LTC4258 datasheet(38/42 Pages) LINER

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LTC4278

4278fc

current will flow through the parasitic body diode of the

internal MOSFET and may cause permanent damage to

the LTC4278.

In order to minimize switching noise and improve output

loadregulation,connecttheGNDpinoftheLTC4278directly

to the ground terminal of the VCC decoupling capacitor,

the bottom terminal of the current sense resistor and the

groundterminaloftheinputcapacitor,usingagroundplane

with multiple vias. Place the VCC capacitor immediately

adjacent to the VCC and GND pins on the IC package. This

capacitor carries high di/dt MOSFET gate drive currents.

Use a low ESR ceramic capacitor.

TakecareinPCBlayouttokeepthetracesthatconducthigh

switching currents short, wide and with minimal overall

loop area. These are typically the traces associated with

the switches. This reduces the parasitic inductance and

also minimizes magnetic field radiation. Figure 19 outlines

the critical paths.

Keep electric field radiation low by minimizing the length

and area of traces (keep stray capacitances low). The drain

of the primary-side MOSFET is the worst offender in this

category. Always use a ground plane under the switcher

circuitry to prevent coupling between PCB planes.

Check that the maximum BVDSS ratings of the MOSFETs

are not exceeded due to inductive ringing. This is done by

viewingtheMOSFETnodevoltageswithanoscilloscope.If

it is breaking down, either choose a higher voltage device,

add a snubber or specify an avalanche-rated MOSFET.

Placethesmall-signalcomponentsawayfromhighfrequency

switching nodes. This allows the use of a pseudo-Kelvin

connection for the signal ground, where high di/dt gate

driver currents flow out of the IC ground pin in one direction

(to the bottom plate of the VCC decoupling capacitor) and

small-signal currents flow in the other direction.

Keep the trace from the feedback divider tap to the FB pin

short to preclude inadvertent pick-up.

For applications with multiple switching power converters

connected to the same input supply, make sure that the

input filter capacitor for the LTC4278 is not shared with

other converters. AC input current from another converter

could cause substantial input voltage ripple which could

interfere with the LTC4278 operation. A few inches of PC

trace or wire (L

@ 100nH) between the CIN of the LTC4278

and the actual source VIN, is sufficient to prevent current

sharing problems.

APPLICATIONS INFORMATION

CVIN

GATE

TURN-ON

GATE

TURN-ON

RSENSE

• •

CVCC

VCC

VIN

GATE

TURN-OFF

GATE

TURN-OFF

COUT

4278 F19

OUT

•

Figure 19. Layout Critical High Current Paths

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