LTC3809-1_15 Datasheet, PDF (15/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

LTC3809-1
APPLICATIONS INFORMATION
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
Schottky Diode Selection (Optional)
The schottky diode D in Figure 9 conducts current dur-
ing the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1%
in efficiency. A 1A Schottky diode is generally a good
size for most LTC3809-1 applications, since it conducts
a relatively small average current. Larger diode results
in additional transition losses due to its larger junction
capacitance. This diode may be omitted if the efficiency
loss can be tolerated.
C_IN and C_OUT Selection
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V_OUT/V_IN). To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
( ) _{CIN RequiredIRMS ≈IMAX •} ^V_OUT ^•
V_IN – V_OUT
V_IN
1/2
This formula has a maximum value at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC3809-1, ceramic capacitors
can also be used for C_IN. Always consult the manufacturer
if there is any question.
The selection of C_OUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (ΔV_OUT) is approximated by:
ΔV_OUT
≈
I_RIPPLE
•
⎛⎝⎜_ESR
+
8
•
f
1
• C_OUT
⎞
⎠⎟
where f is the operating frequency, C_OUT is the output
capacitance and I_RIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I_RIPPLE increase with input voltage.
Setting Output Voltage
The LTC3809-1 output voltage is set by an external
feedback resistor divider carefully placed across the
output, as shown in Figure 3. The regulated output voltage
is determined by:
V_OUT
=
0.6V
•
⎛⎝⎜ _{1 +}
R_B
R_A
⎞
⎠⎟
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