Datasheet

LT3800
9
3800fc
APPLICATIONS INFORMATION
Figure 1. V
CC
Regulator Continuous Operating Conditions
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
LT3800 continuously, provided the V
IN
voltage and/or
MOSFET gate charge currents do not create excessive
power dissipation in the IC. Safe operating conditions
for continuous regulator use are shown in Figure 1. In
applications where these conditions are exceeded, V
CC
must be derived from an external source after start-up.
In LT3800 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 connect-
ing 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.
Charge Pump Doubler
TOTAL FET GATE CHARGE (nC)
0
10
V
IN
(V)
20
30
40
50
60
70
50 100 150 200
3800 F01
SAFE
OPERATING
CONDITIONS
Charge Pump Tripler
V
OUT
1µF
B0520
B0520
1µF
Si1555DL
LT3800
V
CC
BG
V
OUT
1µF
B0520
B0520
1µF
1µF
Si1555DLSi1555DL
LT3800
V
CC
BG
B0520
3800 AI01
Inductor Auxiliary Winding
TG
V
OUT
3800 AI04
V
CC
SW
BG
N
•
•
LT3800