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LT3697
24
3697f
applicaTions inForMaTion
Catch Diode Selection
The catch diode (D
CATCH
from the Block Diagram) conducts
current only during the switch off time. Average forward
current in normal operation can be calculated from:
I
D(AVG)
= I
OUT
•
V
IN
– V
SYS
V
where I
OUT
is the output load current. The current rating of
the diode should be selected to be greater than or equal to
the application’s output load current, so that the diode is
robust for a wide input voltage range. The voltage rating of
the diode is equal to the maximum regulator input voltage
while switching, 37V or less. Use a 3A, 40V Schottky diode.
Do not use a 60V diode due to the high resistive voltage drop.
BST and SYS Pin Considerations
Capacitor C
BST
and Schottky diode D
BST
(see the Block
Diagram) are used to generate a boost voltage that is
higher than the input voltage to drive the internal NPN
power switch. In most cases a 0.47μF capacitor will work
well for C
BST
. For switching frequency below 500kHz, use
1µF. The BST pin must be more than 1.8V above the SW
pin for best efficiency and more than 2.6V above the SW
pin to allow the LT3697 to skip off times to achieve very
high duty cycles.
With the SYS pin connected to the output, a 180mA ac
-
tive load will charge the boost capacitor during light load
start-up and an enforced 600mV minimum dropout voltage
will
keep the boost capacitor charged across operating
conditions (see Minimum Dropout Voltage section).
Enable
The LT3697 is in shutdown with I
VIN
< 1µA when the EN
pin is low and active when the pin is high. The enable
threshold is about 1.5V. The EN pin can be tied to V
IN
if the shutdown feature is not used. The EN pin current
depends on the EN pin voltage for V
EN
< 12V and reaches
about 30µA at 12V.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
Synchronizing the LT3697 oscillator to an external fre
-
quency can be done by connecting a square wave (with
on
and off time greater than 50ns) to the SYNC pin. The
square wave amplitude should have valleys that are below
0.4V and peaks above 1V (up to 6V).
The LT3697 will skip pulses at low output loads while
synchronized to an external clock to maintain regula
-
tion. At very light loads, the part will go to sleep between
groups of pulses, reducing the quiescent current of the
part. Holding the SYNC pin DC high yields no advantages
so
it is not recommended.
The LT3697 may be synchronized over a 300kHz to
2.2MHz range. The R
T
resistor should be chosen to set
the LT3697 switching frequency 10% below the lowest
synchronization input. For example, if the synchroniza
-
tion signal will be 300kHz and higher, the R
T
should be
selected for 270kHz. To ensure reliable and safe operation
the LT3697 will only synchronize when the output voltage
is near regulation. It is therefore necessary to choose a
large enough inductor value to supply the required output
current at the frequency set by the R
T
resistor (see Induc-
tor Selection section). The slope compensation is set by
the
R
T
value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by R
T
, than the slope compensation will be
sufficient for all synchronization frequencies.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively,
the LT3697 will tolerate a shorted output and the
power
dissipation
will be limited by the current limit set by R
LIM
and R
SENSE
(see the Setting the Current Limit section).
There is another situation to consider in systems where
the output will be held high when the input to the LT3697
is absent. This may occur in automotive systems where
the LT3697 output may be connected to the 12V V
BATT
during a fault condition or if a USB peripheral with a
large, charged cap is plugged into the LT3697 output. If
the V
IN
pin is allowed to float and the EN pin is held high
