LT8611
13
8611fa
For more information
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
0.970V
– 1
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
I
Q
=
1.7µA
+
V
OUT
R1
+
R2
V
OUT
V
IN
1
n
(2)
where 1.7µA is the quiescent current of the LT8611 and
the second term is the current in the feedback divider
reflected to the input of the buck operating at its light
load efficiency n. For a 3.3V application with R1 = 1M and
R2 = 412k, the feedback divider draws 2.3µA. With V
IN
=
12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent
current resulting in 2.5µA no-load current from the 12V
supply. Note that this equation implies that the no-load
current is a function of V
IN
; this is plotted in the Typical
Performance Characteristics section.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from V
OUT
to FB.
Setting the Switching Frequency
The LT8611 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.
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
14
28.0
1.6
23.7
1.8
20.5
2.0
18.2
2.2
15.8
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 (~0.3V, ~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 abso-
lute maximum rating of 42V regardless of the R
T
value,
however the LT8611 will reduce switching frequency as
necessary to maintain control of inductor current to as-
sure safe operation.
