December 2009 - Produce High DC/DC Step-Down Ratios in Tight Spaces with 30ns Minimum On-Time Controller in 3mm × 3mm QFN

L DESIGN FEATURES
Produce High DC/DC Step-Down
Ratios in Tight Spaces with 30ns
Minimum On-Time Controller in
3mm × 3mm QFN
by Theo Phillips
Introduction
It can be a challenge to design a DC/
DC converter that takes a high voltage
automotive or industrial power supply
down to the 1.5V or lower voltages
required by today’s microprocessors
and programmable logic chips.
To maintain efficiency and performance, designers are often forced to
create a 2-stage solution, which first
steps down to an intermediate voltage
and uses another converter to produce
the low voltage from there. 2-stage
solutions can perform well, and are
handy if the application can use the
intermediate voltage elsewhere, but
2-stage solutions always take more
space and are more costly than a single
stage solution.
Many regulators can produce high
step-down ratios in a single stage if the
switching frequency of the step-down
converter is slowed considerably. However, this option sacrifices efficiency
and requires larger, more expensive
external components, doing little to
solve the space and cost problems
incurred in 2-stage solutions.
The LTC3775 is a voltage mode
DC/DC regulator with a very low
minimum on-time of 30ns, allowing
very wide step-down ratios at high
switching frequencies without sacrificing performance. Unlike most voltage
mode controllers, the LTC3775 offers
cycle-by-cycle programmable current
limit, excellent short circuit protection
and fast transient response over a wide
input voltage range.
A 1.2V Converter Operating
from 5V–28VIN at 350kHz
The LTC3775 is ideal for generating
low output voltages from high input
voltages, a common requirement for
powering CPUs from wide-ranging
20
application providing a continuous
15A from an 5V–26V input.
Current mode controllers are often
favored for their continuous monitoring of current through the inductor or
switches, protecting these components
and the load against short circuits and
pre-biased outputs during start-up.
To avoid these difficulties, a typical
voltage mode controller requires additional circuitry to monitor current in
the power stage. The LTC3775 requires
no ancillary circuits to oversee the
entire switching cycle.
The current limit is programmed
with two resistors (as shown in the
block diagram of Figure 2), corresponding to the current measured
through the top and bottom switches
Current mode controllers
are often favored for their
continuous monitoring of
current through the inductor
or switches. While a typical
voltage mode controller
requires additional circuitry
to monitor current in the
power stage, the LTC3775
requires no ancillary
circuits to oversee the entire
switching cycle.
rails such as those found in automotive applications. Figure 1 shows an
CF
220pF
DB
RILIMT
732Ω
VIN
ILIMT
RILIMB
57.6k
CVCC
4.7µF
CSS
0.01µF
TG
ILIMB
SENSE
INTVCC
BOOST
SS
SW
FREQ
BG
FB
C2
330pF
+
VIN
5V TO 26V
CIN1
330µF
35V
QT
CB
0.1µF
L1
0.36µH
LTC3775
RSET
38.3k
RA
10k
RSENSE
0.003Ω
+
QB
PGND
COUT
470µF
2.5V
×2
VOUT
1.2V
15A
MODE/SYNC
COMP RUN/SHDN
SGND
RB
10k
R2
C1 4.7k
3.9nF
COUT : SANYO 2R5TPD470M5
DB: CMDSH4E
L1: IHLP-4040DZ-ER-R36-M11
QB: RJK0301DPB-00-J0
QT: RJK0305DPB-00-J0
Figure 1. A 1.2V, 15A converter
Linear Technology Magazine • December 2009
DESIGN FEATURES L
LTC3775
7IN
CTLIM
TURN OFF TG
ILIMT
+
–
100µA
7IN
RILIMT
TG
SENSE
10µA
ILIMB
CBLIM
EXTEND BG
t7ILIMB
–
+
RILIMB
RSENSE
QT
(OPT)
SW
+
during their respective on times. This
arrangement allows cycle-by-cycle
current limit, regardless of the duty
cycle, and ensures that the inductor
is not saturated.
