lt1737is-trpbf Linear Technology Corporation, lt1737is-trpbf Datasheet - Page 20

• Manufacturer Part Number: lt1737is-trpbf
• Description: High Power Isolated Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1737
the power paths (primary and secondary). B field (mag-
netic) radiation is minimized by keeping MOSFET leads,
output diode and output bypass capacitor leads as short
as possible. E field radiation is kept low by minimizing the
length and area of all similar traces. A ground plane
should always be used under the switcher circuitry to
prevent interplane coupling.
The high speed switching current paths are shown sche-
matically in Figure 7. Minimum lead length in these paths
are essential to ensure clean switching and minimal EMI.
The path containing the input capacitor, transformer pri-
mary and MOSFET, and the path containing the trans-
former secondary, output diode and output capacitor
contain “nanosecond” rise and fall times. Keep these
paths as short as possible.
20
U
U
W
+
Figure 7. High Speed Current Switching Paths
PGND
V
CC
V
CC
U
GATE
CHARGE
PATH
GATE
DISCHARGE
PATH
GATE
+
GATE DRIVE RESISTOR CONSIDERATIONS
The gate drive circuitry internal to the LT1737 has been
designed to have as low an output impedance as practi-
cally possible—only a few ohms. A strong L/C resonance
is potentially presented by the inductance of the path
leading to the gate of the power MOSFET and its overall
gate capacitance. For this reason the path from the GATE
package pin to the physical MOSFET gate should be kept
as short as possible, and good layout/ground plane prac-
tice used to minimize the parasitic inductance.
An explicit series gate drive resistor may be useful in some
applications to damp out this potential L/C resonance
(typically tens of MHz). A minimum value of perhaps
several ohms is suggested, and higher values (typically a
few tens of ohms) will offer increased damping. However,
as this resistor value becomes too large, gate voltage rise
time will increase to unacceptable levels, and efficiency
will suffer due to the sluggish switching action.
V
IN
PRIMARY
POWER
PATH
SECONDARY
POWER
PATH
+
1737 F07
1737fa