LT3782 Linear Technology, LT3782 Datasheet - Page 9

• Manufacturer Part Number: LT3782
• Description: 2-Phase Step-Up DC/DC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Layout Considerations
To prevent EMI, the power MOSFETs and input bypass
capacitor leads should be kept as short as possible. A
ground plane should be used under the switching circuitry
to prevent interplane coupling and to act as a thermal
spreading path. Note that the bottom pad of the package
is the heat sink, as well as the IC signal ground, and must
be soldered to the ground plane.
In a boost converter, the conversion gain (assuming
100% efficiency) is calculated as (ignoring the forward
voltage drop of the boost diode):
where D is the duty ratio of the main switch. D can then be
estimated from the input and output voltages:
The Peak and Average Input Currents
The control circuit in the LT3782 measures the input
current by using a sense resistor in each MOSFET source,
so the output current needs to be reflected back to the
input in order to dimension the power MOSFET properly.
Based on the fact that, ideally, the output power is equal to
the input power, the maximum average input current is:
The peak current is:
The maximum duty cycle, D
minimum V
D
I
I
V
IN MAX
IN PEAK
V
OUT
= −
(
(
IN
1
=
)
)
V
=
1
IN
V
= 1 2
OUT
−
IN
1
.
1
I
D
O MAX
–
. •
;
(
D
D
MAX
1
U
MAX
I
O MAX
–
)
(
D
MAX
= −
U
1
)
MAX
V
IN MIN
V
, should be calculated at
OUT
(
W
)
U
Power Inductor Selection
In a boost circuit, a power inductor should be designed to
carry the maximum input DC current. The inductance
should be small enough to generate enough ripple current
to provide adequate signal to noise ratio to the LT3782. An
empirical starting of the inductor ripple current (per phase)
is about 40% of maximum DC current, which is half of the
input DC current in a 2-phase circuit:
where V
voltage and output current, respectively.
And the inductance is estimated to be:
where f
The saturation current level of inductor is estimated to be:
Sense Resistor Selection
During the switch on-time, the control circuit limits the
maximum voltage drop across the sense resistor to about
60mV. The peak inductor current is therefore limited to
60mV/R. The relationship between the maximum load
current, duty cycle and the sense resistor R
∆I
L
L
I
R V
SAT
≅
=
≤
40
f
s
V
s
≥
IN
SENSE MAX
IN
is the switching frequency per phase.
%•
• ∆
, V
∆
•
2
I
OUT
L
D
I
I
L
OUT MAX
(
+
and I
I
IN
2
(
)
2
≅
•
V
OUT
1 2
IN
)
70%•
. •
1
•
–
V
are the DC input voltage, output
OUT
D
I
O MAX
MAX
(
I
OUT MAX
2
=
(
20
V
)
IN MIN
%•
(
)
•
I
OUT MAX
V
)
OUT
LT3782
SENSE
(
V
IN
)
•
is:
V
OUT
3782fa
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