LT8697 datasheet(23/40 Pages) AD

23 / 40 page

LT8698S/LT8698S-1

Rev. A

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APPLICATIONS INFORMATION

current into a tight local loop, minimizing EMI. A second

precaution regarding the ceramic input capacitor con-

cerns the maximum input voltage rating of the LT8698S.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (under damped) tank cir-

cuit. If the LT8698S circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8698S’s voltage rating. This situation is

easily avoided (see ADI Application Note 88).

Output Capacitor and Output Ripple

The output capacitor COUT tied from the OUT/ISP pin to

ground has two essential functions. Along with the induc-

tor, it filters the square wave generated by the LT8698S

to produce the DC output. In this role it determines the

output ripple, thus low impedance at the switching fre-

quency is important. The second function is to store

energy in order to satisfy transient loads and stabilize

the LT8698S’s control loop. Ceramic capacitors have very

low equivalent series resistance (ESR) and provide the

best ripple performance. For good starting values, see

the Typical Applications section.

On a switching regulator with fixed output voltage, tran-

sient performance can generally be improved with a

higher value output capacitor. However, the LT8698S’s

cable drop compensation feature changes the local output

voltage in response to a load step. Since transient load

steps require the LT8698S to slew COUT, larger COUT in

this case can degrade transient response. Only remote

capacitance tied at the end of the cable near the USB

socket can improve transient response. The minimum

local output capacitance required for LT8698S loop stabil-

ity and for output voltage ripple requirements is the best

choice for COUT. See the Typical Applications section in

this data sheet for suggested capacitor values.

The USB 2.0 specification document requires a minimum

of 120µF low-ESR capacitor be tied from VBUS to ground

on a hub. Since the downstream device fixed VBUS capaci-

tance is limited to a maximum of 10µF, this minimum

120µF VBUS capacitance prevents hot plugging of one

downstream device from glitching the VBUS and disrupt-

ing another connected device. If the LT8698S is powering

VBUS on a hub, then this requirement can be satisfied

with extra COUT tied from either OUT/ISP to ground. There

must always be a minimum of 22µF capacitance tied from

OUT/ISP to ground.

Keep the capacitance tied to the BUS/ISN node lower in

magnitude than the capacitance tied to the OUT/ISP node.

Large BUS/ISN node capacitance to ground degrades the

regulator loop phase margin and gives poor cable drop

compensation transient response. If an output capaci-

tance much larger than 22µF is desired, tie this capacitor

to the OUT/ISP node, not to the BUS/ISN node.

When choosing a capacitor, special attention should be

given to the effective capacitance under the relevant oper-

ating conditions of voltage bias and temperature. A physi-

cally larger capacitor or one with a higher voltage rating

may be required.

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT8698S due to their piezoelectric

nature. In pulse-skipping mode operation, the LT8698S’s

switching frequency depends on the load current, and

at very light loads the LT8698S can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT8698S operates at a lower current limit during

pulse-skipping operation, the noise is typically very quiet

to a casual ear. If this is unacceptable, use a high perfor-

mance tantalum or electrolytic capacitor at the output.

Low noise ceramic capacitors are also available.

Table 5. Ceramic Capacitor Manufacturers

VENDOR

WEBSITE

Taiyo Yuden

www.ty-top.com

AVX

www.avxcorp.com

Murata

www.murata.com

TDK

www.tdk.com

EN/UV Pin

The LT8698S is in shutdown when the EN/UV pin is

< 0.25V and active when the pin is high. The falling thresh-

old of the EN/UV comparator is 1.31V, with 150mV of

hysteresis. The EN/UV pin can be tied to VIN if the shut-

down feature is not used, or driven to the desired logic

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