Datasheet
LT3640
18
3640f
The threshold of power-on comparator is 1.15V for the high
voltage channel, and 550mV for the low voltage channel.
Both RST1 and RST2 are open-drain outputs with weak
internal pull-ups (100k to ~2V). The DC characteristics of
the RST1 and RST2 pull-down strength are shown in the
Typical Performance Characteristics section. The weak
Reverse Protection
In battery charging applications or in battery back-up
systems, the output will be held high when the input to the
LT3640 is absent. If the V
IN
pin is floated and the LT3640 is
enabled, the LT3640’s internal circuitry will pull its quiescent
current through the SW1 pin or the SW2 pin. This is fine if
the system can tolerate a few mA in this state. If the LT3640
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 7).
Figure 8. PFM Operation
PFM Operation
To improve efficiency at light loads, the LT3640 auto-
matically 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 LT3640 may have differ-
ent loads, the two channels can have different switching
frequency (Figure 8).
Power-On Reset Timer
Each channel of the LT3640 has a power-on comparator. Both
comparators are enabled when the LT3640 is powered up
and starts monitoring their corresponding feedback voltages.
Figure 7. Diodes Prevent Shorted Inputs from
Discharging a Battery Tied to the Outputs
applicaTions inForMaTion
3640 F07
OUT2
OUT1
GND
FB2
SW2
SW1
EN/UVLO
V
IN
IN
SW
LT3640
BST
V
IN2
FB1
DA
+
–
+
–
IN2
500ns/DIV
I
L1
0.5A/DIV
SW1
10V/DIV
I
L2
0.5A/DIV
SW2
5V/DIV
3640 F08a
V
IN
= 12V
V
OUT1
= 3.3V/25mA
V
IN2
= V
OUT1
V
OUT2
= 1.8V/30mA
2µs/DIV
I
L1
0.5A/DIV
SW1
10V/DIV
I
L2
0.5A/DIV
SW2
5V/DIV
3640 F08b
V
IN
= 12V
V
OUT1
= 3.3V/25mA
V
IN2
= V
OUT1
V
OUT2
= 1.8V/20mA
2µs/DIV
I
L1
0.5A/DIV
SW1
10V/DIV
I
L2
0.5A/DIV
SW2
5V/DIV
3640 F08c
V
IN
= 12V
V
OUT1
= 3.3V/0mA
V
IN2
= V
OUT1
V
OUT2
= 1.8V/30mA
(8a)
(8b)
(8c)