LT1956 데이터 시트보기 (PDF) - Linear Technology
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LT1956 Datasheet PDF : 28 Pages
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LT1956/LT1956-5
APPLICATIO S I FOR ATIO
LAYOUT CONSIDERATIONS
As with all high frequency switchers, when considering
layout, care must be taken in order to achieve optimal
electrical, thermal and noise performance. For maximum
efficiency, switch rise and fall times are typically in the
nanosecond range. To prevent noise both radiated and
conducted, the high speed switching current path, shown
in Figure 5, must be kept as short as possible. This is
implemented in the suggested layout of Figure 6. Shorten-
ing this path will also reduce the parasitic trace inductance
of approximately 25nH/inch. At switch off, this parasitic
inductance produces a flyback spike across the LT1956
switch. When operating at higher currents and input
voltages, with poor layout, this spike can generate volt-
ages across the LT1956 that may exceed its absolute
maximum rating. A ground plane should always be used
under the switcher circuitry to prevent interplane coupling
and overall noise.
LT1956
HIGH
FREQUENCY
VIN
C3 CIRCULATING
PATH
L1
5V
D1 C1
LOAD
1956 F05
Figure 5. High Speed Switching Path
The VC and FB components should be kept as far away as
possible from the switch and boost nodes. The LT1956
pinout has been designed to aid in this. The ground for
these components should be separated from the switch
current path. Failure to do so will result in poor stability or
subharmonic like oscillation.
CONNECT TO
GROUND PLANE
GND
MINIMIZE LT1956
C3-D1 LOOP
GND
D1
L1
D2
C2
FOR THE FE PACKAGE,
C1
SOLDER THE EXPOSED
PAD TO THE COPPER
GROUND PLANE
VOUT
UNDERNEATH THE DEVICE
GND GND
SW
SHDN
KELVIN SENSE
VOUT
C3
VIN
VIN
LT1956
FB
BOOST VC
BIAS
GND GND
SYNC
R2
R1 CFB
CF
RC
PLACE FEEDTHROUGH AROUND
GROUND PINS (4 CORNERS) FOR
GOOD THERMAL CONDUCTIVITY
KEEP FB AND VC COMPONENTS
CC
AWAY FROM HIGH FREQUENCY,
HIGH CURRENT COMPONENTS
1956 F06
Figure 6. Suggested Layout
1956f
17
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