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LT8614EUDC#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT8614
13
8614fc
For more information www.linear.com/LT8614
APPLICATIONS INFORMATION
traces will shield them from the SW and BOOST nodes.
The exposed pad on the bottom of the package should be
soldered to SW to reduce thermal resistance to ambient. To
keep thermal resistance low, extend the ground plane from
GND1 and GND2 as much as possible, and add thermal
vias to additional ground planes within the circuit board
and on the bottom side.
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8614 operates
in low ripple Burst Mode operation, which keeps the out
-
put capacitor
charged to the desired output voltage while
minimizing the input quiescent current and minimizing
output voltage ripple. In Burst Mode operation the LT8614
delivers single small pulses of current to the output capaci
-
tor followed by sleep periods where the output power is
supplied
by the output capacitor. While in sleep mode the
LT8614 consumes 1.7μA.
As the output load decreases, the frequency of single cur
-
rent pulses decreases (see Figure 2a) and the percentage
of
time the LT8614 is in sleep mode increases, resulting in
much higher light load efficiency than for typical convert
-
ers. By maximizing the time between pulses, the converter
quiescent current approaches 2.5µA for
a typical application
when there is no output load. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider must be minimized as it
appears to the output as load current.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8614 can
stay in sleep mode longer between each pulse. This can be
achieved by using a larger value inductor (i.e., 4.7µH), and
should be considered independent of switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch
-
ing frequency
application, if high light load efficiency is
desired, a higher inductor value should be chosen. See
curve in Typical Performance Characteristics.
While in Burst Mode operation the current limit of the top
switch is approximately 600mA resulting in output voltage
ripple shown in Figure 3. Increasing the output capacitance
will decrease the output ripple proportionally. As load ramps
upward from zero the switching frequency will increase
but only up to the switching frequency programmed by
the resistor at the RT pin
as shown in Figure 2a. The out-
put
load at which the LT8614 reaches the programmed
frequency
varies based on input voltage, output voltage,
and inductor choice.
Figure 2. SW Frequency vs Load Information in
Burst Mode Operation (2a) and Pulse-Skipping Mode (2b)
Minimum Load to Full Frequency (SYNC DC High)
Burst Frequency
(2a)
(2b)
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
200
400
600
800
1000
1200
50 100 150 200
8614 F02a
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
INPUT VOLTAGE (V)
5
LOAD CURRENT (mA)
60
80
100
20 30 45
8614 F02b
40
20
0
10 15
25
35 40
FRONT PAGE APPLICATION
V
OUT
= 5V
f
SW
= 1MHz
LT8614
14
8614fc
For more information www.linear.com/LT8614
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 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 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
LT8614
15
8614fc
For more information www.linear.com/LT8614
APPLICATIONS INFORMATION
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.
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 LT8614 will reduce switching frequency as
necessary to maintain control of inductor current to as
-
sure safe operation.
The LT8614 is capable of a maximum duty cycle of greater
than 99%, and the V
IN
-to-V
OUT
dropout is limited by the
R
DS(ON)
of the top switch. In this mode the LT8614 skips
switch cycles, resulting in a lower switching frequency
than programmed by RT.
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)
(5)
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 (~0.3V, ~0.15V,
respectively at maximum load), f
SW
is the switching fre-
quency (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.
Inductor Selection and Maximum Output Current
The LT8614 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8614 safely tolerates opera
-
tion with a saturated inductor through the use of a high
speed peak-current mode architecture.
A good first choice for the inductor value is:
L =
V
OUT
+ V
SW(BOT)
f
SW
(6)
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.15V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled I
SAT
)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
I
L(PEAK)
= I
LOAD(MAX)
+
1
2
ΔI
L
(7)
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LT8614EUDC#PBF

Mfr. #:
Buy LT8614EUDC#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 4A Synchronous Step-Down SILENT SWITCHER with 2.5 A Quiescent Current
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