Mar 2006 Cascadable, 7A Point-of-Load Monolithic Buck Converter

L DESIGN FEATURES
Cascadable, 7A Point-of-Load
by Peter Guan
Monolithic Buck Converter
Introduction
Easy-to-use and compact point-ofload power supplies are necessary
in systems with widely distributed,
high current, low voltage loads. The
LTC3415 provides a compact, simple,
complete and versatile solution. It
includes a pair of integrated complementary power MOSFETs (32mΩ top
and 25mΩ bottom) and requires no
external sense resistor. A complete
design involves choosing an inductor
and input/output capacitors, and
that’s it. The result is a fast, constant
frequency, current mode, 7A DC/DC
switching regulator.
0.1µF
47µF
6.3V
3x
38
1
3
4
5
10pF
6
7
8
9
MODE
10
11
12
37
36
CLKOUT RUN
NC
PVIN
35
PVIN
34
SVIN
33
ITHM
SGND
32
ITH
NC
TRACK
PLLLPF
VFB
PVIN
PVIN
PVIN
PVIN
SW
SW
LTC3415EUHF
SW
SW
SW
SW
SW
SW
MODE
PGOOD
PGND (39)
CLKIN
BSEL
MGN
PHMODE
PGND PGND PGND PGND PGND PGND PGND
13
14
15
16
17
18
31
30
29
27
22pF
30.1k
26
25
24
23
22
21
10k
SVIN
20
19
100µF, 6.3V
2x
0.2µH
15k
SVIN
28
VOUT
1.8V/7A
Figure 1. Typical application of the LTC3415 in a 3.3V to 1.8V/7A step down converter
Operation
Figure 1 shows a typical application
of the LTC3415 in a 3.3V to 1.8V/7A
step down converter. Figure 2 shows
its efficiency and power loss vs load
current. Figure 3 shows its transient
response to a 5A load.
The LTC3415 uses a constant frequency, current mode architecture
to drive an integrated pair of complementary power MOSFETs. An internal
oscillator sets the 1.5MHz operating
95
10000
EFFICIENCY
90
EFFICIENCY (%)
85
VOUT = 1.8V
100mV/DIV
AC COUPLED
1000
80
75
100
70
65
POWER LOSS
60
VIN = 3.3V
VOUT = 1.8V
Burst Mode OPERATION
55
50
frequency of the device. The main
P-channel power MOSFET turns on
with every oscillator cycle and turns
off when the internal current comparator trips, indicating that the inductor
current has reached a level set by the
ITH pin. An internal error amplifier, in
turn, drives the ITH pin by monitoring
the output voltage through an external resistive divider connected to the
VFB pin. While the P-channel power
MOSFET is off, the internal synchronous N-channel power MOSFET turns
on until ether the inductor current
10
100
1000
LOAD CURRENT (mA)
POWER LOSS (mW)
6
2
SGND
Features
The overall solution is extremely compact since the LTC3415’s QFN 5mm ×
7mm package footprint is small while
its high operating frequency of 1.5MHz
allows the use of small low profile
surface mount inductors and ceramic
capacitors. For loads higher than 7A,
multiple LTC3415s can be cascaded
to share the load while running mutually antiphase, which reduces overall
ripple at both the input and the output.
Other features include:
q Spread spectrum operation to
reduce system noise,
q Output tracking for controlled
VOUT ramp-up and ramp-down,
q Output margining for easy system
stress testing,
q Burst Mode® operation to lower
quiescent current and boost efficiency during low loads,
q Low shutdown current of less
than 1µA,
q 100% duty-cycle for low drop out
operation, phase-lock-loop to allow frequency synchronization of
±50%,
q Easily cascadable for multi-device
load sharing with multiphase
operation
q Internal or external ITH compensation for either ease of use or
loop optimization.
