Datasheet
LT3641
16
3641fa
Table 4. Diode Vendors
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
AT 1A
(MV)
V
F
AT 2A
(MV)
V
F
AT 3A
(MV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
1
530 595
Diodes Inc.
B120
B130
B220
B230
DFLS240L
DFLS260
PDS360
PDS3100
20
30
20
30
40
60
60
100
1
1
2
2
2
2
3
3
500
500
500
500
500
620
650
620
760
International Rectifi er
10BQ030
20BQ030
30
30
1
2
420 470
470
BST and SW Pin Considerations
The high voltage channel requires an external capacitor
between the BST and SW pins and an external boost diode
from a voltage source to the BST pin. In most cases, a
0.22μF capacitor will work well. Use a Schottky with fast
reverse recovery for BST diode. The (BST-SW) voltage
cannot exceed 5.5V, and must be more than 2.3V for best
effi ciency. Connect the boost diode to any voltage between
2.7V and 5.5V. The V
IN2
pin is the best choice if the low
voltage channel is used.
The high voltage channel will not start until the (BST-SW)
voltage is 2V or above. When the LT3641 is enabled, an
internal ~5mA current source from V
IN
fl ows out of the
BST pin. The SW pin is disconnected from the SW1 pin,
and is pulled down by an internal current source to ground.
The external boost capacitor can be charged up regard-
less of the output. When the (BST-SW) voltage reaches
2V, the SW pin is connected to the SW1 pin, and the high
voltage channel starts switching. However, the internal
bipolar power switch cannot be fully saturated until the
(BST-SW) voltage is further charged to above 2.3V. To start
up a traditional nonsynchronous buck regulator with very
light load, the input voltage needs to be a couple of volts
higher than the minimum running input voltage if the input
voltage is ramping up slowly. The LT3641’s unique boost
capacitor charging scheme solves this start-up issue.
Figure 2 shows that the minimum input voltage to start
the high voltage channel nonsynchronous buck regulator
of the LT3641 is very close to the minimum input voltage
to regulate the output voltage for most of the load range.
Figure 2. High Voltage Channel Minimum
Input Voltage for V
OUT1
= 3.3V
APPLICATIONS INFORMATION
(2a) F
S
= 2MHz
(2b) F
S
= 500kHz
V
OUT
CURRENT (A)
0.001
4
V
IN
VOLTAGE (V)
0.01 0.1 1
3
2
1
0
5
3641 F03a
START
RUN
V
OUT
CURRENT (A)
0.001
4
V
IN
VOLTAGE (V)
0.01 0.1 1
3
2
1
0
5
3641 F03b
START
RUN