LTC3805 Linear Technology, LTC3805 Datasheet - Page 11

• Manufacturer Part Number: LTC3805
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Many LTC3805 application circuits can be derived from
the topology shown on the fi rst 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
by the ratings of the external power components. The
factors are: Q1 maximum drain-source voltage (B
on-resistance (R
saturation fl ux level and winding insulation breakdown
voltages, C
lent series resistance (ESR), and maximum ripple current
ratings, and D1 and R
V
The V
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 fl exibility, 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
CC
VCC
Bias Power
IN
connected between the input voltage and V
CC
or output voltage V
pin must be bypassed to the GND pin with a
IN
and C
DS(ON)
OUT
36V TO 72V
SENSE
maximum working voltage, equiva-
R
20k
) and maximum drain current, T1
ITH
C
470pF
2.2μF
V
ITH
IN
C
× 2
IN
OUT
power ratings.
. These are all determined
C
0.1μF
SS
Figure 3. Powering the LTC3805 From the Output
R1
221k
R2
8.66k
R
118k
FS
RUN
I
SSFLT
SYNC
FS
FB
CC
TH
VDSS
C
22μF
. The
VCC
LTC3805
GND
V
CC
),
R
100k
START
I
SENSE
GATE
OC
built-in shunt regulator limits the voltage on the V
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 effi ciency 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
the LTC3805 is operated directly from either the input or
output voltage. Figure 3 shows a 5V output converter in
which R
while D2 powers V
is in normal operation. Note that R
the very small 40μA micropower start-up current while
R
1.33k
SLOPE
D2
R
3k
OC
PRI
START
Q1
R
68mΩ
Figure 2. Powering the LTC3805 via the
Internal Shunt Regulator
SENSE
SEC
and C
V
3805 F02
IN
R
CC
VCC
C
VCC
VCC
from the output once the converter
CC
R3
71.5k
R4
13.7k
form a start-up trickle charger
for the LTC3805. In this case,
V
CC
LTC3805
C
100μF
× 2
GND
OUT
V
5V AT 2A
OUT
3805 F02
START
LTC3805
need only supply
11
CC
3805fb
pin