LT8686SJVPBF datasheet(19/26 Pages) AD

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LT8686S

Rev. 0

For more information www.analog.com

However, not all ceramic capacitors are the same. Many

of the higher value capacitors use dielectrics with high

temperature and voltage coefficients. Y5V and Z5U types

lose a large fraction of their capacitance with applied volt-

age and at temperature extremes. Because loop stability,

transient response, ripple and EMI depend on the value of

the input and output capacitors, it is best to use X5R (max

85°C), X7R (max 125°C) or X8R (max 150°C) capacitors

depending on the operating temperature range.

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. Choose 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 value will be larger than a ceramic capacitor that

would give similar ripple performance.

The Typical Applications section provides a reasonable

starting point for output capacitor values. Careful evalua-

tion of each application must be made to ensure adequate

design margin.

BOOST CAPACITOR SELECTION

Connecting a capacitor between each channel’s BST and

SW pins creates an internal approximately 3.4V supply

used to drive the internal power devices. For most applica-

tions, choosing a 0.1μF ceramic capacitor for this function

works well.

Although the SW and BST pins of combined channels are

connected at the board level to drive a single inductor, for

robust operation, each of the combined channels must

have its own boost capacitor connected between their

respective BST and SW pins.

OUTPUT VOLTAGE TRACKING AND SOFT-START

The LT8686S’s programmable channel soft-start fea-

ture, which controls the output voltage ramp time during

startup, combined with channel power good (PG) and

enable (EN) functions, supports flexible startup sequenc-

ing and control. In addition, the soft-start feature can be

used to reduce input surge current and prevent output

voltage overshoot. To program the output voltage soft-

start time, connect a capacitor between the channel’s

TRK/SS pin and ground according to Equation 16.

tSS =

CSS •0.8V

2µA

(16)

where 2µA is the TRK/SS pull-up current and CSS is the

value of the capacitor in Farads.

The channel TRK/SSx pins are pulled down through

approximately 300k, which will discharge the external

soft-start capacitor when the part is shut down or during

certain fault conditions.

For output voltage tracking, the channel TRK/SS pin can

be driven by an external voltage source. More specifically,

when driven with a voltage between 0V and 0.8V, the TRK/

SS pin will override the internal 0.8V reference input to

the error amplifier thus regulating the FB voltage to that

present on the TRK/SS pin. When TRK/SS is above 0.8V,

tracking is disabled, and the feedback voltage will regulate

to the internal reference voltage.

When combining channels, slave channel TRK/SS pins

should be open.

POWER GOOD COMPARATORS

Each LT8686S channel has a power good comparator with

an open drain output pin, PGx. Each PG pin is pulled low

when the corresponding feedback voltage is either above

or below its reference voltage by more than 7.5%. See the

Electrical Characteristics table for more information on

each channel’s power good thresholds. All PG pins will be

pulled low when the part is shut down. Individual channel

PG pins will be pulled low when their corresponding EN

pin is low.

When combining channels, only the master channel PG

pins are valid. Slave PG pins should be open.

APPLICATIONS INFORMATION

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