LT3489
7
3489f
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3489 applications.
Table 3 lists several Schottky diodes that work well with the
LT3489. The diode’s average current rating must exceed
the average output current. The diode’s maximum reverse
voltage must exceed the output voltage. The diode conducts
current only when the power switch is turned off (typically
less than 50% duty cycle), so a 3A diode is suffi cient for
most designs. The companies below also offer Schottky
diodes with high voltage and current ratings.
APPLICATIO S I FOR ATIO
WUU
U
Frequency Compensation
To compensate the feedback loop of the LT3489, a series
resistor-capacitor network should be connected from the
COMP pin to GND. For most applications, a capacitor in
the range of 220pF to 680pF will suffi ce. A good starting
value for the compensation capacitor, C
C
, is 470pF. The
compensation resistor, R
C
, is usually in the range of 20k
to 100k. A thorough analysis of the switching regulator
control loop is not within the scope of this data sheet and
will not be presented here, but values of 20k and 680pF
will be a good choice for many designs.
Table 3. Suggested Diodes
MANUFACTURER
PART NUMBER
MAXIMUM
CURRENT (A)
MAXIMUM
REVERSE
VOLTAGE (V) MANUFACTURER
UPS340
UPS315
3
3
40
15
Microsemi
www.microsemi.com
B220
B230
B240
B320
B330
B340
SBM340
2
2
2
3
3
3
3
20
30
40
20
30
40
40
Diodes, Inc
www.diodes.com
Setting Output Voltage
To set the output voltage, select the values of R1 and R2
(see Figure 1) according to the following equation:
RR
V
V
OUT
12
1 235
1=−
⎛
⎝
⎜
⎞
⎠
⎟
•
.
A good range for R2 is from 5k to 30k.
Board Layout
The high speed operation of the LT3489 demands care-
ful attention to board layout. For high-current switching
regulators like the LT3489, the board layout must have
good thermal performance. Vias located underneath the
part should be connected to an internal ground plane to
improve heat transfer from the LT3489 to the PCB board.
You will not get advertised performance with careless lay-
out. Thermal and noise consideration must be taken into
account. Figure 2 shows the recommended component
placement for a boost converter.
1
2
8
7
3
4
6
5
L1
C2
LT3489
V
OUT
V
IN
GND
SHUTDOWN
R1
R2
MULTIPLE
VIAs
GROUND PLANE
3489 F02
C1
C
SS
C
C
R
C
+
Figure 2. Recommended Component Placement for Boost Converter.
Note Direct High Current Paths Using Wide PC Traces. Minimize
Trace Area at Pin 1 (V
C
) and Pin 2 (FB). Use Multiple Vias to Tie
Pin 4 Copper to Ground Plane. Use Vias at One Location Only to
Avoid Introducing Switching Currents Into the Ground Plane
