Datasheet

LT8614
14
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APPLICATIONS INFORMATION
For some applications it is desirable for the LT8614 to
operate in pulse-skipping mode, offering two major differ-
ences 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, the SYNC
pin is tied high either to a logic output or to the INTV
CC
pin. When a clock is applied to the SYNC pin the LT8614
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
0.970V
–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 effi-
ciency 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 LT8614 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 22pF phase-lead
capacitor should be connected from V
OUT
to FB.
Setting the Switching Frequency
The LT8614 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.
Figure3. Burst Mode Operation
I
L
500mA/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5µs/DIV
FRONT PAGE APPLICATION
12V
IN
TO 5V
OUT
AT 10mA
V
SYNC
= 0V
8614 F03
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