LTC1698IGN datasheet(17/24 Pages) LINER

17 / 24 page

LTC1698

1698f

Once the value of the inductor has been determined, an

inductor with sufficient DC current rating is selected. Core

saturation must be avoided under all operating conditions.

Under start-up conditions, the converter sees a short

circuit while charging the output capacitor. If the inductor

saturates, the peak current will dramatically increase. The

current will be limited only by the primary controller

minimum on time and the circuit impedances.

High efficiency converters generally cannot afford the core

loss found in low cost iron powder cores, forcing the use

of more expensive ferrite, molypermalloy, or Kool M

µ®

cores. As inductance increases, core loss goes down.

Increased inductance requires more turns of wire so

copper losses will increase. The optimum inductor will

have equal core and copper loss.

Ferrite designs have very low core losses and are preferred

at higher switching frequencies. Therefore, design goals

concentrate on minimizing copper loss and preventing

saturation. Kool M

µ is a very good, low-loss powder

material with a “soft” saturation characteristic.

Molypermalloy is more efficient at higher switching fre-

quencies, but is also more expensive. Surface mount

designs are available from many manufacturers using all

of these materials.

Output Capacitor Selection

The output capacitor selection is primarily determined by

the effective series resistance (ESR) to minimize voltage

ripple. In a forward converter application, the inductor

current is constantly flowing to the output capacitor,

therefore, the ripple current at the output capacitor is

small. The output ripple voltage is approximately given by:

ESR

RIPPLE

OUT

≈+









•

••

The output ripple is highest at maximum input voltage

since IRIPPLE increases with input voltage. Typically, once

the ESR requirement for COUT has been satisfied the

capacitance is adequate for filtering and has the required

RMS current rating.

Fast load current transitions at the output will appear as a

voltage across the ESR of the output capacitor until the

feedback loop can change the inductor current to match

the new load current value. As an example: at 3.3V out, a

10A load step with a 0.01

Ω ESR output capacitor would

experience a 100mV step at the output, a 3% output

change. In surface mount applications, multiple capaci-

tors may have to be placed in parallel to meet the ESR

requirement.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1698. These items are also illustrated graphically in

Figure 9. Check the following for your layout:

1. Keep the power circuit and the signal circuit segre-

gated. Place the power circuit, shown in bold, so that

the two MOSFET drain connections are made directly at

the transformer. The two MOSFET sources should be as

close together as possible.

2. Connect PGND directly to the sense resistor with as

short a path as possible. The MOSFET gate drive return

currents flow through this connection.

3. Connect the 4.7

µF ceramic capacitor directly between

VDD and PGND. This supplies the FG and CG drivers and

must supply the gate drive current.

4. Bypass the VAUX supply with a 0.1µF ceramic capacitor

returned to GND.

5. Place all signal components in close proximity to their

associated LTC1698 pins. Return all signal component

grounds directly to the GND pin. One common connec-

tion can be made to VOUT+ from R2, R5 and CCILM.

6. Make the connection between GND and PGND right at

the LTC1698 pins.

7. Use a Kelvin-sense connection from the ISNSandISNSGND

pins to the secondary-side current-limit resistor

RSECSEN.

APPLICATIO S I FOR ATIO

Kool M

µ is a registered trademark of Magnetics, Inc.

LTC1698IGN Datasheet(PDF) 17 Page - Linear Technology

LTC1698IGN Datasheet(HTML) 17 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

Datasheet Download

Link URL