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LT8602

8602fb

For more information www.linear.com/LT8602

applicaTions inForMaTion

Output Capacitor Selection

The output capacitor performs two functions. First, it

filters the inductor current to generate an output with

low voltage ripple. Second, it stores energy to minimize

overshoot during transient loads. Because the LT8602

operatesatahighfrequency,minimaloutputcapacitanceis

necessary. The control loop operates well with or without

the presence of output capacitor series resistance (ESR).

Ceramic capacitors, which achieve very low output ripple

and small circuit size, are therefore an option.

Youcanestimateoutputripplewiththefollowingequations:

VRIPPLE =

ΔIL

8 • fS •COUT

, for ceramic

VRIPPLE = ∆IL • ESR, for aluminum or tantalum.

where VRIPPLE is the peak-to-peak output ripple, fS is the

switching frequency in MHz, ∆IL is the peak-to-peak ripple

current in the inductor, COUT is the output capacitor value

in µF and ESR is the output capacitor series resistance.

Another constraint on the output capacitor is that it must

have greater energy storage than the inductor. When the

load current steps from high to low, the stored energy

in the inductor transfers to the output and the resulting

voltage step should be small compared to the regulation

voltage. For a 5% overshoot, this requirement indicates:

COUT ≥10•L •

ILIM

VOUT

⎛

⎝⎜

⎞

⎠⎟

where ILIM is the maximum switch current limit.

The low ESR and small size of ceramic capacitors make

them the preferred type for LT8602 applications. Not all

ceramic capacitors are the same, however. Many of the

higher value capacitors use poor dielectrics with high

temperature and voltage coefficients. In particular, Y5V

and Z5U types lose a large fraction of their capacitance

withappliedvoltageandattemperatureextremes.Because

loop stability and transient response depend on the value

of COUT, this loss may be unacceptable. Use X7R or X5R

types.

Electrolytic capacitors are also an option. The ESRs of

most aluminum electrolytic capacitors are too large to

deliver low output ripple. Tantalum, as well as newer,

lower-ESR, organic electrolytic capacitors intended for

power supply use are suitable. Chose a capacitor with a

low enough ESR for the required output ripple. Because

the volume of the capacitor determines its ESR, both the

size and the value will be larger than a ceramic capacitor

that would give similar ripple performance. One benefit

is that the larger capacitance may give better transient

response for large changes in load current. Table 3 lists

several capacitor vendors.

Table 3. Low ESR Capacitor Vendors

VENDOR

SERIES

TYPE

Taiyo-Yuden

www.t-yuden.com

Ceramic

TDK

www.tdk.com

Ceramic

Kemet

www.kemet.com

T494, T495

T510, T520, T525, T530

A700

Ceramic

Tantalum

Tantalum Organic Polymer

Alum. Organic Polymer

Panasonic

www.panasonic.com SP-CAP

Ceramic

Alum. Organic Polymer

AVX

www.avx.com

TPS, TES, TCH

Ceramic

Tantalum

BST and SW Pin Considerations

The high voltage channels require a voltage above PVIN

to drive the gates of the top NFET switches. Connect an

external capacitor between the BST and SW pins. An

internal MOS switch connects BST to the internal INTVCC

supply during the switch off cycles. Then BST is boosted

above SW during the switch on cycles. In most cases, a

0.1μF capacitor will work well.

Soft-Start

TheLT8602hasasoft-startpinforeachhighvoltagechan-

nel and internal soft-start for each low voltage channel.

The low voltage channel soft-start is set to 1ms.

On the high voltage channels, the feedback pin voltage

is regulated to the lower of the corresponding TRKSS

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