Datasheet
LT3689/LT3689-5
29
3689fe
For more information www.linear.com/LT3689
ground traces will shield them from the SW and BOOST
nodes. The Exposed Pad on the bottom of the package
must be soldered to ground so that the pad acts as a heat
sink. To keep thermal resistance low, extend the ground
plane as much as possible, and add thermal vias under
and near the LT3689 to additional ground planes within
the circuit board and on the bottom side.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3689
cool. The Exposed Pad on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3689.
Placing additional vias can reduce thermal resistance
further. Because of the large output current capability of
the LT3689, it is possible to dissipate enough heat to raise
the junction temperature beyond the absolute maximum
of 125°C (150°C for H-grade). When operating at high
ambient temperatures, the maximum load current should
be derated as the ambient temperature approaches 125°C
(150°C for H-grade). A board measuring 5cm × 7.5cm with
a top layer layout similar to Figure 15 was evaluated in
still air at 3.3V
OUT
, 700kHz switching frequency. At 700mA
load, the temperature reached approximately 12°C above
ambient for input voltages equal to 12V and 24V. Power
dissipation within the LT3689 can be estimated by calculat-
ing the total power loss from an efficiency measurement
and subtracting the catch diode loss. The die temperature
is calculated by multiplying the LT3689 power dissipation
by the thermal resistance from junction-to-ambient.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
APPLICATIONS INFORMATION
TYPICAL APPLICATIONS
SW
DA
FB
RT
SYNC
f
SW
= 1MHz
WDI
RST
WDO
V
IN
EN/UVLO
V
IN
6.3V TO 36V
TRANSIENT
TO 60V
5V
700mA
R2
102k
L1
12µH
C2
0.1µF
D1
C3
10µF
R1
536k
GND
R
T
12.7k
LT3689
3689 TA02
BST
OUT
C
POR
C
WDT
C1
1µF
I/O
I/O
RESET
µP
W/TWINDOW TIMEOUT
WDEWATCHDOG_DEFEAT
C6
10nF
t
WDU
= 182ms
L1: CDR125NP-12MC
D1: MBRM140
C1, C2, C3: X7R or X5R
C4
5.6pF
C5
68nF
t
RST
= 157ms
5V Step-Down Converter