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LT8610AIMSE-3.3#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT8610A/LT8610AB Series
13
8610abfa
For more information www.linear.com/LT8610A
(2a)
(2b)
APPLICATIONS INFORMATION
resulting in much higher light load efficiency than for typi-
cal converters.
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.
The fixed output versions of the LT8610A/LT8610AB series
have larger internal feedback resistors than can practically
be used externally, so are a good choice for optimizing
quiescent current performance.
While in Burst Mode operation the current limit of the
top switch is approximately 400mA for the LT8610A
resulting in output voltage ripple shown in Figure 2a. The
LT8610AB has a 1A current limit in Burst Mode operation,
which increases the efficiency but also the output voltage
ripple compared to the the LT8610A (Figure 2b). However,
increasing the output capacitance will decrease the output
ripple proportionally (Table 1). 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
1a. The output load at
Figure 1. SW Frequency vs Load Information in
Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
Figure 2. Burst Mode Operation of LT8610A (2a) and
LT8610AB (2b)
Minimum Load to Full Frequency
Burst Frequency
(1a)
(1b)
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
800
8610ab F01a
300
200
0
200 400 600 700100 300 500
100
800
V
IN
= 12V
V
OUT
= 3.3V
L = 4.7µH
LT8610A
LT8610AB
700
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
70
80
15 25
40
8610ab F01b
40
20
0
50
30
10
5 10
20
30 35
V
OUT
= 3.3V
f
SW
= 700kHz
PULSE-SKIPPING MODE
I
L
200mA/DIV
V
OUT
10mV/DIV
5µs/DIVV
SYNC
= 0V
C
OUT
= 47µF
L = 4.7µH
8610ab F02a
LT8610A
I
L
500mA/DIV
V
OUT
20mV/DIV
V
SW
5V/DIV
20µs/DIVV
SYNC
= 0V
C
OUT
= 47µF
L = 4.7µH
8610ab F02b
LT8610AB
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8610A/LT8610AB
operates in low ripple Burst Mode operation, which keeps
the output capacitor charged to the desired output voltage
while minimizing the input quiescent current and minimiz
-
ing output
voltage ripple. In Burst Mode operation the
LT8610A/LT8610AB
delivers single pulses of current to
the output capacitor followed by sleep periods where the
output power is supplied by the output capacitor. While
in sleep mode the LT8610A/LT8610AB consumes 1.7μA.
As the output load decreases, the frequency of single cur
-
rent pulses decreases (see Figure 1a) and the percentage
of time the LT8610A/LT8610AB is in sleep mode increases,
LT8610A/LT8610AB Series
14
8610abfa
For more information www.linear.com/LT8610A
APPLICATIONS INFORMATION
which the LT8610A/LT8610AB reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice. However, the output load required
to reach full frequency will be higher for the LT8610AB
as compared to the LT8610A (Figure 1a).
Inductor value has a very strong effect on Burst Mode ef
-
ficiency. Larger value inductors allow more charge to be
transferred to the output per pulse, which increases both
efficiency and output voltage ripple. This dependence on
inductance is stronger for the LT8610AB than it is for the
LT8610A. If higher efficiency is needed in a Burst Mode ap
-
plication, increasing
inductor
value can be a quick solution.
Table 1. Output Voltage Ripple vs Output Capacitance for
LT8610AB when V
IN
= 12V, V
OUT
= 3.3V, and L = 4.7µH
OUTPUT CAPACITANCE OUTPUT RIPPLE
47µF 40mV
47µF ×2
20mV
47µF ×4
10mV
For some applications it is desirable for the LT8610A/
LT8610AB to operate 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 1b). 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 LT8610A/LT8610AB 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.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from V
OUT
to FB.
The fixed output versions of the LT8610A/LT8610AB
series have the feedback resistor network and phase
lead capacitor integrated within the part. The FB pin is
replaced with a V
OUT
pin for these regulators. The V
OUT
pin can be connected directly to the inductor and output
capacitor. The 3.3V fixed output products (LT8610A-3.3/
LT8610AB-3.3) have a total of 14.3M of internal feedback
divider resistance from the V
OUT
pin to ground. The 5V
fixed output products (LT8610A-5/LT8610AB-5) have a
total of 12.5M of internal feedback divider resistance from
the V
OUT
pin to ground.
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 LT8610A/
LT8610AB and the second term is the current in the feed-
back 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.
Setting the Switching Frequency
The LT8610A/LT8610AB 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)
LT8610A/LT8610AB Series
15
8610abfa
For more information www.linear.com/LT8610A
APPLICATIONS INFORMATION
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 (~0.42V, ~0.21V, 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 LT8610A/LT8610AB will reduce switching
frequency as necessary to maintain control of inductor
current to assure safe operation.
The LT8610A/LT8610AB 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 LT8610A/LT8610AB 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 (~0.42V, ~0.21V,
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 LT8610A/LT8610AB is designed to minimize solution
size by allowing the inductor to be chosen based on the
output load requirements of the application. During over-
load or short-circuit conditions the LT8610A/LT8610AB
safely tolerates operation 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
(~0.21V) 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
∆I
L
(7)
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LT8610AIMSE-3.3#PBF

Mfr. #:
Buy LT8610AIMSE-3.3#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 3.5A Synchronous Step-Down Regulator with 2.5uA Quiescent Current
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