Datasheet

LT8640/LT8640-1
18
Rev.C
For more information www.analog.com
APPLICATIONS INFORMATION
Setting the Switching Frequency
The LT8640/LT8640-1 uses a constant frequency PWM
architecture that can be programmed to switch from
200kHz to 3MHz by using a resistor tied from the RT pin
to ground. A table showing the necessary R
T
value for a
desired switching frequency is in Table 1.
The R
T
resistor required for a desired switching frequency
can be calculated using:
R
T
=
46.5
f
SW
– 5.2
(3)
where R
T
is in kΩ and f
SW
is the desired switching fre-
quency in MHz.
Table 1. SW Frequency vs R
T
Value
f
SW
(MHz) R
T
(kΩ)
0.2 232
0.3 150
0.4 110
0.5 88.7
0.6 71.5
0.7 60.4
0.8 52.3
1.0 41.2
1.2 33.2
1.4 28.0
1.6 23.7
1.8 20.5
2.0 18.2
2.2 15.8
3.0 10.7
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller induc
-
tor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.
The highest switching frequency (f
SW(MAX)
) for a given
application can be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
SW(BOT)
t
ON(MIN)
V
IN
– V
SW(TOP)
+ V
SW(BOT)
( )
(4)
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
SW(TOP)
and V
SW(BOT)
are the internal switch
drops (~0.4V, ~0.15V, respectively at maximum load)
and t
ON(MIN)
is the minimum top switch on-time (see the
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate
a high V
IN
/V
OUT
ratio.
For transient operation, V
IN
may go as high as the absolute
maximum rating of 42V regardless of the R
T
value, however
the LT8640/LT8640-1 will reduce switching frequency
as necessary to maintain control of inductor current to
assure safe operation.
The LT8640/LT8640-1 is capable of a maximum duty cycle
of approximately 99%, and the V
IN
-to-V
OUT
dropout is
limited by the R
DS(ON)
of the top switch. In this mode the
LT8640/LT8640-1 skips switch cycles, resulting in a lower
switching frequency than programmed by RT.
For applications that cannot allow deviation from the pro
-
grammed switching frequency at low V
IN
/V
OUT
ratios use
the following formula to set switching frequency:
V
IN(MIN)
=
V
OUT
+ V
SW(BOT)
1– f
SW
• t
OFF(MIN)
– V
SW(BOT)
+ V
SW(TOP)
(5)
where V
IN(MIN)
is the minimum input voltage without
skipped cycles, V
OUT
is the output voltage, V
SW(TOP)
and
V
SW(BOT)
are the internal switch drops (~0.4V, ~0.15V,
respectively at maximum load), f
SW
is the switching fre-
quency (set by R
T
), and t
OFF(MIN)
is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
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