LTC3772B Datasheet, PDF (11/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

LTC3772B
APPLICATIO S I FOR ATIO
duty cycle–at its worst case the required R_DS(ON) is given
by:
R_{DS(ON)(DC=100%)}
=
P_P
(I_OUT(MAX))² (1+
δ_P)
where P_P is the allowable power dissipation and δ_P is the
temperature dependency of R_DS(ON). (1 + δ_P) is generally
given for a MOSFET in the form of a normalized R_DS(ON) vs
temperature curve, but δ_P = 0.005/°C can be used as an
approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less than
100% and the LTC3772B is in continuous mode, the R_DS(ON)
is governed by:
R_DS(ON)
≅
P_P
(DC)I_OUT2 (1+
δ_P)
where DC is the maximum operating duty cycle of the
LTC3772B.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor ripple
current. However, this is at the expense of efficiency due
to an increase in MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. In normal operation, the ripple current, I_RIPPLE, de-
creases with higher inductance or frequency and increases
with higher V_IN or as V_OUT approaches 1/2 V_IN. The
inductor’s peak-to-peak ripple current is given by:
I_RIPPLE
=
V_IN
− V_OUT
f(L)
⎛
⎝⎜
V_OUT + V_D
V_IN + V_D
⎞
⎠⎟
where f is the operating frequency. V_D is the forward volt-
age drop of the catch diode, 0.5V typical. Accepting larger
values of I_RIPPLE allows the use of low inductances, but re-
sults in higher output voltage ripple and greater core losses.
A reasonable starting point for setting ripple current is
I_RIPPLE = 0.4(I_OUT(MAX)). Remember, the maximum I_RIPPLE
occurs at the maximum input voltage.
Inductor Core Selection
Once the inductance value is determined, the type of induc-
tor must be selected. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in in-
ductor ripple current and consequent output voltage ripple.
Do not allow the core to saturate!
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.
Output Diode Selection
The catch diode carries load current during the off-time. The
average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the diode
conducts most of the time. As V_IN approaches V_OUT the
diode conducts only a small fraction of the time. The most
stressful condition for the diode is when the output is short-
circuited. Under this condition the diode must safely handle
I_PEAK at close to 100% duty cycle. Therefore, it is impor-
tant to adequately specify the diode peak current and av-
erage power dissipation so as not to exceed the diode
ratings.
3772bfa
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