LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 38

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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LTC4269-1
pin should be separated from other high voltage pins, like
V
shutting down the LTC4269-1. If not used, tie SHDN to
V
V
fully charged. The design of a PD must ensure that this
energy is not inadvertently dissipated in the LTC4269-1.
The polarity-protection diodes prevent an accidental short
on the cable from causing damage. However if, V
is shorted to V
is charged, current will fl ow through the parasitic body
diode of the internal MOSFET and may cause permanent
damage to the LTC4269-1.
In order to minimize switching noise and improve output
load regulation, connect the GND pin of the LTC4269-1
directly to the ground terminal of the V
capacitor, the bottom terminal of the current sense resistor
and the ground terminal of the input capacitor, using a
ground plane with multiple vias. Place the V
immediately adjacent to the V
package. This capacitor carries high di/dt MOSFET gate
drive currents. Use a low ESR ceramic capacitor.
Take care in PCB layout to keep the traces that conduct high
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 fi eld radiation. Figure 19 outlines
the critical paths.
APPLICATIONS INFORMATION
38
PORTN
PORTP
NEG
of the LTC4269-1 can store signifi cant energy when
, V
. The load capacitor connected between V
NEG
, to avoid the possibility of leakage currents
PORTP
V
CC
inside the PD while capacitor C1
V
C
CC
VCC
SG
V
CC
CC
and GND pins on the IC
V
CC
PG
Figure 19. Layout Critical High Current Paths
TURN-OFF
TURN-ON
GATE
CC
GATE
CC
•
decoupling
T2
PORTP
capacitor
•
R
SENSE
PORTN
and
MP
+
V
IN
Keep electric fi eld 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 BV
are not exceeded due to inductive ringing. This is done by
viewing the MOSFET node voltages with an oscilloscope. If
it is breaking down, either choose a higher voltage device,
add a snubber or specify an avalanche-rated MOSFET.
Place the small-signal components away from high frequen-
cy switching nodes. This allows the use of a pseudo-Kelvin
connection for the signal ground, where high di/dt gate
driver currents fl ow out of the IC ground pin in one direction
(to the bottom plate of the V
small-signal currents fl ow 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 fi lter capacitor for the LTC4269-1 is not shared with
other converters. AC input current from another converter
could cause substantial input voltage ripple which could
interfere with the LTC4269-1 operation. A few inches of PC
trace or wire (L ≅ 100nH) between the C
and the actual source V
sharing problems.
C
VIN
+
•
•
C
T1
R
•
Q4
Q3
TURN-OFF
TURN-ON
GATE
GATE
IN
, is suffi cient to prevent current
CC
DSS
MS
decoupling capacitor) and
ratings of the MOSFETs
+
IN
42691 F19
of the LTC4269-1
C
OUT
OUT
42691f