LT3640EFE LINER [Linear Technology], LT3640EFE Datasheet - Page 18

• Manufacturer Part Number: LT3640EFE
• Description: Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3640
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
enabled, the LT3640’s internal circuitry will pull its quiescent
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 LT3640
is disabled, the SW1 pin and the SW2 pin current will drop
to essentially zero. However, if the V
the high voltage channel output is held high, an external
diode is required at the V
pulled out of the V
the low voltage channel output is held high, an external
diode is required at the V
pulled out of the V
PFM Operation
To improve effi ciency 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.
18
IN2
IN
Figure 7. Diodes Prevent Shorted Inputs from
Discharging a Battery Tied to the Outputs
V
EN/UVLO
V
IN
IN2
IN
IN2
SW
pin. If the V
0.1μF
LT3640
pin (Figure 7).
BST
IN
IN2
IN
GND
SW1
SW2
pin is fl oated and the LT3640 is
3640 F07
FB1
FB2
DA
pin to prevent current being
pin to prevent current being
IN2
IN
pin is grounded while
pin is grounded while
+
–
+
–
OUT2
OUT1
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
0.5A/DIV
0.5A/DIV
0.5A/DIV
0.5A/DIV
0.5A/DIV
0.5A/DIV
10V/DIV
10V/DIV
10V/DIV
5V/DIV
5V/DIV
5V/DIV
SW1
SW2
SW1
SW2
SW1
SW2
I
I
I
I
I
I
L1
L2
L1
L2
L1
L2
V
V
V
V
V
V
IN
OUT1
IN
OUT1
IN
OUT1
Figure 8. PFM Operation
= 12V
= 12V
= 12V
= 3.3V/25mA
= 3.3V/25mA
= 3.3V/0mA
(8a)
(8b)
(8c)
2μs/DIV
2μs/DIV
2μs/DIV
V
V
V
V
V
V
IN2
OUT2
IN2
OUT2
IN2
OUT2
= V
= V
= V
= 1.8V/30mA
= 1.8V/30mA
= 1.8V/30mA
OUT1
OUT1
OUT1
3640 F08a
3640 F08b
3640 F08c
3640p