LTC4007 datasheet(14/20 Pages) LINER

14 / 20 page

LTC4007

4007i

Highest possible voltage rating on the capacitor will mini-

mize problems. Consult with the manufacturer before use.

Alternatives include new high capacity ceramic (at least

µF) from Tokin, United Chemi-Con/Marcon, et al. Other

alternative capacitors include OS-CON capacitors from

Sanyo.

The output capacitor (C3) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

RMS

BAT

DCIN

()







()( )

029

.–

For example:

VDCIN = 19V, VBAT = 12.6V, L1 = 10µH, and

f = 300kHz, IRMS = 0.41A.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads, and beads or inductors

may be added to increase battery impedance at the 300kHz

switching frequency. Switching ripple current splits be-

tween the battery and the output capacitor depending on

the ESR of the output capacitor and the battery imped-

ance. If the ESR of C3 is 0.2

Ω and the battery impedance

is raised to 4

Ω with a bead or inductor, only 5% of the

current ripple will flow in the battery.

Inductor Selection

Higher operating frequencies allow the use of smaller

inductor and capacitor values. A higher frequency gener-

ally results in lower efficiency because of MOSFET gate

charge losses. In addition, the effect of inductor value on

ripple current and low current operation must also be

considered. The inductor ripple current

∆IL decreases

with higher frequency and increases with higher VIN.

∆= ()( )









OUT

1–

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆IL = 0.4(IMAX). In no case should

∆IL exceed 0.6(IMAX) due to limits imposed by IREV and

CA1. Remember the maximum

∆IL occurs at the maxi-

mum input voltage. In practice 10

µH is the lowest value

recommended for use.

Lower charger currents generally call for larger inductor

values. Use Table 4 as a guide for selecting the correct

inductor value for your application.

Table 4

MAX AVERAGE

MINIMUM INDUCTOR

CURRENT (A)

INPUT VOLTAGE (V)

VALUE (

µH)

≤20

±20%

1>20

±20%

≤20

±20%

2>20

±20%

≤20

±20%

3>20

±20%

≤20

±20%

4>20

±20%

Charger Switching Power MOSFET

and Diode Selection

Two external power MOSFETs must be selected for use

with the charger: a P-channel MOSFET for the top (main)

switch and an N-channel MOSFET for the bottom (syn-

chronous) switch.

The peak-to-peak gate drive levels are set internally. This

voltage is typically 6V. Consequently, logic-level threshold

MOSFETs must be used. Pay close attention to the BVDSS

specification for the MOSFETs 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), total gate capacitance QG, reverse

transfer capacitance CRSS, input voltage and maximum

output current. The charger is operating in continuous

mode at moderate to high currents so the duty cycles for

the top and bottom MOSFETs are given by:

Main Switch Duty Cycle = VOUT/VIN

Synchronous Switch Duty Cycle = (VIN – VOUT)/VIN.

APPLICATIO S I FOR ATIO

LTC4007 датащи(PDF) 14 Page - Linear Technology

LTC4007 датащи(HTML) 14 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

датащи скачать

ссылки URL