LT3509
18
3509fd
For more information www.linear.com/LT3509
Diode Selection
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
D AVG OUT IN OUT IN()
=
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current limit.
If transient input voltages exceed 40V
, use a Schottky
diode with a reverse voltage rating of 45V or higher. If
the maximum transient input voltage is under 40V, use a
Schottky diode with a reverse voltage rating greater than
the maximum input voltage. Table 3 lists several Schottky
diodes and their manufacturers:
Table 3. Schottky Diodes
MANUFACTURER/
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
at 1A
(mV)
On Semiconductor
MBRM140 40 1 550
MicroSemi
UPS140 40 1 450
Diodes Inc.
DFLS140L 40 1 550
1N5819HW 40 1 450
Short and Reverse Protection
Provided the inductors are chosen to not go deep into
their saturation region at the maximum I
LIMIT
current the
LT3509 will tolerate a short circuit on one or both outputs.
The excess current in the inductor will be detected by the
DA comparator and the frequency will reduced until the
valley current is below the limit. This shouldn’t affect the
other channel unless the channel that is shorted is also
providing the boost supply to the BD pin. In this case the
voltage drop of the other switch will increase and lower
the efficiency. This could eventually cause the part to reach
the thermal shutdown limit. One other important feature
of the part that needs to be considered is that there is a
parasitic diode in parallel with the power switch. In normal
operation this is reverse biased but it could conduct if the
load can be powered from an alternate source when the
LT3509 has no input. This may occur in battery charging
applications or in battery backup systems where a bat
-
tery or some other supply is diode ORed with one of the
LT3509 regulated outputs. If
the SW pin is at more than
about 4V the V
IN
pin can attain sufficient voltage for LT3509
control circuitry to power-up to the quiescent bias level
and up to 2mA could be drawn from the backup supply.
This can be minimized if some discrete FETs or open-drain
buffers are used to pull down the RUN/SS pins. Of course
the gates need to be driven from the standby or battery
backed supply. If there is the possibility of a short circuit
at the input or just other parallel circuits connected to V
IN
it would be best to add a protection diode in series with
V
IN
. This will also protect against a reversed input polarity.
These concepts are illustrated in Figure 12.
applicaTions inForMaTion
L1
D1
SLEEP
D2
V
OUT
C
OUT
C
BOOST
C
IN
V
IN
3509 F12
LT3509
GND
V
IN
RUN/SS1
RUN/SS2
BD
BOOST
SW
DA
Figure 12. Reverse Bias Protection