1Ω
VIN
IINDUCTOR
STEP =
0A TO 5A
5A/DIV
IOUT
STEP =
0A TO 5A
5A/DIV
10
1
10000
Figure 2. Efficiency and power loss vs
load current for the circuit in Figure 1
VIN = 3.3V
40µs/DIV
L = 0.2µH
COUT = 2 × 100µF
Figure 3. Transient response to a
5A load for the circuit in Figure 1
Linear Technology Magazine • March 2006
DESIGN FEATURES L
10k
0.1µF
RUN
100pF
1000pF
0.1µF
1Ω
1Ω
VIN
VIN
38
1
SGND
2
3
4
5
6
7
8
9
10
11
SVIN
12
37
36
CLKOUT RUN PVIN
NC
35
PVIN
34
SVIN
33
ITHM
SGND
32
ITH
NC
TRACK
PLLLPF
VFB
PVIN
PVIN
PVIN
PVIN
SW
SW
LTC3415EUHF
SW
SW
SW
SW
SW
SW
MODE
PGOOD
PGND (39)
CLKIN
BSEL
PHMODE
MGN
PGND PGND PGND PGND PGND PGND PGND
13
14
15
16
17
18
38
22µF
6x
31
1
15k
30
VTRACK
29
10k
SGND
2
3
4
28
27
30.1k
10pF
100pF
26
5
6
25
7
24
8
23
9
10k
22
21
10
SVIN
11
BSEL
20
MGN
SVIN
12
36
35
PVIN
34
SVIN
33
ITHM
SGND
32
ITH
NC
TRACK
PLLLPF
VFB
PVIN
PVIN
PVIN
PVIN
SW
SW
LTC3415EUHF
SW
SW
SW
SW
SW
SW
MODE
PGOOD
PGND (39)
CLKIN
BSEL
PHMODE
MGN
PGND PGND PGND PGND PGND PGND PGND
13
19
37
CLKOUT2 RUN PVIN
NC
14
15
16
17
18
31
30
29
28
27
26
25
24
23
22
21
20
BSEL
MGN
19
100µF
6.3V
2x
VOUT
1.8V/14A
0.2µH
0.2µH
Figure 4. Dual-phase single output 3.3V to 1.8V 15A application using two LTC3415s running 180° out of phase with respect to each other.
Modes of Operation:
Burst, Pulse-Skip, and
Forced Continuous
Three modes of operation can be selected through the MODE pin. Tying
it to VIN enables Burst Mode operation
for highest efficiency. During low output loads, the peak inductor current
limit is clamped to about a quarter of
the maximum value and the ITH pin is
monitored to determine whether the
device will go into a power-saving Sleep
mode. Quiescent current is reduced
to 450µA in Burst Mode operation
because most of the internal circuitry
is turned off.
For applications that aim to reduce
output ripple and strive to maximize
operating time at constant frequency,
pulse-skip mode is a good solution.
Pulse-skip mode is enabled by letting
the Mode pin float or tying it to VIN /2
or VOUT. Inductor current is still not
allowed to reverse as in Burst Mode operation, but the peak inductor current
limit is no longer clamped internally.
ITH has full control of output current
until ITH drops so low (at low output
Linear Technology Magazine • March 2006
loads) that minimum on-time of the
device is reached and the LTC3415
begins to skip cycles.
For applications that require constant frequency operation even at no
load, the LTC3415 can be put into
forced continuous mode operation
by tying the Mode pin to ground. In
this mode, inductor current is allowed
to reverse while the internal power
MOSFETs are always driven at the
same frequency.
Output Tracking
For applications that require controlled output voltage tracking between
100
95
90
EFFICIENCY (%)
starts to reverse, as indicated by the
SW pins going below ground, or until
the beginning of the next cycle.
85
80
1
75
2
3 4
6
70
65
60
55
50
1
10
LOAD CURRENT (A)
100
Figure 5. Combined efficiency of loadsharing in 1-phase, 2-phase, 3-phase,
4-phase, and 6-phase operation.
their various outputs in order to prevent excessive current draw or even
latch-up during turn-on and turnoff, the LTC3415 includes a Track
pin that allows the user to program
how its output voltage ramps during
start-up and shutdown. During startup, if the voltage on the Track pin is
less than 0.57V, the feedback voltage
regulates to this tracking voltage, thus
programming the output voltage to
follow along. Inductor current is not
allowed to reverse during tracking,
ensuring monotonic voltage rise. When
the tracking voltage exceeds 0.57V,
tracking is disabled and the feedback
voltage regulates to the internal reference voltage (0.596V). In other words,
output voltage is controlled by the
Track voltage until the output is in
regulation.
