LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 11

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Many LTC3805-5 application circuits can be derived from
the topologies shown on the fi rst page or in the Typical
Applications section of this data sheet.
The LTC3805-5 itself imposes no limits on allowed input
voltage V
by the ratings of the external power components. In Figure 8,
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 LTC3805-5 is designed so
that it can be operated from voltages well beyond the
LTC3805-5’s absolute maximum ratings. Figure 2 shows
the simplest case, in which the LTC3805-5 is powered
with a resistor R
and V
on the V
shunt regulator is not forced to sink more than 25mA.
This powering scheme has the drawback that the power
loss in the resistor reduces converter effi ciency and the
CC
Bias Power
CC
CC
. The built-in shunt regulator limits the voltage
IN
CC
pin must be bypassed to the GND pin with a
IN
or output voltage V
pin to around 9.5V as long as the internal
and C
VCC
DS(ON)
OUT
connected between the input voltage
SENSE
maximum working voltage, equiva-
) and maximum drain current, T1
power ratings.
OUT
. These are all determined
Figure 2. Powering the LTC3805-5 via the
Internal Shunt Regulator
V
IN
R
VDSS
VCC
C
VCC
),
V
25mA shunt regulator maximum may limit the maximum-
to-minimum range of input voltage.
The typical application circuit in Figure 9 shows a different
fl yback converter bias power strategy for a case in which
neither the input or output voltage is suitable for providing
bias power to the LTC3805-5. A small NPN preregulator
transistor and a zener diode are used to accelerate the
rise of V
The fl yback transformer has an additional bias winding to
provide bias power. Note that this topology is very power-
ful because, by appropriate choice of transformer turns
ratio, the output voltage can be chosen without regard to
the value of the input voltage or the V
the LTC3805-5. The number of turns in the bias winding
is chosen according to
where N
N
V
is the converter output voltage, V
drop of D1 and V
Note that since V
loop, V
“off-the-shelf” transformer with excessive bias windings
is used, the resistor, R
limit the current.
LTC3805-5
CC
CC
SEC
GND
N
is the desired voltage to power the LTC3805-5, V
BIAS
38055 F02
is the number of turns in the secondary winding,
CC
CC
BIAS
=
is also regulated although not as precisely. If an
and reduce the value of the V
N
is the number of turns in the bias winding,
SEC
OUT
V
D2
V
OUT
CC
is regulated by the converter control
is the forward voltage drop of D2.
+
BIAS
+
V
V
D
D
2
in Figure 9, can be added to
1
D1
is the forward voltage
LTC3805-5
CC
CC
bias power for
bias capacitor.
11
38055fa
OUT