Datasheet

LT8609/LT8609A/LT8609B
14
Rev. H
For more information www.analog.com
APPLICATIONS INFORMATION
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1= R2
V
OUT
0.782V
– 1
⎛
⎝
⎜
⎞
⎠
⎟
1% resistors are recommended to maintain output volt-
age accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible when good low load
efficiency is desired: The resistor divider generates a
small load on the output, which should be minimized to
optimize the quiescent current at low loads.
When using large FB resistors, a 10pF phase lead capaci
-
tor should be connected from V
OUT
to FB.
Setting the Switching Frequency
The LT8609/LT8609A/LT8609B uses a constant frequency
PWM architecture that can be programmed to switch from
200kHz to 2.2MHz 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. When in spread
spectrum modulation mode, the frequency is modulated
upwards of the frequency set by R
T
.
Table1. SW Frequency vs RT Value
f
SW
(MHz) R
T
(kΩ)
0.2 221
0.300 143
0.400 110
0.500 86.6
0.600 71.5
0.700 60.4
0.800 52.3
0.900 46.4
1.000 40.2
1.200 33.2
1.400 27.4
1.600 23.7
1.800 20.5
2.000 18.2
2.200 16.2
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 inductor 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)
( )
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, ~0.25V, respectively at max load) and t
ON(MIN)
is
the minimum top switch on-time (see Typical Applications).
This equation shows that 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 Abs Max
rating regardless of the RT value, however the LT8609/
LT8609A/LT8609B will reduce switching frequency as nec-
essary to maintain control of inductor current to assure
safe operation.
The LT8609/LT8609A/LT8609B is capable of maximum
duty cycle approaching 100%, and the V
IN
to V
OUT
dropout is limited by the R
DS(ON)
of the top switch. In
this mode the LT8609/LT8609A /LT8609B skips switch
cycles, resulting in a lower switching frequency than pro-
grammed by R
T
.
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)
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, ~0.25V,
respectively at max load), f
SW
is the switching frequency
(set by RT), and t
OFF(MIN)
is the minimum switch off-time.
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