LT3508
APPLICATIONS INFORMATION
High Temperature Considerations
was 13°C; for 24V to 3.3V
the rise was 18°C; for
IN
OUT
12V to 5V
the rise was 14°C and for 24V to 5V
IN
OUT
IN OUT
The die temperature of the LT3508 must be lower than the
maximum rating of 125°C (150°C for the H grade). This is
generally not a concern unless the ambient temperature is
above85°C.Forhighertemperatures,careshouldbetaken
in the layout of the circuit to ensure good heat sinking of
the LT3508. The maximum load current should be derated
as the ambient temperature approaches 125°C (150°C
for the H grade). The die temperature is calculated by
multiplying the LT3508 power dissipation by the thermal
resistance from junction to ambient. Power dissipation
within the LT3508 can be estimated by calculating the total
power loss from an efficiency measurement and subtract-
ing the catch diode loss. 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, 6.5cm × 7.5cm circuit board in still air at a
the rise was 19°C.
Outputs Greater Than 6V
For outputs greater than 6V, add a resistor of 1k to 2.5k
across the inductor to damp the discontinuous ringing of
the SW node, preventing unintended SW current. The 12V
output circuit in the Typical Applications section shows
the location of this resistor.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for step-down regu-
lators and other switching regulators. The LT1376 data
sheet has a more extensive discussion of output ripple,
loop compensation and stability testing. Design Note 318
showshowtogenerateadualpolarityoutputsupplyusing
a step-down regulator.
load current of 1.4A (f = 700kHz). For a 12V input to
SW
3.3V output the die temperature elevation above ambient
TYPICAL APPLICATIONS
1MHz, 3.3V and 1.8V Outputs with Sequencing
V
IN
ON OFF
3.9V TO 16V
C1
4.7ꢀF
D1
V
V
SHDN
D2
IN1 IN2
OUT2
3.3V
1.4A
OUT2
BOOST1
BOOST2
C2
C3
0.1ꢀF
L1 3.3ꢀH
L2 4.7ꢀH
OUT1
1.8V
1.4A
0.1ꢀF
SW1
SW2
D3
D4
LT3508
R1
18.7k
R2
35.7k
FB1
FB2
V
V
C2
C1
R3
R5
R4
11.5k
R6
39k
C5
10ꢀF
TRACK/SS1
TRACK/SS2
PG1
PG2
15.0k
47k
R7
100k
C6
330pF
C4
47ꢀF
GND R /SYNC
T
C7
150pF
R8
33.2k
C8
1nF
POWER
GOOD
C1 TO C5: X5R OR X7R
D1, D2: MMSD4148
f
= 1MHz
SW
3508 TA02
D3: DIODES INC. B140
D4: DIODES INC. B240A
3508fb
19
Linear [ Linear ]
