TC110 Datasheet, PDF (6/16 Pages) Microchip Technology – PFM/PWM Step-Up DC/DC Controller

TC110
3.5 Output Capacitor
The effective series resistance of the output capacitor
directly affects the amplitude of the output voltage
ripple. (The product of the peak inductor current and
the ESR determines output ripple amplitude.) There-
fore, a capacitor with the lowest possible ESR should
be selected. Smaller capacitors are acceptable for light
loads or in applications where ripple is not a concern.
The Sprague 595D series of tantalum capacitors are
among the smallest of all low ESR surface mount
capacitors available. Table 4-1 lists suggested
components and suppliers.
3.6 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
faster and could overshoot the desired peak value. This
not only reduces efficiency, but could also cause the
current rating of the external components to be
exceeded. 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.
A 22µH inductor is recommended for the 300kHz
versions and a 47µH inductor is recommended for the
100kHz versions. Inductors with a ferrite core (or
equivalent) are also recommended. For highest
efficiency, use inductors with a low DC resistance (less
than 20 mΩ).
The inductor value directly affects the output ripple
voltage. Equation 3-3 is derived as shown below, and
can be used to calculate an inductor value, given the
required output ripple voltage and output capacitor
series resistance:
EQUATION 3-1:
V_RIPPLE ≈ ESR(di)
where ESR is the equivalent series resistance of the
output filter capacitor, and V_RIPPLE is in volts.
Expressing di in terms of switch ON resistance and
time:
EQUATION 3-2:
V_RIPPLE ≈
ESR [(V_IN – V_SW)t_ON]
L
Solving for L:
EQUATION 3-3:
L ≈ ^{ESR [(VIN – VSW)tON]}
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 3-10) 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 3-6 calculates the average DC current. Note
that minimum input voltage and maximum load current
values should be used:
EQUATION 3-4:
Output Power
Input Power = _Efficiency
Re-writing in terms of input and output currents and
voltages:
EQUATION 3-5:
(V_INMIN) (I_INMAX) =
(V_OUTMAX) (I_OUTMAX)
Efficiency
Solving for input curent:
EQUATION 3-6:
I_INMAX =
(V_OUTMAX) (I_OUTMAX)
(Efficiency) (V_INMAX)
The sawtooth current is centered on the DC current
level; swinging equally above and below the DC current
calculated in Equation 3-6. The peak inductor current is
the sum of the DC current plus half the AC current.
Note that minimum input voltage should be used when
calculating the AC inductor current (Equation 3-9).
EQUATION 3-7:
L(di)
V = _dt
EQUATION 3-8:
di
=
V(dt)
dt
EQUATION 3-9:
di = ^{[(VINMIN – VSW)tON]}
L
where: V_SW = V_CESAT of the switch (note if a CMOS
switch is used substitute V_CESAT for r_DSON x I_IN)
Combining the DC current calculated in Equation 3-6,
with half the peak AC current calculated in Equation 3-
9, the peak inductor current is given by:
EQUATION 3-10:
I_PK = I_INMAX + 0.5(di)
DS21355B-page 6
© 2002 Microchip Technology Inc.