Minimize Standby Current in Automotive DDR Supplies

July 2013
I N
T H I S
I S S U E
straighten out your power
source priorities 8
µModule DC/DC combines
Volume 23 Number 2
Minimize Standby Current in
Automotive DDR Supplies
David Gilbert
high efficiency switching
with low noise linear
regulation 18
reference clock distribution
for a 325MHz IF sampling
system with over 30MHz
bandwidth, 64dB SNR and
80dB SFDR 26
near noiseless ADC drivers
for imaging 31
When you turn on a laptop or a smart phone, you expect
to wait for it to boot up, but you are less patient when you
turn on your car. With a car, consumers expect immediate
access to computer electronics, including navigation and
infotainment systems, and automobile manufacturers strive to
meet this desire with design strategies that shorten start-up
time. One such strategy is to keep dynamic memory (RAM)
active at all times, even during the ignition-off state.
The DDR3 memory used in automobiles operates on a 1.5V rail with peak load currents over
2A—preferably produced by a high efficiency DC/DC converter to minimize heat dissipation. In
these applications, light load efficiency is just as important to preserve battery life when the automobile is not
running. DDR memory can consume 1m A–10m A from the
1.5V rail in standby, but drawing 10m A from the battery
is unacceptable when the car is parked for long periods.
This constraint rules out the use of a linear regulator, where
input and output current are equal. On the other hand, a
switching step-down (buck) regulator draws less input current than load current in proportion to the step-down ratio:
IIN =
1 VOUT • IOUT
•
VIN
η
where h is the efficiency factor (0 to 1).
Figure 1 shows that the LT®8610AB synchronous stepdown regulator achieves ~83% efficiency at a 1m A load.
For a battery voltage of 12V and load current of 1m A at
1.5V, calculated input current is only 151µ A.
The LT8610 keeps automotive electronics running.
Caption
w w w. li n ea r.com
(continued on page 4)
The LT8610A and LT8610AB have a low component count, low
minimum input voltage, low quiescent current and high efficiency across
a wide load range. These features make them the preferred solution for
providing standby power to DDR memory in automotive applications.
(LT8610A/AB continued from page 1)
100
DIRECT DC/DC CONVERSION
FROM CAR BATTERY TO
1.5V DDR MEMORY
95
EFFICIENCY (%)
90
LT8610A and LT8610AB are monolithic,
synchronous step-down regulators
designed specifically for automotive
systems. They deliver 3.5A while consuming only 2.5µ A quiescent current. Building
a circuit around them is easy. No additional semiconductors are required, they
work with inexpensive ceramic capacitors, and the MSOP package has leads that
are easy to solder and inspect. With a
typical minimum on-time of 30ns (45ns
guaranteed maximum), one can design
compact, high switching-frequency
buck regulators with large step-down
ratios. Figure 2 shows an application
that delivers 3.5A at 1.5V. The operating
frequency is 475kHz to optimize efficiency
and remain below the AM radio band.
Figure 2. This LT8610A or
LT8610AB step-down converter
circuit accepts automotive
battery and generates 1.5V at
3.5A. Low quiescent current and
synchronous rectification result in
high efficiency across the entire
load range.
4 | July 2013 : LT Journal of Analog Innovation
4.7µF
80
75
70
65
60
55
50
1
0.1
1k
10k
Figure 1. LT8610AB efficiency versus load
input is worst-case 3.4V, the maximum
duty cycle is above 99% and the dropout voltage is typically 200mV at 1A, all
of which keep the output in regulation
through cold-crank. The typical minimum
input voltage is plotted in Figure 3.