Taking the LTC3415’s Run pin
to below 1.5V would normally shut
down the part, but if the output also
needs tracking during shutdown,
then the LTC3415 must remain active even if the Run pin is low. So,
during shutdown, if Track is 0.5V or
more below SVIN, then even if Run is
low, the LTC3415 will not shutdown
until Track has fallen below 0.18V,
thus allowing the output to properly
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L DESIGN FEATURES
0°
180°
90°
CLKIN CLKOUT
+90°
PHMODE
CLKIN CLKOUT
+90°
CLKIN CLKOUT
+90°
PHMODE
PHMODE
CLKIN CLKOUT
PHASE 4
PHASE 7
PHASE 10
120°
210°
300°
(420°)
60°
+90°
PHMODE
CLKIN CLKOUT
PHMODE
PHASE 5
PHASE 8
CLKIN CLKOUT
PHMODE
PHASE 11
+120°
PHASE 3
+90°
PHMODE
PHASE 2
150°
CLKIN CLKOUT
PHMODE
CLKIN CLKOUT
+90°
CLKIN CLKOUT
+90°
PHMODE
PHASE 6
330°
240°
CLKIN CLKOUT
+90°
+120°
PHMODE
PHASE 1
CLKIN CLKOUT
(390°)
30°
270°
+90°
CLKIN CLKOUT
PHMODE
PHMODE
PHASE 9
PHASE 12
Figure 6. Cascaded, 12-phase operation makes for efficient high current load sharing
track the master voltage during its
ramp-down. For applications that
do not require tracking or externally
controlled soft start, simply tie the
Track pin to SVIN.
Output Margining
For convenient and accurate system
stress test on the LTC3415’s output,
the user can program the LTC3415’s
output to ±5, ±10, and ±15% of its
nominal operational voltage. The MGN
pin, when left floating, forces normal
operation. When MGN is tied to GND, it
forces negative margining, in which the
output voltage is below the regulation
point. When MGN is tied to SVIN, then
the output voltage is forced to above the
regulation point. The amount of output voltage margining is determined
by the BSEL pin. When BSEL is low,
it’s 5%. When BSEL is high, it’s 10%.
When BSEL is left floating, margin
percentage is 15%. To prevent system
glitches while margining, the internal
output overvoltage and undervoltage
comparators are disabled and thus
PGOOD remains pulled high by the
external resistor.
Multiphase Operation
For output loads that demand more
than 7A of current, multiple LTC3415s
can be cascaded to run out of phase
while equally sharing output load
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current. Figure 4 shows a dual-phase
single output 3.3V to 1.8V 14A application using two LTC3415s running
180° out of phase with respect to each
other. Figure 5 shows the combined efficiency of 1-phase, 2-phase, 3-phase,
4-phase, and 6-phase operation.
The CLKIN pin allows the LTC3415
to synchronize to an external frequency
(between 0.75Mhz and 2.25Mhz) and
the internal phase-lock-loop allows
the LTC3415 to lock on to CLKIN’s
phase as well. The CLKOUT signal
can be connected to the CLKIN pin of
the following LTC3415 stage to line up
both the frequency and the phase of
the entire system. Tying the PHMODE
pin to SVIN, SGND, or SVIN/2 (floating)
respectively generates a phase difference (between CLKIN and CLKOUT) of
180°, 120°, or 90°, which respectively
corresponds to 2-phase, 3-phase, or
4-phase operation. A total of 12 phases
can be programmed by setting the
PHMODE pin of each phase to different
levels. For instance, a slave stage that’s
180° out of phase from the master can
generate a 120° CLKOUT signal that’s
300° (PHMODE = SGND) away from
the master for the next stage, which
then can generate a CLKOUT signal
that’s 420°, or 60° (PHMODE = SVIN/2)
away from the master for its following
stage. See Figure 6.
A multiphase power supply significantly reduces the amount of ripple
current in both the input and output
capacitors. The RMS input ripple current is divided by, and the effective
ripple frequency is multiplied by, the
number of phases used (assuming
that the input voltage is greater than
the number of phases used times the
output voltage). The output ripple
amplitude is also reduced by, and the
effective ripple frequency is increased
by, the number of phases used.
Output Current Sharing
When multiple LTC3415s are cascaded
to drive a common load, accurate
output current sharing is essential
to achieve optimal performance and
efficiency. Otherwise, if one stage is
delivering more current than another,
then the temperature between the
two stages is different, and that can
translate into higher switch RDS(ON),
lower efficiency, and higher RMS
ripple. Each LTC3415 has a trimmed
peak current limit such that when the
ITH pins of multiple LTC3415s are tied
together, the amount of output current delivered from each LTC3415 is
nearly the same.
