LTC3864 Datasheet, PDF (13/28 Pages) Linear Technology – 60V Low IQ Step-Down DC/DC Controller with 100% Duty Cycle Capability

LTC3864
APPLICATIONS INFORMATION
The free-running switching frequency can be programmed
from 50kHz to 850kHz by connecting a resistor from FREQ
to signal ground. The resulting switching frequency as a
function of resistance on FREQ pin is shown in Figure 2.
1000
900
800
700
600
500
400
300
200
100
0
15 25 35 45 55 65 75 85 95 105 115 125
FREQ PIN RESISTOR (kΩ)
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Figure 2. Switching Frequency vs Resistor on FREQ
Set the free-running frequency to the desired synchroni-
zation frequency using the FREQ pin so that the internal
oscillator is prebiased to approximately the synchronization
frequency. While it is not required that the free-running
frequency be near the external clock frequency, doing so
will minimize synchronization time.
Inductor Selection
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. A higher frequency
generally results in lower efficiency because of MOSFET
gate charge and transition losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
Given the desired input and output voltages, the inductor
value and operation frequency determine the ripple current:
∆I_L
=


V_OUT
f •L




1–
V_OUT
V_IN


Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and results in lower
output ripple. Highest efficiency operation is obtained at
low frequency with small ripple current. However, achieving
this requires a large inductor. There is a trade-off between
component size, efficiency, and operating frequency.
A reasonable starting point for ripple current is 40% of
I_OUT(MAX) at nominal V_IN. The largest ripple current occurs
at the highest V_IN. To guarantee that the ripple current does
not exceed a specified maximum, the inductance should
be chosen according to:
L
=


f
•
V_OUT 
∆I_L(MAX) 


1–
V_OUT 
V_IN(MAX) 
Once the inductance value has been determined, the type
of inductor must be selected. Core loss is independent of
core size for a given inductor value, but it is very depen-
dent on the inductance selected. As inductance increases,
core losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
High efficiency converters generally cannot tolerate the
core loss of low cost powdered iron cores, forcing the use
of more expensive ferrite materials. Ferrite designs have
very low core loss and are preferred at high switching
frequencies, so design goals can concentrate on cop-
per loss and preventing saturation. Ferrite core material
saturates hard, which means that inductance collapses
abruptly when the peak design current is exceeded. This
will result in an abrupt increase in inductor ripple current
and output voltage ripple. Do not allow the core to saturate!
A variety of inductors are available from manufacturers
such as Sumida, Panasonic, Coiltronics, Coilcraft, Toko,
Vishay, Pulse, and Würth.
Current Sensing and Current Limit Programming
The LTC3864 senses the inductor current through a cur-
rent sense resistor, R_SENSE, placed across the V_IN and
SENSE pins. The voltage across the resistor, V_SENSE, is
proportional to inductor current and in normal operation
is compared to the peak inductor current setpoint. A
current limit condition is detected when V_SENSE exceeds
95mV. When the current limit threshold is exceeded, the
P-channel MOSFET is immediately turned off by pulling
the GATE voltage to V_IN regardless of the controller input.
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