Datasheet

LTC3862-1
24
38621f
Figure 18. A Typical 2-Phase, Single Output Boost Converter Application Circuit
Typical Boost Applications Circuit
A basic 2-phase, single output LTC3862-1 application
circuit is shown in Figure 18. External component selec-
tion is driven by the characteristics of the load and the
input supply.
Duty Cycle Considerations
For a boost converter operating in a continuous conduction
mode (CCM), the duty cycle of the main switch is:
D=
V
O
+ V
F
–V
IN
V
O
+ V
F






= t
ON
•f
where V
F
is the forward voltage of the boost diode. The
minimum on-time for a given application operating in
CCM is:
t
ON(MIN)
=
1
f
V
O
+ V
F
–V
IN(MAX)
V
O
+ V
F






For a given input voltage range and output voltage, it is
important to know how close the minimum on-time of the
application comes to the minimum on-time of the control
IC. The LTC3862-1 minimum on-time can be programmed
from 210ns to 375ns using the BLANK pin.
Minimum On-Time Limitations
In a single-ended boost converter, two steady-state condi-
tions can result in operation at the minimum on-time of
the controller. The fi rst condition is when the input voltage
is close to the output voltage. When V
IN
approaches V
OUT
the voltage across the inductor approaches zero during
the switch off-time. Under this operating condition the
converter can become unstable and the output can experi-
ence high ripple voltage oscillation at audible frequencies.
For applications where the input voltage can approach
or exceed the output voltage, consider using a SEPIC or
buck-boost topology instead of a boost converter.
The second condition that can result in operation at the
minimum on-time of the controller is at light load, in deep
discontinuous mode. As the load current is decreased,
the on-time of the switch decreases, until the minimum
on-time limit of the controller is reached. Any further de-
crease in the output current will result in pulse skipping,
a typically benign condition where cycles are skipped in
order to maintain output regulation.
APPLICATIONS INFORMATION
SENSE1
+
3V8
SLOPE
BLANK
CLKOUT
SYNC
PLLFLTR
PHASEMODE
D
MAX
10nF
SENSE1
–
SENSE2
–
SENSE2
+
RUN
FREQ
SS
V
IN
66.5k 24.9k
150k
V
IN
8.5V TO 36V
D1
PDS760
D2
PDS760
10
L2
19µH
PA2050-193
ITH
SGND
V
OUT
12.4k
FB
39.2k
475k
0.1µF
1µF
1nF
6.8µF 50V
0.005
1W
Q1
HAT2279H
Q2
HAT2279H
0.005
1W
V
OUT
48V
3A TO 5A
38621 F18
100µF
63V
100µF
63V
6.8µF 50V
6.8µF 50V
6.8µF 50V
LTC3862-1
10nF
100pF
INTV
CC
GATE1
GATE2
PGND
4.7µF
10nF
6.8µF 50V
6.8µF 50V
6.8µF 50V
L1
19µH
PA2050-193
10
+
+