LT8685SRVPBF datasheet(18/24 Pages) AD

18 / 24 page

LT8685S

Rev. 0

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APPLICATIONS INFORMATION

The worst-case ripple current occurs when VOUT is one-

half VIN. Under this condition, the ripple current is:

ICIN(RMS) =

IOUT

Reasonable starting values for the input capacitance are

2.2µF for channels 1 and 2 and 2.2µF for channels 3 and 4.

When combining channels, the shared channel VIN pins

must be connected together, and the input capacitor

should be chosen based on the total current supplied by

the combined channels.

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

droop and overshoot during transient loads. Because the

LT8685S buck converters are able to operate at a high

frequency, required output capacitance is minimal. The

internally compensated current mode control loops are

stable without requiring a minimum series resistance

(ESR) in the output capacitor. Therefore, ceramic capaci-

tors may be used and will result in very low output ripple.

An estimate for the output ripple is as follows for a given

capacitor type:

VRIPPLE =

ΔIL

8 • fSW •COUT

for ceramic capacitors, and

VRIPPLE= ΔIL • ESR

for aluminum or tantalum capacitors. VRIPPLE is the peak-

to-peak output ripple, fSW is the switching frequency in

MHz, ΔIL is the peak-to-peak ripple current in the inductor,

COUT is the output capacitor value in µF and ESR is the

output capacitor effective series resistance.

The low ESR and small size of ceramic capacitors make

them the preferred type for LT8685S applications.

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 LT8685S’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 out-

put voltage overshoot. To program the output voltage

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