LTC1735 Linear Technology, LTC1735 Datasheet - Page 11

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
capacitor reaches 1.19V, C
process then repeats.
The value of C
frequency assuming no external clock input on the FCB
pin:
A graph for selecting C
Figure 2. The maximum recommended switching fre-
quency is 550kHz .
The internal oscillator runs at its nominal frequency (f
when the FCB pin is pulled high to INTV
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 0.9f
1.3V for at least 0.3 s and the clock low level must be less
than 0.3V for at least 0.3 s. The top MOSFET turn-on will
synchronize with the rising edge of the clock.
Attempting to synchronize to too high an external fre-
quency (above 1.3f
pensation and possible loop instability. If this condition
exists simply lower the value of C
to Figure 2.
When synchronized to an external clock, Burst Mode
operation is disabled but the inductor current is not
C
OSC
( )
pF
100.0
87.5
75.0
62.5
50.0
37.5
25.0
12.5
0
O
0
OSC
Figure 2. Timing Capacitor Value
and 1.3f
Frequency
1 61 10
100
is calculated from the desired operating
. (
OPERATING FREQUENCY (kHZ)
O
U
) can result in inadequate slope com-
O
200
OSC
. The clock high level must exceed
7
U
)
versus frequency is shown in
300
OSC
–
11
OSC
400
is reset to ground. The
W
so f
500
CC
EXT
1735 F02
or connected to
600
= f
O
according
U
O
)
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 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
tance or frequency and increases with higher V
Accepting larger values of I
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is I
the maximum I
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
Lower inductor values (higher I
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.
I
L
( )( )
f L
1
V
OUT
L
occurs at the maximum input voltage.
1
–
L
L
V
V
= 0.3 to 0.4(I
OUT
decreases with higher induc-
IN
L
L
) will cause this to occur
allows the use of low
MAX
LTC1735
). Remember,
IN
or V
11
SENSE
OUT
:
.