LT8580 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT8580

Boost/SEPIC/Inverting DC/DC Converter with 1A, 65V Switch, Soft-Start and Synchronization

Linear Technology 

LT8580

Applications Information

For uncoupled dual inductor topologies, the peak output

inductor current is given by:

IL2-PEAK

= IOUT +

VOUT • (1− DC)

2 • L2 • f

For the coupled inductor topologies:

IL2-PEAK

= IOUT

⎡⎣⎢1+

VOUT

h • VIN

⎤

⎦⎥

+

VIN • DC

2 •L • f

Note: Inductor current can be higher during load transients.

It can also be higher during start-up if inadequate soft-start

capacitance is used.

Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multilayer ceramic capacitors are an excellent choice, as

they have an extremely low ESR and are available in very

small packages. X5R or X7R dielectrics are preferred, as

these materials retain their capacitance over wider voltage

and temperature ranges. A 0.47µF to 10µF output capacitor

is sufficient for most applications. Always use a capacitor

with a sufficient voltage rating. Many ceramic capacitors,

particularly 0805 or 0603 case sizes, have greatly reduced

capacitance at the desired output voltage. Solid tantalum

or OS-CON capacitors can be used, but they will occupy

more board area than a ceramic and will have a higher

ESR with greater output ripple.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as closely

as possible to the VIN pin of the LT8580 as well as to the

inductor connected to the input of the power path. If it is

not possible to optimally place a single input capacitor,

then use one at the VIN pin of the chip (CVIN) and one at

the input of the power path (CPWR). See equations in

Table 4, Table 5 and Table 6 for sizing information. A 1µF

to 2.2µF input capacitor is sufficient for most applications.

Table 2 shows a list of several ceramic capacitor manu-

facturers. Consult the manufacturers for detailed informa-

tion on their entire selection of ceramic parts.

Table 2. Ceramic Capacitor Manufacturers

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

TDK

www.tdk.com

Compensation—Adjustment

To compensate the feedback loop of the LT8580, a series

resistor-capacitor network in parallel with a single capacitor

should be connected from the VC pin to GND. For most

applications, the series capacitor should be in the range

of 470pF to 2.2nF with 1nF being a good starting value.

The parallel capacitor should range in value from 10pF to

100pF with 47pF a good starting value. The compensation

resistor, RC, is usually in the range of 5k to 50k. A good

technique to compensate a new application is to use a

100kΩ potentiometer in place of series resistor RC. With

the series capacitor and parallel capacitor at 1nF and 47pF

respectively, adjust the potentiometer while observing

the transient response and the optimum value for RC can

be found. Figure 3 (3a to 3c) illustrates this process for

the circuit of Figure 4 with a load current stepped be-

tween 60mA and 160mA. Figure 3a shows the transient

response with RC equal to 2k. The phase margin is poor,

as evidenced by the excessive ringing in the output

voltage and inductor current. In Figure 3b, the value of

RC is increased to 3k, which results in a more damped

response. Figure 3c shows the results when RC is increased

further to 6k. The transient response is nicely damped and

the compensation procedure is complete.

Compensation—Theory

Like all other current mode switching regulators, the

LT8580 needs to be compensated for stable and efficient

operation. Two feedback loops are used in the LT8580—

a fast current loop which does not require compensation,

and a slower voltage loop which does. Standard bode plot

analysis can be used to understand and adjust the voltage

feedback loop.

8580f

12

For more information www.linear.com/LT8580

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