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LT3782EFE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT3782
13
3782fg
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P
FET
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(CA)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
Input Capacitor Choice
The input capacitor must have high enough voltage and
ripple current ratings to handle the maximum input voltage
and RMS ripple current rating. The input ripple current in
a boost circuit is very small because the input current is
continuous. With 2-phase operation, the ripple cancellation
will further reduce the input capacitor ripple current rating.
The ripple current is plotted in Figure 5. Please note that
the ripple current is normalized against
I
norm
=
V
IN
L•f
s
Output Capacitor Selection
The voltage rating of the output capacitor must be greater
than the maximum output voltage with suffi cient derat-
ing. Because the ripple current in output capacitor is a
pulsating square wave in a boost circuit, it is important
that the ripple current rating of the output capacitor be
high enough to deal with this large ripple current. Figure
6 shows the output ripple current in the 1- and 2-phase
designs. As we can see, the output ripple current of a
2-phase boost circuit reaches almost zero when the duty
cycle equals 50% or the output voltage is twice as much as
the input voltage. Thus the 2-phase technique signifi cantly
reduces the output capacitor size.
Figure 6. Normalized Output RMS Ripple Currents in Boost
Converter: 1-Phase and 2-Phase. I
OUT
Is the DC Output Current.
0.1
I
ORIPPLE
/I
OUT
0.9
3782 F05
0.3
0.5
0.7
0.8
0.2
0.4
0.6
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
DUTY CYCLE OR (1-V
IN
/V
OUT
)
1-PHASE
2-PHASE
Figure 5. Normalized Input Peak-to-Peak Ripple Current
DUTY CYCLE
0
ΔI
IN
/I
NORM
1.00
0.90
0.80
0.60
0.70
0.50
0.40
0.30
0.20
0.10
0
0.8
3782 F04
0.2
0.4
0.6
1.0
1-PHASE
2-PHASE
APPLICATIONS INFORMATION
I
norm
=
V
IN
L•f
s
The RMS Ripple Current is About 29% of
the Peak-to-Peak Ripple Current.
LT3782
14
3782fg
For a given V
IN
and V
OUT
, we can calculate the duty cycle D
and then derive the output RMS ripple current from Figure
6. After choosing output capacitors with suffi cient RMS
ripple current rating, we also need to consider the ESR
requirement if electrolytic caps, tantulum caps, POSCAPs
or SP CAPs are selected. Given the required output ripple
voltage spec ΔV
OUT
(in RMS value) and the calculated RMS
ripple current ΔI
OUT
, one can estimate the ESR value of
the output capacitor to be
ESR≤
ΔV
OUT
ΔI
OUT
External Regulator to Bias Gate Drivers
For applications with V
IN
higher than 24V, the IC temperature
may get too high. To reduce heat, an external regulator
between 12V to 14V should be used to override the internal
V
GBIAS
regulator to supply the current needed for BGATE1
and BGATE2 (see Figure 7).
Effi ciency Considerations
The effi ciency of a switching regulator is equal to the out-
put power divided by the input power (¥100%). Percent
effi ciency can be expressed as:
% Effi ciency = 100% – (L1 + L2 + L3 + …),
where L1, L2, etc. are the individual loss components
as a percentage of the input power. It is often useful to
analyze individual losses to determine what is limiting
the effi ciency and which change would produce the most
improvement. Although all dissipative elements in the
circuit produce losses, four main sources usually ac-
count for the majority of the losses in LT3782 application
circuits:
1. The supply current into V
IN
. The V
IN
current is the sum
of the DC supply current I
Q
(given in the Electrical Char-
acteristics) and the MOSFET driver and control currents.
The DC supply current into the V
IN
pin is typically about
7mA and represents a small power loss (much less
than 1%) that increases with V
IN
. The driver current
results from switching the gate capacitance of the power
MOSFET; this current is typically much larger than the
DC current. Each time the MOSFET is switched on and
then off, a packet of gate charge Q
G
is transferred from
GBIAS to ground. The resulting dQ/dt is a current that
must be supplied to the GBIAS capacitor through the
V
IN
pin by an external supply. In normal operation:
I
Q(TOT)
≈ I
Q
= f • Q
G
P
IC
= V
IN
• (I
Q
+ f • Q
G
)
Figure 7
3782 F07
2μF
12V
+
GBIAS
GBIAS1
GBIAS2
LT3782
APPLICATIONS INFORMATION
LT3782
15
3782fg
2. Power MOSFET switching and conduction losses:
P
FET
=
I
O(MAX)
2
1–D
MAX
2
•R
DS(ON)
•D
MAX
•
T
+ k•V
O
2
•
I
O(MAX)
2
1–D
MAX
•C
RSS
•f
3. The I
2
R losses in the sense resistor can be calculated
almost by inspection.
P
R(SENSE)
=
I
O(MAX)
2
1–D
MAX
2
•R•D
MAX
4. The losses in the inductor are simply the DC input cur-
rent squared times the winding resistance. Expressing
this loss as a function of the output current yields:
P
R(WINDING)
=
I
O(MAX)
2
1–D
MAX
2
•R
W
5. Losses in the boost diode. The power dissipation in the
boost diode is:
P
DIODE
=
I
O(MAX)
2
•V
D
The boost diode can be a major source of power loss
in a boost converter. For 13.2V input, 42V output at 3A,
a Schottky diode with a 0.4V forward voltage would
dissipate 600mW, which represents about 1% of the
input power. Diode losses can become signifi cant at
low output voltages where the forward voltage is a
signifi cant percentage of the output voltage.
6. Other losses, including C
IN
and C
O
ESR dissipation and
inductor core losses, generally account for less than
2% of the total losses.
PCB Layout Considerations
To achieve best performance from an LT3782 circuit, the
PC board layout must be carefully done. For lower power
applications, a two-layer PC board is suffi cient. However,
at higher power levels, a multiplayer PC board is recom-
mended. Using a solid ground plane under the circuit is
the easiest way to ensure that switching noise does not
affect the operation.
In order to help dissipate the power from MOSFETs and
diodes, keep the ground plane on the layers closest to the
layers where power components are mounted. Use power
planes for MOSFETs and diodes in order to improve the
spreading of the heat from these components into the
PCB.
APPLICATIONS INFORMATION
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT3782EFE#PBF

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Buy LT3782EFE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 2-Phase Step-Up Controller
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