The difference between the LT8610A and
LT8610AB is that the LT8610AB features
higher efficiency at light loads. This is
achieved by using an increased Burst
Mode current limit, allowing more
energy to be delivered during each
switch cycle and lowering the switching frequency for a given load. Because
a fixed amount of energy is required to
SAVE THE BATTERY WITH LOW
RIPPLE Burst Mode OPERATION AND
MINIMAL QUIESCENT CURRENT
VIN
BST
EN/UV
LT8610A/
PG
SW
LT8610AB
BIAS
SYNC
FB
TR/SS
INTVCC
RT
93.1k
fSW = 475kHz
GND
0.1µF
3.3µH
549k
4.7pF
1M
3.6
3.4
3.2
INPUT VOLTAGE (V)
The LT8610A and LT8610AB are designed
to minimize output voltage ripple over
the entire load range. At light loads, they
10nF
1µF
10
100
ILOAD (mA)
VIN = 12V
VOUT = 1.5V
RT = 93.1k (475kHz)
L = COILCRAFT XAL6030-332ME 3.3µH
Both parts feature excellent fault tolerance
to automotive environments. A maximum input of 42V handles load dumps.
A robust switch design and high speed
current comparator protect the device
during output shorts. The minimum
VIN
12V
85
maintain efficiency by reducing their operating frequency and entering Burst Mode®
operation. Fast transient response is maintained even at very low loads. This feature
combined with the very low quiescent
current of 2.5µ A means that, even at a few
µ A of load, the LT8610A and LT8610AB are
more efficient than a linear regulator
with zero quiescent current. For systems
where low frequency operation must be
avoided, Burst Mode operation can be
turned off by applying either a logic high
signal or clock signal to the SYNC pin.
3.0
2.8
2.6
2.4
VOUT
1.5V
47µF 3.5A
×3
2.2
2.0
–55 –25
95
65
35
TEMPERATURE (°C)
5
125
155
Figure 3. Keeping the memory alive in cold-crank
or start-stop events. The LT8610A and LT8610AB
operate down to a typical minimum input voltage of
2.9V at 25°C, with 3.4V maximum guaranteed over
temperature.
design features
The LT8610A and LT8610AB are designed to minimize output voltage
ripple over the entire load range. At light loads, they maintain efficiency by
reducing their operating frequency and entering Burst Mode operation.
Fast transient response is maintained even at very low loads.
EFFICIENCY (%)
90
OUTPUT VOLTAGE RIPPLE (mVP–P)
95
LT8610AB
85
80
LT8610A
75
70
65
60
55
50
1
0.1
10
100
ILOAD (mA)
1k
10k
70
70
60
60
50
40
30
20
10
0
VIN = 12V
VOUT = 1.5V
RT = 93.1k (475kHz)
L = COILCRAFT XAL6030-332ME 3.3µH
switch the MOSFET on and off, a lower
switching frequency reduces gatecharge losses and increases efficiency.
Figure 4 shows the efficiency difference
between the LT8610A and LT8610AB.
For loads between 1m A and 100m A, the
LT8610AB improves efficiency by more
than 10% compared to the LT8610A.
The trade-off to the increased Burst
Mode current limit is that more energy
is delivered in each switch cycle, so
VIN
4.7µF
BST
EN/UV
LT8610A/
PG
SW
LT8610AB
BIAS
SYNC
10nF
FB
TR/SS
1µF
INTVCC
RT
18.2k
fSW = 2MHz
GND
LT8610A
2
1
3
NUMBER OF 1210 47µF OUTPUT CAPACITORS
VIN = 12V
VOUT = 1.5V
RT = 93.1k (475kHz)
L = COILCRAFT XAL6030-332ME 3.3µH
Figure 4. An increased Burst Mode current limit on
the LT8610AB results in substantial efficiency gains
at light load compared to the LT8610A. VIN
12V
LT8610AB
0.1µF
1µH
549k
(a)
50
40
30
20
LT8610AB
10
0
LT8610A
2
1
3
NUMBER OF 1210 47µF OUTPUT CAPACITORS
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
L = COILCRAFT XAL4020-102ME 1µH
(b)
Figure 5. Output voltage ripple versus number of 1210 size 47µF output capacitors for two inductor values, at
10mA load. (a) Ripple for the 475kHz application in Figure 2. (b) Ripple for the 2MHz application in Figure 6.
more output capacitance is required
to keep output voltage ripple low.
Figure 5 compares the output ripple for
the LT8610A and LT8610AB as a function of the output capacitance for
two inductor values, at 10m A load.
one. If high efficiency at light loads is
paramount, then the inductor value
can be increased beyond the starting
value recommended in the data sheet.
