Datasheet

LT3641
18
3641fa
Reverse Protection
In battery charging applications or in battery back-up
systems, the output will be held high when the input to the
LT3641 is absent. If the V
IN
pin is fl oated and the LT3641
is enabled, the LT3641’s internal circuitry will pull its qui-
escent current through the SW1 pin or the SW2 pin. This
is fi ne if the system can tolerate a few mA in this state.
If the LT3641 is disabled, the SW1 pin and the SW2 pin
current will drop to essentially zero. However, if the V
IN
pin is grounded while the high voltage channel output is
held high, an external diode is required at the V
IN
pin to
prevent current being pulled out of the V
IN
pin. If the V
IN2
pin is grounded while the low voltage channel output is
held high, an external diode is required at the V
IN2
pin to
prevent current being pulled out of the V
IN2
pin (Figure 6).
PFM Operation
To improve effi ciency at light loads, the LT3641 automatically
switches to pulse frequency modulation (PFM) operation
which minimizes the switching loss and keeps the output
voltage ripples small.
Because the two channels of the LT3641 may have differ-
ent loads, the two channels can have different switching
frequency (Figure 7).
Figure 6. Diodes Prevent Shorted Inputs from
Discharging a Battery Tied to the Outputs
APPLICATIONS INFORMATION
3641 F07
OUT2
OUT1
GND
FB2
SW2
SW1
EN/UVLO
V
IN
IN
SW
LT3641
BST
V
IN2
FB1
DA
+
–
+
–
IN2
Figure 4. The High Voltage Channel Reduces Frequency
to Protect Against Shorted Output with 55V Input
Figure 5. The Low Voltage Channel Operates in Valley
Current Limit Mode to Protect Against Shorted Output
1μs/DIV
SW1
20V/DIV
I
L1
1A/DIV
3641 F05
V
IN
= 55V
V
OUT1
= SHORT
1μs/DIV
SW2
2V/DIV
SW1
20V/DIV
I
L2
1A/DIV
3641 F06
V
IN2
= 5V
V
OUT2
= SHORT