TC115_13 Datasheet, PDF (6/14 Pages) Microchip Technology – PFM/PWM Step-Up DC/DC Converter

TC115
5.0 APPLICATIONS
5.1 Input Bypass Capacitors
Using an input bypass capacitor reduces peak current
transients drawn from the input supply and reduces the
switching noise generated by the regulator. The source
impedance of the input supply determines the size of
the capacitor that should be used.
V_IN
+C₁
5
L₁
4
D₁
C₂⁺
V_OUT
GND
LX
TC115
NC PS SHDN
1
2
3
OFF ON
(Tie to V_IN or V_OUT
if not used)
FIGURE 5-1:
TC115 Typical Application.
5.2 Inductor Selection
Selecting the proper inductor value is a trade-off
between physical size and power conversion require-
ments. Lower value inductors cost less, but result in
higher ripple current and core losses. They are also
more prone to saturate since the coil current ramps to
a higher value. Larger inductor values reduce both
ripple current and core losses, but are larger in physical
size and tend to increase the start-up time slightly.
Practical inductor values, therefore, range from 50 µH
to 300 µH. Inductors with a ferrite core (or equivalent)
are recommended. For highest efficiency, use an
inductor with a series resistance less than 0.1).
The inductor value directly affects the output ripple
voltage. Equation 5-3 is derived as shown below, and
can be used to calculate an inductor value, given the
required output ripple voltage (V_RIPPLE) and output
capacitor series resistance:
EQUATION 5-1:
Where:
V_RIPPLE  ESRdi
ESR: the equivalent series resistance of the
output filter capacitor; V_RIPPLE is in
volts.
di: represents the peak to peak ripple
current in the inductor.
Expressing di in terms of switch ON resistance and
time:
EQUATION 5-2:
Where:
V_RIPPLE

_---_--V_{----I--N-----}–_----V_{----S---W----}_---t_-O----N----
L
V_SW = voltage drop across the switch.
T_ON = the amount of time the switch is ON.
Solving for L:
EQUATION 5-3:
L  ^---^--^V----^I--^N-----^–----^V----^S---^W----^---^t--^O---^N----^
V_RIPPLE
Care must be taken to ensure the inductor can handle
peak switching currents, which can be several times
load currents. Exceeding rated peak current will result
in core saturation and loss of inductance. The inductor
should be selected to withstand currents greater than
I_PK (Equation 5-10) without saturating.
Calculating the peak inductor current is straightforward.
Inductor current consists of an AC (sawtooth) current
centered on an average DC current (i.e., input current).
Equation 5-6 calculates the average DC current. Note
that minimum input voltage and maximum load current
values should be used:
EQUATION 5-4:
Input Power = ^{O-----u---t--p---u---t---P---o---w----e---r-}
Efficiency
DS21361D-page 6
 2002-2012 Microchip Technology Inc.