LT3782 Linear Technology, LT3782 Datasheet - Page 11

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

APPLICATIO S I FOR ATIO
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
The R
the R
the case to the ambient temperature (R
of T
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 volt-
age 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 cancella-
The RMS Ripple Current is About 29% of the Peak-to-Peak
Figure 5. Normalized Input Peak-to-Peak Ripple Current:
T
J
J
TH(JC)
can then be compared to the original, assumed value
= T
TH(JA)
A
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
+ P
0
for the device plus the thermal resistance from
0
to be used in this equation normally includes
Normalized Peak-to-Peak Input
FET
0.2
• R
I
Ripple Current.
U
norm
TH(JA)
1-PHASE
Ripple Current
0.4
DUTY CYCLE
=
U
L f
V
•
IN
0.6
s
2-PHASE
0.8
W
TH(CA)
3782 F04
1.0
). This value
U
tion 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
Output Capacitor Selection
The voltage rating of the output capacitor must be greater
than the maximum output voltage with sufficient derating.
Because the ripple current in output capacitor is a pulsat-
ing 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 Ripple Currents in Boost Converter:
I
norm
1-Phase and 2-Phase. I
=
L f
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
V
•
IN
0
0.1
s
0.2
Normalized Output RMS
DUTY CYCLE OR (1-V
0.3
Ripple Currents
OUT
0.4
1-PHASE
Is the DC Output Current.
0.5
0.6
IN
/V
2-PHASE
0.7
OUT
)
0.8
3782 F05
LT3782
0.9
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3782fa