In addition to the current limit, the
inductor choice affects the efficiency
and switching frequency in Burst Mode
operation. This is because for a fixed
current limit, a larger inductor value
can store more energy than a smaller
For most automotive systems, 9V to
16V is the typical input voltage, so application circuits are usually optimized
for this range. The 475kHz application
in Figure 2 operates at the designed
frequency over the entire input range
of 3.5V to 42V. However, if we restrict
the normal operating voltage to
16V (42V transient), the operating frequency can be increased and the value
and size of the inductor reduced. With
a worst-case minimum on-time of 45ns,
the LT8610A and LT8610AB can be programmed to 2MHz as shown in Figure 6.
VOUT
1.5V
47µF 3.5A
×3
4.7pF
1M
OUTPUT VOLTAGE RIPPLE (mVP–P)
100
Figure 6. Similar 12V to 1.5V application as in
Figure 2, but the operating frequency of the LT8610A
and LT8610AB is increased to 2MHz for reduced
inductor value and size.
GO FASTER FOR A SMALLER
SOLUTION
July 2013 : LT Journal of Analog Innovation | 5
100
100
95
95
90
90
COILCRAFT
XAL5030-222ME 2.2µH
85
EFFICIENCY (%)
EFFICIENCY (%)
An important feature is that this internal regulator can draw current from either the VIN pin
or the BIAS pin. If a voltage of 3.1V or higher is tied to the BIAS pin, gate drive current
is drawn from BIAS. If the BIAS voltage is lower than VIN, the internal linear regulator will
run more efficiently using the lower voltage supply, thus increasing overall efficiency.
80
75
COILCRAFT
XAL4020-102ME 1µH
70
65
80
75
65
55
55
1
10
100
ILOAD (mA)
1k
50
10k
COILCRAFT
XAL4020-102ME 1µH
70
60
0.1
The LT8610A and LT8610AB use two internal nMOSFETs specifically optimized for
automotive applications. In particular, the
gate drive circuitry requires less than 3V to
fully enhance the FETs. To generate the
gate drive supply, the LT8610A/AB includes
an internal linear voltage regulator, the
output of which is the INTVCC pin (do not
load INTVCC with external circuitry).
85
60
50
BIAS PIN OPTIMIZES EFFICIENCY
LT8610AB
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
COILCRAFT
XAL5030-222ME 2.2µH
0.1
1
10
100
ILOAD (mA)
1k
10k
An important feature is that this internal
regulator can draw current from either
the VIN pin or the BIAS pin. If the BIAS pin
is left open, then gate drive current is
drawn from VIN . However, if a voltage
of 3.1V or higher is tied to the BIAS pin,
gate drive current is drawn from BIAS.
If the BIAS voltage is lower than VIN, the
internal linear regulator will run more
efficiently using the lower voltage supply, thus increasing overall efficiency.
LT8610A
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
Figure 7. LT8610A and LT8610AB efficiency versus load at 2MHz with two inductor values
100
100
95
95
90
BIAS = 3.3V
GATE DRIVE DRAWN FROM
85% EFFICIENT SUPPLY
85
80
75
EFFICIENCY (%)
EFFICIENCY (%)
90
BIAS = 0V
GATE DRIVE DRAWN FROM VIN
70
65
85
80
70
65
60
60
55
55
50
0.1
1
10
100
ILOAD (mA)
1k
10k
LT8610AB
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
L = COILCRAFT XAL4020-102ME 1µH
BIAS = 3.3V
GATE DRIVE DRAWN FROM
85% EFFICIENT SUPPLY
75
50
BIAS = 0V
GATE DRIVE DRAWN FROM VIN
0.1
1
10
100
ILOAD (mA)
1k
10k
LT8610A
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
L = COILCRAFT XAL4020-102ME 1µH
Figure 8. Efficiency can be increased if the BIAS pin is tied to an external 3.3V supply. (External supply
efficiency of 85% is assumed and factored into overall efficiency shown here.)
