11
Rev. C
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
0.778V
–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 LT8607 uses a constant frequency PWM architec-
ture 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 R
T
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 disadvan-
tages 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 (~0.25V, ~0.125V, respectively at max load) and
t
ON(MIN)
is the minimum top switch on-time (see Electrical
Characteristics). This equation shows that slower switch-
ing 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 R
T
value, however the LT8607
will reduce switching frequency as necessary to maintain
control of inductor current to assure safe operation.
The LT8607 is capable of maximum duty cycle approach-
ing 100%, and the V
IN
to V
OUT
dropout is limited by the
R
DS(ON)
of the top switch. In this mode the LT8607 skips
switch cycles, resulting in a lower switching frequency
than programmed 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 (~0.25V, ~0.125V,
respectively at max load), f
SW
is the switching frequency
(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.