Different ground potentials among
LTC3415 stages, caused by physical
distances and switching noises, could
cause an offset to the absolute ITH value
Linear Technology Magazine • March 2006
DESIGN FEATURES L
Internal/External
ITH Compensation
During single phase operation, the
user can simplify the compensation
of the internal error amplifier loop
(on the ITH pin) by tying it to SVIN to
enable internal compensation, which
connects an internal 50k resistor in
series with a 50pF cap to the internal
ITH compensation point. This is a tradeoff for simplicity and ease of use at the
expense of OPTI-LOOP® optimization,
where external ITH components can
be selected to optimize the loop transient response with minimum output
capacitance.
In multi-phase operation where
all the ITH pins of each phase are tied
together to achieve accurate load
sharing, internal ITH compensation is
disabled and external compensation
components need to be properly selected for optimal transient response
and stable operation.
Master/Slave Operation
In multiphase single-output operation, the user has the option to run
in multi-master mode where all the
FB, ITH, and output pins of the stages
are tied to each other. All the error
amplifiers are effectively operating in
parallel and the total transconductance (gm) of the system is increased
by the number of stages. The ITH
value, which dictates how much
current is delivered to the load from
each stage, is averaged and smoothed
out by the external ITH compensation
components. Nevertheless, in certain
applications, the resulting higher gm
from multiple 3415s can make the
system loop harder to compensate;
in this case, the user can choose an
alternative mode of operation.
Linear Technology Magazine • March 2006
–10
–20
–30
VIN = 5V
VOUT = 1.8V
RBW = 100Hz
–10
–14.1dBm
–20
–30
–40
AMPLITUDE (dBm)
AMPLITUDE (dBm)
seen by each stage. To ensure that
the ground level doesn’t affect the ITH
value, the LTC3415 uses a differential amplifier that takes as input not
just the ITH pin, but also the ITHM pin,
which doesn’t connect to any other
circuitry except for the ITH differential
amplifier. Therefore, the ITHM pins of
all the LTC3415 stages should be tied
together and then connected to the
SGND pin at only one point.
–50
–60
–70
–80
–50
–60
–70
–80
–90
–100
–100
a. The LTC3415’s output noise spectrum
analysis in free-running constant frequency
operation
–37.3dBm
–40
–90
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
FREQUENCY (MHz)
VIN = 5V
VOUT = 1.8V
RBW = 100Hz
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
FREQUENCY (MHz)
b. The LTC3415’s output noise spectrum
analysis in spread spectrum operation
Figure 7. Spread spectrum operation reduces EMI peaks
by spreading the EMI energy over a range of frequencies
The second mode of operation is
single-master operation where only
the error amplifier of the master stage
is used while the error amplifiers of
the other stages (slaves) are disabled.
The slave’s error amplifier is disabled
by tying its FB pin to SVIN, which
also disables the internal overvoltage
comparator and power-good indicator.
The master’s error amplifier senses the
output through its FB pin and drives
the ITH pins of all the stages. To account for ground voltage differences
among the stages, the user should tie
all ITHM pins together and then tie it to
the master’s signal ground. As a result,
not only is it easier to do loop compensation, this single-master operation
should also provide for more accurate
current sharing among stages because
it prevents the error amplifer’s output
(ITH) of each stage from interfering with
that of another stage.
Spread Spectrum Operation
Switching r egulators can be
troublesome where electromagnetic interference (EMI) is a concern.
Switching regulators operate on a
cycle-by-cycle basis to transfer power
to an output. In most cases, the frequency of operation is fixed or is a
constant based on the output load.
This method of conversion creates
large components of noise at the frequency of operation (fundamental) and
multiples of the operating frequency
(harmonics).
To reduce this noise, the LTC3415
can run in spread spectrum operation by tying the CLKIN pin to SVIN.
In spread spectrum operation, the
LTC3415’s internal oscillator is designed to produce a clock pulse whose
period is random on a cycle-by-cycle
basis but fixed between 60% and 140%
of the nominal frequency. This has
the benefit of spreading the switching
noise over a range of frequencies, thus
significantly reducing the peak noise.
Figures 7 and 8 show how the spread
spectrum feature of the LTC3415
significantly reduces the peak harmonic noise vs free-running constant
frequency operation. Spread spectrum
operation is disabled if CLKIN is tied to
ground or if it’s driven by an external
frequency synchronization signal.
Conclusion
With its many operational features
and compact total solution size, the
LTC3415 is an ideal fit for today’s
point-of-load power supplies. It also
has the advantage of simple upgradeability: if the load requirement of a
power supply increases, no need to
dread the redesign of the whole system,
just stack on another LTC3415 and
keep going. L
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the latest information
on LTC products,
visit
www.linear.com
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