LTC4449 datasheet(21/38 Pages) LINER

21 / 38 page

LTC3774

3774fc

For more information www.linear.com/LTC3774

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requires

moreturnsofwireandthereforecopperlosseswillincrease.

Ferrite designs have very low core loss and are preferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

PWM and PWMEN Pins

The PWM pins are three-state compatible outputs, de-

signed to drive MOSFET drivers, DrMOSs, etc which do

not represent a heavy capacitive load. An external resistor

divider may be used to set the voltage to mid-rail while in

the high impedance state.

ThePWMENoutputshaveanopen-drainpull-uptoINTVCC

and require an appropriate external pull-down resistor.

This pin is intended to drive the enable pins of the MOS-

FET drivers that do not have three-state compatible PWM

inputs. PWMEN is low only when PWM is high impedance,

and high at any other PWM state.

When selecting a DrMOS or gate driver to use with the

LTC3774, care must be taken to ensure that the absolute

maximum voltage rating for the DrMOS or gate driver’s

PWM input is not exceeded. The LTC3774’s PWM output

driver is biased from INTVCC, which is typically 5.5V,

while the DrMOS or gate driver is generally biased from

a 5V supply. If the DrMOS or gate driver has a maximum

PWM rating less than 5.5V then tie the VIN and INTVCC

pins of the LTC3774 together and tie the combined pins

to the 5V supply with a 1Ω or 2.2 Ω resistor. Please a

4.4µF capacitor from the combined VIN and INTVCC pins

to ground. Refer to Figure 11 for an example. Contact

factory applications support for assistance.

APPLICATIONS INFORMATION

Power MOSFET and Schottky Diode

(Optional) Selection

At least two external power MOSFETs need to be selected:

One N-channel MOSFET for the top (main) switch and one

or more N-channel MOSFET(s) for the bottom (synchro-

nous) switch. The number, type and on-resistance of all

MOSFETsselectedtakeintoaccountthevoltagestep-down

ratio as well as the actual position (main or synchronous)

in which the MOSFET will be used. A much smaller and

much lower input capacitance MOSFET should be used

for the top MOSFET in applications that have an output

voltage that is less than one-third of the input voltage. In

applications where VIN >> VOUT, the top MOSFETs’ on-

resistance is normally less important for overall efficiency

than its input capacitance at operating frequencies above

300kHz. MOSFET manufacturers have designed special

purposedevicesthatprovidereasonablylowon-resistance

with significantly reduced input capacitance for the main

switch application in switching regulators.

The peak-to-peak MOSFET gate drive levels are set by the

internal regulator voltage, VINTVCC, requiring the use of

logic-level threshold MOSFETs in most applications. Pay

close attention to the BVDSSspecificationfortheMOSFETs

as well; many of the logic-level MOSFETs are limited to

30V or less. Selection criteria for the power MOSFETs

include the on-resistance, RDS(ON), input capacitance,

inputvoltageandmaximumoutputcurrent.MOSFETinput

capacitance is a combination of several components but

can be taken from the typical gate charge curve included

on most data sheets (Figure 8). The curve is generated by

forcing a constant input current into the gate of a common

source, current source loaded stage and then plotting the

gate voltage versus time.

Figure 8. Gate Charge Characteristic

–

VDS

VIN

3774 F08

VGS

MILLER EFFECT

QIN

CMILLER = (QB – QA)/VDS

VGS

–

LTC4449 Datenblatt(PDF) 21 Page - Linear Technology

LTC4449 Datenblatt(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Datenblatt Download

Link URL