LT8604 datasheet(14/20 Pages) AD

14 / 20 page

LT8604

Rev. 0

For more information www.analog.com

to store energy in order to satisfy transient loads and sta-

bilize the LT8604’s control loop. Ceramic capacitors have

very low equivalent series resistance (ESR) and provide

the best ripple performance. A good starting value is:

COUT =

VOUTfSW

where fSW is the switching frequency in MHz, VOUT is

the output voltage, and COUT is the recommended output

capacitance in μF. 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 capaci-

tor 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 LT8604 due to their piezoelectric

nature. When in Burst Mode operation, the LT8604’s

switching frequency depends on the load current, and at

very light loads the LT8604 can excite the ceramic capacitor

at audio frequencies, generating audible noise. Since the

LT8604 operates at a lower current limit during Burst Mode

operation, the noise is typically very quiet. If this is unac-

ceptable, use a high performance tantalum or electrolytic

capacitor at the output.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT8604. As

previously mentioned, a ceramic input capacitor com-

bined with trace or cable inductance forms a high qual-

ity (under damped) tank circuit. If the LT8604 circuit is

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

twice its nominal value, possibly exceeding the LT8604’s

rating. This situation is easily avoided (see Analog Devices

Application Note 88).

EN/UV Pin

The LT8604 is in shutdown when the EN/UV pin is low

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

the EN/UV comparator is 1.05V, with 50mV of hysteresis.

The EN/UV 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/UV programs

the LT8604 to regulate the output only when VIN is above

a desired voltage (see Block Diagram). Typically, this

threshold, VIN(EN/UV), 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/UV) 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 the following equation:

VIN(EN/UV)

–1

⎛

⎝⎜

⎞

⎠⎟ •R4

where the LT8604 will remain off until VIN isaboveVIN(EN/UV).

Due to the comparator’s hysteresis, switching will not stop

until the input falls slightly below VIN(EN/UV).

For light-load currents, the current through the VIN(EN/UV)

resistor network can easily be greater than the supply current

consumed by the LT8604. Therefore, the VIN(EN/UV) resis-

tors 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 LT8604’s circuitry and must be bypassed to

APPLICATIONS INFORMATION

LT8604 データシート(PDF) 14 Page - Analog Devices

LT8604 データシート(HTML) 14 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

データシートダウンロード