LTC1735
U
W U U
APPLICATIO S I FOR ATIO
The internal oscillator runs at its nominal frequency (fO)
when the FCB pin is pulled high to INTVCC or connected to
ground. Clocking the FCB pin above and below 0.8V will
cause the internal oscillator to injection lock to an external
clock signal applied to the FCB pin with a frequency
between 0.9fO and 1.3fO. The clock high level must exceed
1.3V for at least 0.3µs and the clock low level must be less
than 0.3V for at least 0.3µs. The top MOSFET turn-on will
synchronize with the rising edge of the clock.
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic trade
off, the effect of inductor value on ripple current and low
current operation must also be considered.
Theinductorvaluehasadirecteffectonripplecurrent.The
inductor ripple current ∆IL decreases with higher induc-
Attempting to synchronize to too high an external fre-
quency (above 1.3fO) can result in inadequate slope com-
pensation and possible loop instability. If this condition
existssimplylowerthevalueofCOSC sofEXT =fO according
to Figure 2.
tance or frequency and increases with higher VIN or VOUT:
1
V
OUT
V
IN
∆I =
V
1–
L
OUT
(f)(L)
Accepting larger values of ∆IL allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is∆IL = 0.3 to 0.4(IMAX). Remember,
the maximum ∆IL occurs at the maximum input voltage.
100.0
87.5
75.0
62.5
50.0
37.5
25.0
12.5
0
The inductor value also has an effect on low current
operation. The transition to low current operation begins
when the inductor current reaches zero while the bottom
MOSFET is on. Burst Mode operation begins when the
averageinductorcurrentrequiredresultsinapeakcurrent
below 25% of the current limit determined by RSENSE
.
0
100
200
300
400
500
600
OPERATING FREQUENCY (kHZ)
Lower inductor values (higher ∆IL) will cause this to occur
at higher load currents, which can cause a dip in efficiency
in the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
1735 F02
Figure 2. Timing Capacitor Value
When synchronized to an external clock, Burst Mode
operation is disabled but the inductor current is not
allowed to reverse. The 25% minimum inductor current
clamp present in Burst Mode operation is removed,
providing constant frequency discontinuous operation
over the widest possible output current range. In this
mode the synchronous MOSFET is forced on once every
10 clock cycles to recharge the bootstrap capacitor. This
minimizes audible noise while maintaining reasonably
high efficiency.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ® cores. Actual core loss is independent of
coresizeforafixedinductorvalue, butitisverydependent
on inductance selected. As inductance increases, core
losses decrease. Unfortunately, increased inductance re-
quires more turns of wire and therefore copper losses will
increase.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
Kool Mµ is a registered trademark of Magnetics, Inc.
1735fc
12
Linear [ Linear ]
