Datasheet

LTC3805
11
3805fg
applications inForMation
Many LTC3805 application circuits can be derived from
the topology shown on the first page of this data sheet
and from the topology shown in Figure 2.
The LTC3805 itself imposes no limits on allowed input volt-
age V
IN
or output voltage V
OUT
. These are all determined
by the ratings of the external power components. The
factors are: Q1 maximum drain-source voltage (B
VDSS
),
on-resistance (R
DS(ON)
) and maximum drain current, T1
saturation flux level and winding insulation breakdown
voltages, C
IN
and C
OUT
maximum working voltage, equiva-
lent series resistance (ESR), and maximum ripple current
ratings, and D1 and R
SENSE
power ratings.
V
CC
Bias Power
The V
CC
pin must be bypassed to the GND pin with a
minimum 1µF ceramic or tantalum capacitor located im-
mediately adjacent to the two pins. Proper supply bypassing
is necessary to supply the high transient currents required
by the MOSFET gate driver.
For maximum flexibility, the LTC3805 is designed so that it
can be operated from voltages well beyond the LTC3805’s
absolute maximum ratings. Figure 2 shows the simplest
case, in which the LTC3805 is powered with a resistor
R
VCC
connected between the input voltage and V
CC
. The
built-in shunt regulator limits the voltage on the V
CC
pin
to around 9.5V as long as the internal shunt regulator is
not forced to sink more than 25mA. This powering scheme
has the drawback that the power loss in the resistor re-
duces converter efficiency and the 25mA shunt regulator
maximum may limit the maximum-to-minimum range of
input voltage.
In some cases, the input or output voltage is within the
operational range of V
CC
for the LTC3805. In this case,
the LTC3805 is operated directly from either the input
or output voltage. Figure 3 shows a 5V output converter
in which R
START
and C
VCC
form a start-up trickle charger
while D2 powers V
CC
from the output once the converter
is in normal operation. Note that R
START
need only supply
the very small 40µA micropower start-up current while
Figure 3. Powering the LTC3805 from the Output
LTC3805
I
TH
GND
SSFLT
RUN
GATE
OC
I
SENSE
SYNC
FS
FB
V
CC
R
FS
118k
R
SLOPE
3k
R
SENSE
68mΩ
C
IN
2.2µF
× 2
C
VCC
22µF
R
OC
1.33k
Q1
SECPRI
D2
R2
8.66k
R1
221k
R
START
100k
V
IN
36V TO 72V
V
OUT
5V AT 2A
R
ITH
20k
C
ITH
470pF
C
SS
0.1µF
R4
13.7k
R3
71.5k
C
OUT
100µF
× 2
3805 F02
Figure 2. Powering the LTC3805 via the
Internal Shunt Regulator
LTC3805
V
CC
R
VCC
C
VCC
3805 F02
V
IN
GND