ADP5080 datasheet(34/64 Pages) AD

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ADP5080

Data Sheet

APPLICATIONS INFORMATION

This section provides component and PCB layout guidelines

to ensure optimal device performance, efficiency, stability, and

minimal switching noise and crosstalk.

COMPONENT SELECTION FOR THE BUCK AND

BUCK BOOST REGULATORS

Setting the Output Voltage (Adjustable Mode Channels)

Channel 3, Channel 4, and Channel 6 can be configured for an

adjustable output voltage. Table 9 provides the adjustable output

voltage range for these channels. When any of these channels is

configured for adjustable mode, connect a resistor divider to the

FBx pin between VOUT and GND, as shown in Figure 56.

Figure 56. Feedback Resistors for Adjustable Output

The resistor values can be calculated as follows, where 0.8 V

is the typical FB voltage, and 20 kΩ is a good typical value for

RFB_BOT.

(

)

BOT

OUT

TOP

−

Note that changing the output voltage often requires a change

to the inductor (L) and output capacitor (COUT) values. After the

VOUT value is selected, calculate and test the L and COUT values

(see the Selecting the Inductor section and the Selecting the

Output Capacitor section).

Selecting the Inductor

The required inductor value can be determined by the input and

output voltages, the switching frequency, and the ripple current,

as shown in Equation 1.

OUT

RIPPLE

OUT

−

(1)

where:

L is the inductor value.

fSW is the switching frequency.

IRIPPLE is a peak-to-peak value for the ripple current.

In general, the recommended ripple current is 30% of the maxi-

mum load current. Therefore, Equation 1 can be rewritten as

follows:

OUT

LOAD

OUT

−

(2)

Note that ripple current varies with input voltage. The typical

input voltage can be used to determine the inductor value.

However, to avoid inductor saturation and current limit, also

calculate the inductor value with the worst-case input voltage

(VIN max).

The maximum rated current of the selected inductor (both rms

current and saturation current) must be greater than the peak

inductor current (IPEAK) at the maximum load current. If the rating

of the inductor is not sufficient, the inductor may saturate due to

inductor value degradation, causing it to reach the current limit,

even in a lower load condition than expected.

The peak current can be estimated using Equation 3.

IPEAK = ILOAD + (IRIPPLE/2)

(3)

If the 30% ripple guideline is followed, typical peak current is

simplified as follows:

IPEAK = (ILOAD + 0.15) × ILOAD = 1.15 × ILOAD

(4)

Another important specification to consider is the parasitic

series resistance in the inductor: dc resistance (DCR). A larger

DCR decreases efficiency, but a larger size inductor typically has

lower DCR. Therefore, the trade-off between available space on

the PCB and device performance must be considered carefully.

Equation 1 to Equation 4 apply to the buck regulators. Although

Channel 6 is a buck boost regulator, the inductor value can be

determined using the buck regulator mode of operation given

that the available step-up ratio in boost mode is relatively small

(4 V at the PVIN6 pin to 5.5 V at the VOUT6 pin) compared to

the available step-down ratio. Therefore, an inductor value selected

for buck regulator mode typically works equally well in boost

regulator mode.

Table 13 lists recommended inductor values for a range of volt-

ages and frequencies. The values provided are based on a wide

operating range and assume the maximum load current for each

channel. In the actual application, larger or smaller values may be

more appropriate. In general, the inductor value can be increased

or decreased by one standard value from the recommended 30%

ripple guideline. A larger inductance provides higher efficiency,

whereas smaller values results in better transient response and

a smaller footprint. Note that inductor values much smaller or

larger than the ones recommended in Table 13 may cause control

loop instability.

It is also important to note that because the current-limit pro-

tection monitors peak or valley current, the selected inductance

affects the load current level at which current limit is triggered.

VOUT

FBx

RFB_TOP

RFB_BOT

Rev. A | Page 34 of 64

ADP5080 Datasheet(PDF) 34 Page - Analog Devices

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