In a current mode converter, the
voltage on the output of the error
amplifier controls the peak switch
current, such that the switch current
must always be monitored, allowing
the introduction of noise. This may be
most pronounced around 50% duty
cycle in some current mode designs.
Contrast this with a voltage mode
converter, where the error voltage
on VOUT is compared to a saw-tooth
ramp, which in turn controls duty
cycle; the larger the error voltage, the
longer the top switch stays on. The
LTC3775 senses current through both
MOSFETs to assure that they do not
exceed programmed limits. During
normal operation, these limits do not
come into play, and noise-free operation is assured.
A high side current limit would be of
little value if the circuit was operated
at its maximum VIN, since the bottom
switch would be on most of the time,
and nothing would protect the synchronous MOSFET. Fortunately, the
low side current limit, programmed by
ILIMB, can limit the current through
the bottom switch. Conversely, a fault
at low VIN during the on-time of the
top switch requires a high side current limit for immediate response. The
LTC3775 uses both top- and bottomside current limit circuits to provide
PGND
7OUT
BG
QB
SGND
Figure 2. The LTC3775 features high and low side programmable current limits, for cycle-by-cycle
short circuit protection.
optimum protection for the MOSFETs
and inductor.
This current limit approach is effective, as shown by the short circuit
behavior in Figure 3. A hard short
could spell disaster for an unprotected
voltage mode converter. But here, the
inductor does not saturate, and the
input rail maintains its integrity while
the output gracefully drops.
Output voltage is monitored using
an inverting summing amplifier topology, with the FB pin configured as a
virtual ground. The reference voltage
is accurate to within ±0.75% over
temperature. The LTC3775 uses a true
operational error amplifier with 80dB
of open loop gain, and a 25MHz gainbandwidth product. Feedback gain
can be tightly controlled by external
components, allowing the use of “Type
3” compensation, which provides a
phase boost at the LC double pole
frequency and significantly improves
control loop phase margin. Figure 4
shows a characteristically fast load
transient response.
The modulator consists of the
PWM generator, the output MOSFET
drivers and the external MOSFETs
themselves. The modulator gain varies linearly with the input voltage. The
line feedforward circuit compensates
for this change in gain, and provides a
constant gain from the error amplifier
output to the inductor input regardless
of input voltage.
The application in Figure 1 demands a minimum on time of just
86ns at the maximum input voltage
of 28V. Many controllers turn the top
gate on at the beginning of the clock
cycle and must wait for the response
time of the PWM comparator before
turning off the top gate. This response
time is typically around 100ns or more.
In addition, those controllers would
make the decision at a noisy interval,
VSW
20V/DIV
IL
20A/DIV
VOUT(AC)
100mV/DIV
VSW
10V/DIV
0A LOAD
IL
10A/DIV
IL
10A/DIV
VSS
1V/DIV
VOUT(AC)
RIPPLE
20mV/DIV
ILOAD
10A/DIV
VIN = 12V
VOUT = 1.2V
CSS = 0.01µF
20µs/DIV
Figure 3. Short circuit behavior for the
converter of Figure 1
Linear Technology Magazine • December 2009
VIN = 12V
5µs/DIV
VOUT = 1.2V
LOAD STEP = 0A TO 10A
MODE/SYNC = 0V
SW FREQ = 500kHz
Figure 4. Load transient response for the
converter of Figure 1
VIN = 26V
VOUT = 1.2V
NO LOAD
RSET = 38.3k
1µs/DIV
Figure 5. The converter of Figure 1
demonstrates a clean switching waveform
with a razor-thin on-time.