Note that when the input voltage goes
above 16V, the output remains in regulation although the switching frequency
decreases to maintain safe operation. The
2MHz solution is identical to the circuit
in Figure 2, except for the RT resistor
6 | July 2013 : LT Journal of Analog Innovation
changed to 18.2kΩ and the inductor
value and size reduced in order to save
space. Figure 7 shows efficiency versus load for two inductor choices.
The efficiency data in Figures 1, 4 and 7
was recorded with the BIAS pin open. After
all, if the 1.5V output is the only rail alive,
then there is likely no good place to tie
the BIAS pin. However, if there is a 3.3V or
5V supply, tie it to the BIAS pin, even if
the supply is not available in standby or
ignition-off conditions. Figure 8 shows the
efficiency with and without a 3.3V supply
connected to BIAS. In calculating the total
efficiency, we have included the power
drawn from the 3.3V rail and assumed
that it was generated with 85% efficiency.
Note that the benefit to externally powering BIAS is greater at higher operating
frequencies because the gate drive current is higher. The LT8610A also benefits
design features
An important consideration for automotive applications is the behavior of the
power supply during cold crank and idle-stop transients, when the voltage from
the 12V battery may drop below 4V. The LT8610AB operates up to 99% duty
cycle, providing output regulation at the lowest possible input voltage.
more from external bias compared to
the LT8610AB—the AB’s increased Burst
Mode current limit results in a lower
operating frequency for a given load.
and reliable output voltage as a function of input voltage. Figure 10(b) shows
the output voltage as the input supply is
ramped from zero to 10V and back to zero.
NOT JUST FOR MEMORY
CONCLUSION
The LT8610AB is an excellent regulator
for other automotive supplies, including 3.3V and 5V supplies, with efficiency
above 90%, as shown in Figure 9.
The LT8610A and LT8610AB have a low
component count, low minimum input
voltage, low quiescent current and high
efficiency across a wide load range.
These features make them the preferred solutions for providing standby
PARAMETER
LT8610
LT8610A
LT8610AB
Max Load Current (A)
2.5
3.5
3.5
Minimum On-Time
(ns) (Typ)
50
30
30
V IN = 12V, V OUT = 1.5V, I LOAD = 10mA,
f SW=475kHz, L = 3.3µH
73.9
73.9
85.5
Output Voltage Ripple
(mV P-P)
I LOAD = 10mA, C OUT = 47µF, L = 3.3µH
8.4
8.4
52.5
800
90
700
80
600
60
50
40
30
fSW = 700kHz
VIN = 12V
L = 4.7µH
20
0.001
0.1
VOUT = 3.3V
VOUT = 5V
1
10
100
LOAD CURRENT (mA)
1000
Figure 9. Efficiency for 3.3V and 5V outputs is above
90%, reducing total power dissipation and keeping
temperature under control.
CONDITIONS
Efficiency (%)
100
70
Visit www.linear.com/LT8610 for
data sheets, demo boards and other
applications information. n
Table 1. Comparison of features for the LT8610 family of monolithic, synchronous buck converters
DROPOUT VOLTAGE (V)
EFFICIENCY (%)
An important consideration for automotive applications is the behavior of the
power supply during cold crank and
idle-stop transients, when the voltage from
the 12V battery may drop below 4V. The
LT8610AB operates up to 99% duty cycle,
providing output regulation at the lowest possible input voltage. Figure 10(a)
shows the dropout voltage. This is the
difference between VIN and VOUT as the
input voltage decreases towards the
intended output regulation voltage. The
LT8610AB also has excellent start-up and
dropout behavior, resulting in predictable
power to DDR memory in automotive
applications. Table 1 summarizes the
performance of the LT8610 family.
VIN
2V/DIV
VIN
500
400
VOUT
2V/DIV
300
VOUT
200
100
0
0
0.5
1.5
2
2.5
1
LOAD CURRENT (A)
(a)
3
3.5
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
(b)
Figure 10. The LT8610AB operates to 99% duty cycle, providing smooth start-up and low dropout voltage.
July 2013 : LT Journal of Analog Innovation | 7