LT3430-1 Datasheet, PDF (17/28 Pages) Linear Technology – High Voltage, 3A,200kHz/100kHz Step-Down Switching Regulators

LT3430/LT3430-1
APPLICATIONS INFORMATION
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-
LT3430/
LT3430-1
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 LT3430/
LT3430-1 switch. When operating at higher currents and
input voltages, with poor layout, this spike can generate
voltages across the LT3430/LT3430-1 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.
L1
5V
HIGH
FREQUENCY
V_IN
C3 _CIRCULATING
PATH
D1 C1
LOAD
1 GND
3430 F05
Figure 5. High Speed Switching Path
2 SW
3 ^V_{IN LT3430/}
4 ^VIN ^LT3430-1
5 SW
6 BOOST
V_IN PINS 3 AND 4
ARE SHORTED TOGETHER.
SW PINS 2 AND 5 ARE ALSO
MINIMIZE
LT3430/LT3430-1
C3-D1 LOOP
SHORTED TOGETHER (USING
AVAILABLE SPACE UNDERNEATH
GND
D1
THE DEVICE BETWEEN PINS AND
GND PLANE)
C3
V_IN
L1
D2
C2
1 GND GND 16
2 SW
15
3 VIN
14
4 ^VIN ^LT3430/ 13
LT3430-1
5 SW
FB 12
6 BOOST V_C 11
7
BIAS 10
8 GND GND 9
PLACE FEEDTHROUGH AROUND
GROUND PINS (4 CORNERS) FOR
GOOD THERMAL CONDUCTIVITY
CONNECT TO
GROUND PLANE
GND
C1
SHDN
SYNC
V_OUT
SOLDER THE EXPOSED PAD
(PIN 17) TO THE ENTIRE COPPER
GROUND PLANE UNDERNEATH
THE DEVICE. NOTE: THE BOOST
AND BIAS COPPER TRACES ARE
ON A SEPARATE LAYER FROM
THE GROUND PLANE
KELVIN SENSE
V_OUT
R2
R1 C_FB
C_F
R_C
C_C
KEEP FB AND V_C COMPONENTS
AWAY FROM HIGH FREQUENCY,
HIGH CURRENT COMPONENTS
3430 F06
Figure 6. Suggested Layout
34301fa
17