LT3782 Linear Technology, LT3782 Datasheet - Page 12

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

APPLICATIO S I FOR ATIO
LT3782
For a given V
D and then derive the output RMS ripple current from
Figure 6. After choosing output capacitors with sufficient
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 ∆V
calculated RMS ripple current ∆I
ESR value of the output capacitor to be
External Regulator to Bias Gate Drivers
If V
LT3782 to drive large MOSFETs 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
regulator to supply the current needed for BGATE1 and
BGATE2 (see Figure 7).
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power (¥100%). Per-
cent efficiency can be expressed as:
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
efficiency and which change would produce the most
improvement. Although all dissipative elements in the
circuit produce losses, four main sources usually account
for the majority of the losses in LT3782 application
circuits:
12
% Efficiency = 100% – (L1 + L2 + L3 + …),
ESR
IN
is higher than 24V and the applications require the
≤
∆
∆
V
I
OUT
IN
OUT
and V
3782 F07
LT3782
U
OUT
GBIAS1
GBIAS2
GBIAS
, we can calculate the duty cycle
Figure 7
U
2µF
OUT
OUT
12V
(in RMS value) and the
W
, one can estimate the
+
U
GBIAS
1. The supply current into V
2. Power MOSFET switching and conduction losses:
3. The I
4. The losses in the inductor are simply the DC input
of the DC supply current I
Characteristics) and the MOSFET driver and control
currents. The DC supply current into the V
typically about 7mA and represents a small power loss
(much less than 1%) that increases with V
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
from GBIAS to ground. The resulting dQ/dt is a current
that must be supplied to the GBIAS capacitor through
the V
almost by inspection.
current squared times the winding resistance. Express-
ing this loss as a function of the output current yields:
I
P
P
P
P
Q(TOT)
FET
R SENSE
R WINDING
IC
IN
2
(
(
R losses in the sense resistor can be calculated
= V
pin by an external supply. In normal operation:
+
=
IN
k V
≈ I
⎛
⎜
⎜
⎜
⎝
•
1
)
• (I
I
Q
O MAX
–
=
O
)
(
= f • Q
D
Q
2
⎛
⎜
⎜
⎜
⎝
2
=
MAX
1
+ f • Q
•
I
⎛
⎜
⎜
⎜
⎝
O MAX
–
1
)
1
I
(
O MAX
–
D
I
O MAX
–
2
G
⎞
⎟
⎟
⎟
⎠
(
D
MAX
(
2
2
D
2
G
MAX
•
)
MAX
)
IN
R
)
⎞
⎟
⎟
⎟
⎠
)
. The V
DS ON
2
Q
•
⎞
⎟
⎟
⎟
⎠
• •
(
C
2
(given in the Electrical
R D
RSS
•
)
R
IN
•
W
D
MAX
•
current is the sum
MAX
f
G
is transferred
IN
•
ρ
. The driver
T
IN
pin is
3782fa