature and dc bias characteristics.

Calculate the worst-case capacitance accounting for capacitor

variation over temperature, component tolerance, and voltage

using the following equation:

CEFFECTIVE = CNOMINAL × (1 − TEMPCO) × (1 − DCBIASCO) ×

(1 − Tolerance)

where:

CEFFECTIVE is the effective capacitance at the operating voltage.

CNOMINAL is the nominal data sheet capacitance.

TEMPCO is the worst-case capacitor temperature coefficient.

DCBIASCO is the dc bias derating at the output voltage.

Tolerance is the worst-case component tolerance.

To guarantee the performance of the device, it is imperative that

the effects of dc bias, temperature, and tolerances on the

behavior of the capacitors be evaluated for each application.

Capacitors with lower effective series resistance (ESR) and

effective series inductance (ESL) are preferred to minimize

output voltage ripple.

The use of large output capacitors can require a slower soft start

to prevent current limit during startup. A 10 μF capacitor is

suggested as a good balance between performance and size.

Input Capacitor

Higher value input capacitors help to reduce the input voltage

ripple and improve transient response.

To minimize supply noise, place the input capacitor as close as

possible to the AVIN pin and the PVIN pin. A low ESR capacitor

is recommended.

The effective capacitance needed for stability is a minimum of 10 μF.

If the power pins are individually decoupled, it is recommended

to use an effective minimum of a 5.6 μF capacitor on the PVIN pin

and a 3.3 μF capacitor on the AVIN pin. The minimum values

specified exclude dc bias, temperature, and tolerance effects that

are application dependent and must be taken into

consideration.

VREF Capacitor

A 1.0 μF ceramic capacitor (CVREF) is required between the

VREF pin and the AGND pin.

Soft Start Resistor

A resistor can be connected between the SS pin and the AGND pin

to increase the soft start time. The soft start time can be set by the

resistor between 4 ms (268 kΩ) and 32 ms (50 kΩ). Leaving the

SS pin open selects the fastest time of 4 ms. Figure 43 shows the

behavior of this operation. Calculate the soft start time using the

following formula:

tSS = 38.4 × 10−3 − 1.28 × 10−7 × RSS (Ω)

where 50 kΩ ≤ RSS ≤ 268 kΩ.

SS PIN OPEN

SOFT START

TIMER

SOFT START

RESISTOR

32ms

4ms

Figure 43. Soft Start Behavior

Diodes

Use a Schottky diode with low junction capacitance for diode 1

(D1) and diode 2 (D2). At higher output voltages and especially at

higher switching frequencies, the junction capacitance is a

significant contributor to efficiency. Higher capacitance diodes

also generate more switching noise. As a guide, a diode with less

than 40 pF junction capacitance is preferred when the output

voltage is above 5 V.

Inductor Selection for the Boost Regulator

The inductor stores energy during the on time of the power

switch and transfers that energy to the output through the

output rectifier during the off time. To balance the tradeoffs

between small inductor current ripple and efficiency, inductance

values in the range of 1 μH to 22 μH are recommended. In general,

lower inductance values have higher saturation current and

lower series resistance for a given physical size. However, lower

inductance results in a higher peak current that can lead to reduced

efficiency and greater input and/or output ripple and noise. A

peak-to-peak inductor ripple current that is close to 30% of the

maximum dc input current for the application typically yields

an optimal compromise.

For the inductor ripple current in continuous conduction mode

(CCM) operation, the VIN and output voltage (VPOS) determine

the switch duty cycle (DUTY1) using the following equation:



















DIODE1

POS

DIODE1

POS

DUTY

where VDIODE1 is the forward voltage drop of D1.

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