LT8640 datasheet(20/28 Pages) AD

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LT8640A

Rev. 0

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APPLICATIONS INFORMATION

the LT8640A to produce the DC output. In this role it

determines the output ripple, thus low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT8640A’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.

Use X5R or X7R types. This choice will provide low output

ripple and good transient response. Transient performance

can be improved with a higher value output capacitor and the

addition of a feedforward capacitor placed between VOUT and

FB. Increasing the output capacitance will also decrease the

output voltage ripple. A lower value of output capacitor can

be used to save space and cost but transient performance

will suffer and may cause loop instability. See the Typical

Applications in this data sheet for suggested capacitor values.

When choosing a capacitor, special attention should be

given to the data sheet to calculate the effective capaci-

tance under the relevant operating conditions of voltage

bias and 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 LT8640A due to their piezoelectric

nature. When in Burst Mode operation, the LT8640A’s

switching frequency depends on the load current, and

at very light loads the LT8640A can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT8640A operates at a lower current limit during

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.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT8640A. As

previously mentioned, a ceramic input capacitor com-

bined with trace or cable inductance forms a high qual-

ity (underdamped) tank circuit. If the LT8640A circuit is

plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT8640A’s

rating. This situation is easily avoided (see ADI Note 88).

Enable Pin

The LT8640A is in shutdown when the EN pin is low and

active when the pin is high. The rising threshold of the EN

comparator is 1.0V, with 40mV of hysteresis. The EN pin

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 programs the

LT8640A to regulate the output 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 current limited, or has a relatively high source

resistance. A switching regulator draws constant power

from the source, 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 R3 and R4 such that

they satisfy Equation 11.

VIN(EN) =

+ 1

⎛

⎝⎜

⎞

⎠⎟

• 1.0V

(11)

where the LT8640A will remain off until VIN is above

VIN(EN). Due to the comparator’s hysteresis, switching

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 LT8640A. Therefore, the VIN(EN) resistors should

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

INTVCC Regulator

An internal low dropout (LDO) regulator produces the

3.4V supply from VIN that powers the drivers and the

internal bias circuitry. The INTVCC can supply enough cur-

rent for the LT8640A’s circuitry and must be bypassed to

ground with a minimum of 1µF ceramic capacitor. Good

bypassing is necessary to supply the high transient cur-

rents required by the power MOSFET gate drivers. To

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