lt3688fe Linear Technology Corporation, lt3688fe Datasheet - Page 24

• Manufacturer Part Number: lt3688fe
• Description: Lt3688 - Dual 800ma Step-down Switching Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

LT3688
APPLICATIONS INFORMATION
Figure 13. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3688 Runs Only When the Input
Is Present
these components should be as small as possible. These
components, along with the inductor and output capacitor,
should be placed on the same side of the circuit board.
Place a local, unbroken ground plane below these com-
ponents. The SW and BST nodes should be as small as
possible. Finally, keep the FB node small so that the ground
traces will shield them from the SW and BST 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
24
V
IN
Figure 14. Top Layer PCB Layout in the LT3688
Demonstration Board
D4
V
EN/UVLO
IN
LTC3688
BIAS
GND
BOOST
SW
DA
FB
+
3688 F13
V
OUT
3688 F14
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3688 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 LT3688
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 lay-
ers will spread the heat dissipated by the LT3688. Placing
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
to ambient can be reduced to θ
100 LFPM airfl ow, this resistance can fall by another 25%.
Further increases in airfl ow will lead to lower thermal re-
sistance. Because of the large output current capability of
the LT3688, 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). Power dissipation within the LT3688
can be estimated by calculating the total power loss from
an effi ciency measurement and subtracting the catch diode
loss. The die temperature is calculated by multiplying the
LT3688 power dissipation by the thermal resistance from
junction-to-ambient. Thermal resistance depends on the
layout of the circuit board, but values from 30°C/W to
60°C/W are typical. Die temperature rise was measured
on a 4-layer, 5cm • 7.5cm circuit board in still air at a load
current of 0.8A (f
output the die temperature elevation above ambient was
14°C; for 12V
to 5V
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.
OUT
and 3.3V
IN
to 5V
SW
OUT
= 800kHz). For a 12V input to 3.3V
OUT
the rise was 30°C.
the rise was 15°C and for 12V
JA
= 40°C/W or less. With
3688f
IN