LTC3805-5_15 Datasheet, PDF (11/20 Pages) Linear Technology – Adjustable Frequency Current Mode Flyback/ Boost/SEPIC DC/DC Controller

LTC3805-5
APPLICATIONS INFORMATION
Many LTC3805-5 application circuits can be derived from
the topologies shown on the first page or in the Typical
Applications section of this data sheet.
The LTC3805-5 itself imposes no limits on allowed input
voltage V_IN or output voltage V_OUT. These are all determined
by the ratings of the external power components. In Figure 8,
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 LTC3805-5 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_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 pow-
ering scheme has the drawback that the power loss in the
resistor reduces converter efficiency and the 25mA shunt
regulator maximum may limit the maximum-to-minimum
range of input voltage.
V_IN
R_VCC
C_VCC
LTC3805-5
V_CC
GND
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Figure 2. Powering the LTC3805-5 via the
Internal Shunt Regulator
The typical application circuit in Figure 9 shows a different
flyback 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_CC and reduce the value of the V_CC bias capacitor.
The flyback 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_CC bias power for
the LTC3805-5. The number of turns in the bias winding
is chosen according to
N_BIAS
= N_SEC
V_CC + V_D2
V_OUT + V_D1
where N_BIAS is the number of turns in the bias winding,
N_SEC is the number of turns in the secondary winding,
V_CC is the desired voltage to power the LTC3805-5, V_OUT
is the converter output voltage, V_D1 is the forward voltage
drop of D1 and V_D2 is the forward voltage drop of D2.
Note that since V_OUT is regulated by the converter control
loop, V_CC is also regulated although not as precisely. If an
“off-the-shelf” transformer with excessive bias windings
is used, the resistor, R_BIAS in Figure 9, can be added to
limit the current.
Transformer Design Considerations
Transformer specification and design is perhaps the most
critical part of applying the LTC3805-5 successfully. In
addition to the usual list of caveats dealing with high fre-
quency power transformer design, the following should
prove useful.
Turns Ratios
Due to the use of the external feedback resistor divider
ratio to set output voltage, the user has relative freedom
in selecting transformer turns ratio to suit a given ap-
plication. Simple ratios of small integers, e.g., 1:1, 2:1,
3:2, etc. can be employed which yield more freedom in
setting total turns and transformer inductance. Simple
integer turns ratios also facilitate the use of “off-the-shelf”
configurable transformers. Turns ratio can be chosen on
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