LTC4269-1
37
42691fc
pin should be separated from other high voltage pins, like
V
PORTP
, V
NEG
, to avoid the possibility of leakage currents
shutting down the LTC4269-1. If not used, tie SHDN to
V
PORTN
. The load capacitor connected between V
PORTP
and
V
NEG
of the LTC4269-1 can store signifi cant energy when
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
PORTN
is shorted to V
PORTP
inside the PD while capacitor C1
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
CC
decoupling
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
CC
capacitor
immediately adjacent to the V
CC
and GND pins on the IC
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.
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
DSS
ratings of the MOSFETs
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
CC
decoupling capacitor) and
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
IN
of the LTC4269-1
and the actual source V
IN
, is suffi cient to prevent current
sharing problems.
APPLICATIONS INFORMATION
T2
T1
C
R
C
VIN
MS
MP
GATE
TURN-ON
GATE
TURN-ON
R
SENSE
••
C
VCC
SG
V
CC
PG
V
CC
V
CC
V
CC
V
IN
GATE
TURN-OFF
GATE
TURN-OFF
Q4
Q3
C
OUT
42691 F19
OUT
•
•
•
+
+
+
Figure 19. Layout Critical High Current Paths
