LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 18

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

LT3437
APPLICATIO S I FOR ATIO
inductance of approximately 25nH/inch. At switch off, this
parasitic inductance produces a flyback spike across the
LT3437 switch. When operating at higher currents and
input voltages, with poor layout, this spike can generate
voltages across the LT3437 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.
18
+
V
IN
Figure 8. High Speed Switching Path
C2
Figure 9. Suggested Layouts
C2
C1
V
D1
IN
C1
U
C2
CIRCULATION
DD PACKAGE TOPSIDE METAL
FE PACKAGE TOPSIDE METAL
LT3437
FREQUENCY
D1
L1
PATH
HIGH
SW
U
D1
L1
W
L1
C1
3437 F08
LOAD
3437 F09a
3437 F09b
V
OUT
U
The V
possible from the switch and boost nodes. The LT3437
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 oscillation.
Board layout also has a significant effect on thermal
resistance. Pin 4/Pin 10 and the exposed die pad, Pin 11/
Pin 17, are connected by a continuous copper plate that
runs under the LT3437 die. This is the best thermal path
for heat out of the package. Reducing the thermal resis-
tance from Pin 4 and the exposed pad onto the board will
reduce die temperature and increase the power capability
of the LT3437. This is achieved by providing as much
copper area as possible around the exposed pad. Adding
multiple solder filled feedthroughs, under and around this
pad, to an internal ground plane will also help. Similar
treatment to the catch diode and coil terminations will
reduce any additional heating effects.
THERMAL CALCULATIONS
Power dissipation in the LT3437 chip comes from four
sources: switch DC loss, switch AC loss, boost circuit
current, and input quiescent current. The following formu-
las show how to calculate each of these losses. These
formulas assume continuous mode operation, and should
not be used for calculating efficiency at light load currents.
Switch loss:
Boost current loss:
Quiescent current loss:
P
R
t
P
P
EFF
SW
BOOST
Q
SW
C
= V
= effective switch current/voltage overlap time
and FB components should be kept as far away as
=
= switch resistance (≈1 when hot )
IN
R
=
SW OUT
(500µA) + V
(
V
( ) ( )
OUT
I
V
IN
) (
2
V
2
IN
I
OUT
V
OUT
OUT
/
30
(800µA)
+
)
t
EFF
( )( )( )( )
1 2 /
I
OUT
V
IN
3437f
f