21
L DESIGN FEATURES
R4
69.8k
R5
10k
RILIMT
1.24k
SENSE
INTVCC
CB
0.1µF
BOOST
L1
4.7µH
SS
SW
FREQ
BG
RUN/SHDN
FB
PGND
C2
330pF
VIN
24V
CIN1
330µF
35V
QT
LTC3775
RSET
39.2k
RA
191k
TG
ILIMB
CSS
0.01µF
C3
330pF
VIN
ILIMT
RILIMB
56.2k
CVCC
4.7µF
R3
2.05k
+
DB
QB
+
COUT
68µF
16V
×2
VOUT
12V
5A
MODE/SYNC
COMP
RB
10k
SGND
R2
C1 7.68k
3.3nF
COUT : SANYO 16TQC68M
DB: CMDSH4E
L1: IHLP-4040DZ-ER-4R7-M11
QB, QT: RJK0305DPB-00-JO
Figure 6. A 12V, 5A converter operating at 500kHz from 24VIN.
because ringing persists for some time
after the top gate turns on. Thus even
though the minimum on-time could
be as low as 100ns, practical design
considerations such as noise and jitter
would require a nominal on-time of
no less than 150ns to guarantee that
there will be no pulse-skipping mode
at maximum input voltage.
The LTC3775’s leading-edge voltage mode architecture and very low
minimum on-time of 30ns makes it
practical to run with on-times as low
as 40ns, even in noisy environments.
The LTC3775’s leading edge modulation architecture turns on the top
2.5
VIN = 24V
90 VOUT = 12V
CONTINUOUS
MODE
80
SW FREQ = 500kHz
70
60
50
2.0
1.5
EFFICIENCY
40
30
POWER LOSS
20
1.0
0.5
10
0
0.01
1
0.1
LOAD CURRENT (A)
Figure 7. Efficiency for the
converter of Figure 6
22
0
10
POWER LOSS (W)
EFFICIENCY (%)
100
gate when the PWM comparator trips
and turns off the top gate when the
clock signal goes high. The switching
waveform shows no skipped pulses
and is free from erratic behavior, even
with very short on-times. Figure 5 illustrates waveforms for a 36V input,
1.2V output converter operating in
continuous conduction mode (CCM)
at 350kHz with no load. The waveform
shows constant frequency operation
and extremely low switch node jitter.
A 12V Converter Operating
from 24VIN at 500kHz
Because the LTC3775 separately
monitors the current through the top
and bottom switches, its comparators
do not need extended common mode
ranges. Some current mode converters use a sense resistor in series with
the inductor, restricting the range of
VOUT due to the common mode range
of their current comparators. Without
this restriction, the LTC3775 is useful
for powering higher output voltages,
as exemplified by the 12V converter of
Figure 6. Also, the absence of a sense
resistor means no loss in efficiency,
and no low amplitude current mode
sense signal which can be a source of
jitter in the SW node waveform.
Current sensing through the top
MOSFET can be measured across a
sense resistor for the highest possible
accuracy. To boost efficiency and save
a component, just omit the sense resistor and measure directly across the
top MOSFET. Figure 6 demonstrates
a high efficiency 12V converter that
uses this arrangement. When using
the top MOSFET RDS(ON) to measure
current, care must be taken to Kelvinconnect the VIN pin of the IC to the
drain terminal of the power MOSFET
and the SENSE pin to the source of
the MOSFET. Likewise, when a sense
resistor is used for improved current
limit accuracy, Kelvin-connect the
VIN and SENSE pins of the IC to the
positive and negative terminals of the
sense resistor, respectively.
The circuits featured here operate in
forced continuous mode, for constant
frequency operation at any load. If
higher light-load efficiency is desired,
the MODE/SYNC pin can be tied to a
voltage above 1.2V (typically INTVCC)
for pulse skippin operation. This pin
can also be the input for a sync signal,
from 250kHz to 1MHz. No external PLL
components are required for syncing.
The synchronization feature operates
within ±20% of the free-running frequency defined by RSET.
Conclusion
The LTC3775 provides unprecedented
performance for today’s demanding
high current, low voltage power supply systems. With a minimum on-time
of 30ns and a high bandwidth true
operational amplifier, the controller can operate at frequencies up to
1MHz, maintaining a very small power
supply PCB footprint and reducing
output ripple.
The LTC3775 offers numerous
features in a tiny 3mm × 3mm QFN,
including cycle-by-cycle current
limit, synchronization capability over a
±20% range, a wide input and output
operating voltage range, internal line
feed-forward compensation, a high
bandwidth operational error amplifier,
strong internal gate drivers and very
tight output voltage tolerance (±0.75%
over temperature). L
Linear Technology Magazine • December 2009
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