LTC3862 Datasheet, PDF (24/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
APPLICATIONS INFORMATION
Typical Boost Applications Circuit
Minimum On-Time Limitations
A basic 2-phase, single output LTC3862 application circuit is
shown in Figure 18. External component selection 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_O +
– V_IN ⎞
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 minimum on-time can be programmed
from 180ns to 340ns using the BLANK pin.
In a single-ended boost converter, two steady-state condi-
tions can result in operation at the minimum on-time of
the controller. The first 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.
66.5k
10nF
10nF
68.1k
100pF
12.4k
475k
V_OUT
D_MAX
SLOPE
3V8
SENSE1⁺
BLANK
PHASEMODE SENSE1^–
FREQ
RUN
SS
LTC3862 ^VIN
ITH
FB
SGND
INTV_CC
GATE1
PGND
CLKOUT
SYNC
PLLFLTR
GATE2
SENSE2^–
SENSE2⁺
V_IN
5V TO 36V
L1
19.4μH
PA2020-193
1nF
10Ω
10nF
24.9k
1μF
84.5k
4.7μF
6.8μF 50V
6.8μF 50V
6.8μF 50V
10Ω
10nF
L2
19.4μH
PA2020-193
D1
PDS760
Q1
HAT2266H
0.006Ω
1W
100μF
63V
100μF _{6.8μF 50V}
63V
0.006Ω
1W
Q2
HAT2266H
D2
PDS760
6.8μF 50V
6.8μF 50V
6.8μF 50V
V_OUT
48V
2A TO 5A
3862 F18
Figure 18. A Typical 2-Phase, Single Output Boost Converter Application Circuit
3862fb
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