LT8640 datasheet(17/22 Pages) AD

17 / 22 page

LT8644S

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

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 LT8644S due to their piezoelectric

nature. When in Burst Mode operation, the LT8644S’s

switching frequency depends on the load current, and

at very light loads the LT8644S can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT8644S 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 LT8644S. As

previously mentioned, a ceramic input capacitor com-

bined with trace or cable inductance forms a high qual-

ity (underdamped) tank circuit. If the LT8644S circuit is

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

twice its nominal value, possibly exceeding the LT8644S’s

rating. This situation is easily avoided (see ADI Application

Note 88).

Enable Pin

The LT8644S is in shutdown when the EN pin is low and

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

comparator is 0.98V, 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

LT8644S 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 10.

VIN(EN) =

+ 1

⎛

⎝

⎜

⎞

⎠

⎟ • 0.98V

(10)

where the LT8644S 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 LT8644S. 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

3.6V supply from SVIN that powers the drivers and the

internal bias circuitry. The INTVCC can supply enough cur-

rent for the LT8644S’s circuitry. Voltage on INTVCC will

vary between 2.6V and 3.6V when SVIN is between 2.7V

and 3.7V. If SVIN is connected to a different supply than

PVIN, be sure to bypass with a local ceramic capacitor. Do

not connect an external load to the INTVCC pin.

Frequency Compensation

Loop compensation determines the stability and transient

performance, and is provided by the components tied to

the VC pin. Generally, a capacitor (CC) and a resistor (RC)

in series to ground are used. Designing the compensation

network is a bit complicated and the best values depend

on the application. A practical approach is to start with

one of the circuits in this data sheet that is similar to your

application and tune the compensation network to opti-

mize the performance. LTpowerCAD® simulations can help

in this process. Stability should then be checked across all

operating conditions, including load current, input voltage

and temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

how to test the stability using a transient load.

LT8640 Datasheet(PDF) 17 Page - Analog Devices

LT8640 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL