11
LTC1736
APPLICATIO S I FOR ATIO
W
U
U
U
C
pF
Frequency
OSC
(
)
. (
)
–
=
1 61 10
11
7
A graph for selecting C
OSC
versus frequency is given in
Figure 2. The maximum recommended switching fre-
quency is 550kHz .
The internal oscillator runs at its nominal frequency (f
O
)
when the FCB pin is pulled high to INTV
CC
or connected to
ground. Clocking the FCB pin above and below 0.8V will
cause the internal oscillator to lock to an external clock
signal with a frequency between 0.9f
O
and 1.3f
O
. The clock
high level must exceed 1.3V for at least 0.3
µ
s, and the
clock low level must be less than 0.3V for at least 0.3
µ
s.
The top MOSFET turn-on will synchronize with the rising
edge of the external clock.
Attempting to synchronize to too high an external fre-
quency (above 1.3f
O
) can result in inadequate slope com-
pensation and possible loop instability at high duty cycles.
If this condition exists simply lower the value of C
OSC
so
f
EXT
= f
O
according to Figure 2.
cycles to recharge the bootstrap capacitor. This minimizes
audible noise while maintaining reasonably high efficiency.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
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.
The inductor value has a direct effect on ripple current. The
inductor ripple current
∆
I
L
decreases with higher induc-
tance or frequency and increases with higher V
IN
or V
OUT
:
∆
I
f L
V
V
V
L
OUT
OUT
IN
=
1
1
( )( )
–
Accepting larger values of
∆
I
L
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
∆
I
L
= 0.3 to 0.4(I
MAX
). Remember,
the maximum
∆
I
L
occurs at the maximum input voltage.
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
average inductor current required results in a peak current
below 25% of the current limit determined by R
SENSE
.
Lower inductor values (higher
∆
I
L
) 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.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron
cores, forcing the use of more expensive ferrite,
OPERATING FREQUENCY (kHz)
0
100
200
300
400
500
600
C
OSC
VALUE (pF)
1736 F02
100.0
87.5
75.0
62.5
50.0
37.5
25.0
12.5
0
When synchronized to an external clock, Burst Mode op-
eration 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 wid-
est possible output current range. In this mode the
synchronous MOSFET is forced on once every 10 clock
Figure 2. Timing Capacitor Value
