Datasheet

LT8641
15
Rev B
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APPLICATIONS INFORMATION
For some applications it is desirable for the LT8641 to oper-
ate in pulse-skipping mode, offering two major differences
from Burst Mode operation. First is the clock stays awake
at all times and all switching cycles are aligned to the clock.
In this mode much of the internal circuitry is awake at all
times, increasing quiescent current to several hundred µA.
Second is that full switching frequency is reached at lower
output load than in Burst Mode operation (see Figure 2b).
To enable pulse-skipping mode, float the SYNC/MODE pin.
When a clock is applied to the SYNC/MODE pin the LT8641
will also operate in pulse-skipping mode.
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
0.81V
– 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 LT8641 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 = 324k, the feedback divider draws 2.5µA. With V
IN
=
12V and n = 85%, 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 22pF phase-lead
capacitor should be connected from V
OUT
to FB.
Setting the Switching Frequency
The LT8641 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.
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