lt3825 Linear Technology Corporation, lt3825 Datasheet - Page 26

• Manufacturer Part Number: lt3825
• Description: Isolated No-opto Synchronous Flyback Controller With Wide Input Supply Range
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3825
The total IC dissipation is computed as:
V
Junction temperature is computed as:
where:
PC Board Layout Considerations
In order to minimize switching noise and improve output
load regulation, connect the GND pin of the LT3825 directly
to the ground terminal of the V
the bottom terminal of the current sense resistor, the
ground terminal of the input capacitor, and the ground
plane (multiple vias). Place the V
26
CC
P
T
T
θ
impedance (40°C/W).
A
J
JA
D(TOTAL)
is the worst-case LT3825 supply voltage.
= T
is the ambient temperature
is the FE16 package junction-to-ambient thermal
A
+ P
= V
D
• θ
CC
JA
• (I
LT3825
V
CC
CC
+ f
C
VCC
V
GND
V
CC
LT3825
CC
OSC
SG
GND
• (Q
CC
CC
V
CC
PG
capacitor immediately
decoupling capacitor,
GPRI
TURN-OFF
+ C
GATE
TURN-ON
GATE
S
•
• V
T2
Figure 9. High Current Paths
•
R
SGMAX
SENSE
MP
))
C
R
V
IN
adjacent to the V
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 9 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
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.
C
VIN
Q4
Q3
TURN-OFF
•
•
TURN-ON
GATE
T1
GATE
•
CC
and GND pins on the IC package. This
MS
DSS
3825 F09
ratings of the MOSFETs
C
OUT
OUT
3525fa