LT8650SPJVPBF datasheet(20/34 Pages) AD

20 / 34 page

LT8650SP

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

When choosing a capacitor, special attention should be

giventothedatasheettocalculatetheeffectivecapacitance

undertherelevantoperatingconditionsofvoltagebiasand

temperature. A physically 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 LT8650SP due to their piezoelectric

nature. When in Burst Mode operation, the LT8650SP’s

switching frequency depends on the load current, and

at very light loads the LT8650SP can excite the ceramic

capacitor at audio frequencies, generating audible noise.

SincetheLT8650SPoperatesatalowercurrentlimitduring

Burst Mode operation, the noise is typically very quiet to a

casual ear. If this is unacceptable, use a high performance

tantalum or electrolytic capacitor at the output. Low noise

ceramic capacitors are also available.

Table 3. Ceramic Capacitor Manufacturers

MANUFACTURER

WEB

Taiyo Yuden

www.t-yuden.com

AVX

www.avxcorp.com

Murata

www.murata.com

TDK

www.tdk.com

Enable Pin

The LT8650SP is in shutdown when both EN/UV pins are

low and active when either pin is high. The rising threshold

of the EN/UV comparator is 0.74V, with 30mV of hyster-

esis. The EN/UV pins can be tied to VIN if the shutdown

feature is not used, or tied to a logic level if shutdown

control is required.

Adding a resistor divider from VIN to EN/UV programs the

LT8650SP to operate only when VIN is above a desired

voltage (see the Block Diagram). Typically, this threshold,

VIN(EN), is used in situations where the input supply is cur-

rent limited, or has a relatively high source resistance. A

switchingregulatordrawsconstantpowerfromthesource,

so source current increases as source voltage drops. This

looks like a negative resistance load to the source and can

cause the source to current limit or latch low under low

source voltage conditions. The VIN(EN) threshold prevents

the regulator from operating at source voltages where the

problems might occur. This threshold can be adjusted by

setting the values R5 and R6 (R7 and R8 for channel 2)

such that they satisfy the following equation:

VIN(EN) =

⎛

⎝⎜

⎞

⎠⎟

•0.74V

wherethecorrespondingchannelwillremainoffuntilVINis

above VIN(EN).Duetothecomparator’shysteresis,switch-

ing will not stop until the input falls slightly below VIN(EN).

When operating in Burst Mode operation for light load

currents, the current through the VIN(EN) resistor network

can easily be greater than the supply current consumed

by the LT8650SP. Therefore, the VIN(EN) resistors should

be large to minimize their effect on efficiency at low loads.

VCC Regulator

Aninternallowdropout(LDO)regulatorproducesthe3.4V

supply from VIN1 that powers the drivers and the internal

bias circuitry. For this reason, VIN1 must be present and

valid to use either channel. The VCC can supply enough

currentfortheLT8650SP’scircuitryandmustbebypassed

to ground with a 1μF ceramic capacitor. Good bypassing is

necessary to supply the high transient currents required

by the power MOSFET gate drivers. To improve efficiency

the internal LDO can also draw current from the BIAS pin

when the BIAS pin is at 3.1V or higher. Typically the BIAS

pin can be tied to the lowest output or external supply

above 3.1V. If BIAS is connected to a supply other than

VOUT, be sure to bypass with a local ceramic capacitor. If

the BIAS pin is below 3V, the internal LDO will consume

current from VIN1.

Applications with high input voltage and high switching

frequency where the internal LDO pulls current from VIN1

will increase die temperature because of the higher power

dissipation across the LDO. Do not connect an external

load to the VCC pin.

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