Datasheet

LT3845A
11
3845afa
Because the LT3845A uses a linear regulator to generate
V
CC
, power dissipation can become a concern with high
V
IN
voltages. Gate drive currents are typically in the range
of 5mA to 15mA per MOSFET, so gate drive currents can
create substantial power dissipation. It is advisable to
derive V
CC
and V
BOOST
power from an external source
whenever possible.
The onboard V
CC
regulator will provide gate drive power
for start-up under all conditions with total MOSFET gate
charge loads up to 180nC. The regulator can operate the
LT3845A continuously, provided the power dissipation
of the regulator does not exceed 250mW. The power
dissipation of the regulator is calculated as follows:
P
D(REG)
= (V
IN
– 8V) • (f
SW
• Q
G(TOTAL)
+ 3mA)
where Q
G(TOTAL)
is the total MOSFET gate charge of the
TG and BG.
In applications where these conditions are exceeded, V
CC
must be derived from an external source after start-up.
Maximum continuous regulator power dissipation may be
exceeded for short duration V
IN
transients.
In LT3845A converter applications with output voltages in
the 9V to 20V range, back-feeding V
CC
and V
BOOST
from
the converter output is trivial, accomplished by connecting
diodes from the output to these supply pins. Deriving
these supplies from output voltages greater than 20V will
require additional regulation to reduce the feedback voltage.
Outputs lower than 9V will require step-up techniques to
increase the feedback voltage to something greater than
the 8V V
CC
regulated output. Low power boost switchers
are sometimes used to provide the step-up function, but
a simple charge-pump can perform this function in many
instances.
Burst Mode Operation
The LT3845A employs low current Burst Mode functionality
to maximize efficiency during no load and low load
conditions. Burst Mode operation is enabled by shorting
the BURST_EN pin to SGND. Burst Mode operation can be
disabled by shorting BURST_EN to either V
FB
or V
CC
.
When the required switch current, sensed via the V
C
pin voltage, is below 15% of maximum, the Burst Mode
operation is employed and that level of sense current is
APPLICATIONS INFORMATION
V
OUT
1µF
B0520
B0520
1µF
Si1555DL
LT3845A
Charge Pump Doubler
Charge Pump Tripler
V
CC
BG
V
OUT
1µF
B0520
B0520
1µF
1µF
Si1555DLSi1555DL
LT3845A
V
CC
BG
B0520
3845A AI01
TG
V
OUT
3845A AI04
V
CC
SW
BG
N
•
•
LT3845A
Inductor Auxiliary Winding