Datasheet

LT8608/LT8608B
12
Rev. D
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
0.778V
– 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 capacitor
should be connected from V
OUT
to FB.
Setting the Switching Frequency
The LT8608 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 modu-
lation 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 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 (M AX )
=
V
OUT
+
V
SW (BOT )
t
ON(M IN )
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 (~0.55V, ~0.35V, 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 RT value, however the LT8608
will reduce switching frequency as necessary to maintain
control of inductor current to assure safe operation.
The LT8608 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 LT8608 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(M IN)
=
V
OUT
+
V
SW (BOT )
1– f
SW
• t
OFF (M IN )
– 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 (~0.55V, ~0.35V,
respectively at max load), f
SW
is the switching frequency
(set by RT), 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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