2MHz Dual DC/DC Controller Halves Settling Time of Load Release Transients, Features 0.67% Differential VOUT Accuracy and Is Primed for High Step-Down Ratios

design features
2MHz Dual DC/DC Controller Halves Settling Time of
Load Release Transients, Features 0.67% Differential VOUT
Accuracy and Is Primed for High Step-Down Ratios
Shuo Chen and Terry Groom
Electrical conditions once considered extreme are now
the norm. Modern electronic systems demand high
currents and very low voltages that can appear to a
DC/DC converter as an intermittent electrical short. It is
not uncommon for sub-0.9V power supply rails to demand
25A or more. In this environment, tight total differential
regulation accuracy is critical to achieve the demanding
voltage tolerances required to power core processors
and large ASICs. In addition, PCB and component
size constraints have driven up converter operating
frequencies to enable the use of smaller components.
VIN
4.5V TO 38V
+
CIN1
100µF
CIN2
10µF
×3
LTspice IV
circuits.linear.com/546
VIN
0.1µF
3.57k
+
COUT2
330µF
×2
SENSE1–
SENSE2–
SENSE1+
SENSE2+
BOOST1
BOOST2
0.1µF
TG1
MT1
3.57k
TG2
MT2
DB2
SW1
1µF
15k
0.1µF
DB1
2.2Ω
COUT1
100µF
×2
LTC3838
0.1µF
15k
VOUT1
1.2V
15A
In addition to architectural advantages,
the proprietary detect transient release
(DTR) feature improves the transient
2.2Ω
1µF
L1
0.56µH
The LTC3838 and LTC3839 controllers are
designed to meet the needs of the most
demanding low output voltage, high load
current applications. Both feature superior
differential regulation accuracy and fast
transient response. The controlled on-time
architecture yields minimum on-times as
low as 30ns and is capable of switching
frequencies from 200kHz to 2MHz with
synchronization to an external clock.
4.7µF
DRVCC1
INTVCC
DRVCC2
EXTVCC
BG2
MB2
PGND
10k
VOUTSENSE1+
PGOOD1
0.01µF
Figure 1. 4.5V to 38V input, 1.2/15A,
1.5V/15A dual output, 350MHz step-down
converter. With the output sensed directly
through a resistor divider network, the
remote sensing scheme in channel 1
mimics the traditional feedback used in
channel 2. The LTC3838’s novel remote
sensing scheme eliminates the diff amp
output pin required in other parts.
40.2k
220pF
115k
COUT3
100µF
×2
10k
VOUTSENSE1–
PGOOD1
PGOOD2
PGOOD2
ITH1
DTR1
VRNG1
RT
SGND
RUN1
ITH2
DTR2
VRNG2
100k
0.01µF
TRACK/SS1 TRACK/SS2
22pF
COUT4 +
330µF
×2
15k
VFB2
10k
100k
VOUT2
1.5V
15A
SW2
BG1
MB1
L2
0.56µH
22pF
220pF
40.2k
CIN1: NICHICON UCJ1H101MCL165
CIN2: MURATA GRM32ER71H106K
COUT2, COUT4: SANYO 2R5TPE330M9
COUT1, COUT3: MURATA GRM31CR60J107ME39L
DB1, DB2: DIODES INC. SDM10K45
L1, L2: TOKO FDA1055-R56M
MT1, MT2: INFINEON BSC093N04LSG
MB1, MB2: INFINEON BSC035N04LSG
PHASMD
MODE/PLLIN
CLKOUT
RUN2
April 2012 : LT Journal of Analog Innovation | 19
The LTC3838 and LTC3839 controllers are designed to meet
the needs of the most demanding low output voltage, high
load current applications. Both feature superior differential
regulation accuracy and fast transient response.
ILOAD
10A/DIV
PLLIN
5V/DIV
ILOAD
10A/DIV
VOUT
50mV/DIV
AC-COUPLED
VOUT
50mV/DIV
AC-COUPLED
SW1
10V/DIV
IL
10A/DIV
SW2
10V/DIV
IL
10A/DIV
0°
180°
CLKOUT
5V/DIV
5µs/DIV
LOAD STEP = 0A TO 15A
VIN = 12V
VOUT = 1.2V
FORCED CONTINUOUS MODE
5µs/DIV
LOAD RELEASE = 15A TO 0A
VIN = 12V
VOUT = 1.2V
FORCED CONTINUOUS MODE
60°
500ns/DIV
VIN = 12V
VOUT1 = 5V, VOUT2 = 3.3V
LOAD = 0A
MODE/PLLIN = 333kHz EXTERNAL CLOCK
PHASMD = GND
Figure 2. Switching frequency is constant and phase locked during steady state, but fast transient performance is achieved
by momentarily adjusting the switching frequency: increasing it on a load step; decreasing it on a load release.
performance in high step-down ratio,
low output voltage applications. This
enables the LTC3838/LTC3839 to maintain accuracy and respond to load transients faster than other topologies.
In high output current supplies applications, it is important that overall regulation accuracy is well understood. To this
end, the LTC3838 and LTC3839 internally
combine the output differential amplifier and error amplifier and specify DC,
line and load regulation output voltage
errors as a single lumped parameter.
This allows the LTC3838 and LTC3839 to
achieve a level of total differential accuracy unavailable in other controllers.
The LTC3838 and LTC3839 make high frequency switching practical in a high input
voltage, low output voltage converter.
Both devices can produce high step-down
ratios at high switching frequencies while
maintaining high efficiency at heavy load
20 | April 2012 : LT Journal of Analog Innovation
currents—previously challenging due
to greater switching losses and limitations inherent in other architectures.
For instance, in the typical 12V input
to 3.3V/25A output application shown
in Figure 3, the LTC3838/LTC3839 delivers a peak efficiency of 93% at 2MHz.
FLEXIBLE DUAL/SINGLE OUTPUT,
HIGH ACCURACY REMOTE SENSE
The LTC3838’s dual channels can be
configured for either dual- or singleoutput applications, whereas the LTC3839
is dedicated for single-output applications. Both convert an input of 4.5V to
38V (40V abs max) down to outputs of
0.6V to 5.5V (6V abs max) in applications
with per-channel currents up to 25A.
Their remotely sensed differential feedback has a voltage regulation accuracy
of ±0.67%—where the remote power
ground can deviate as much as ±500mV.
The LTC3838’s second channel can provide an independent ±1% output, or
together with the first channel, serve as
one of the PolyPhase® channels for a
single-output, higher current application.
For higher load currents, or to maximize
efficiency, multiple LTC3838s and LTC3839s
can be paralleled for up to 12-phases.
FAST TRANSIENT PERFORMANCE,
CONSTANT FREQUENCY
The LTC3838 and LTC3839 employ the new
controlled on-time, valley current mode
architecture, primed for fast transient
performance. This architecture retains the
benefits of a constant on-time controller: it responds to sudden load increases
by a sequence of consecutive on-time
pulses with a very short 90ns off-time in
between, without having to wait until the
next switching cycle like that of a fixed
frequency controller. During a load release,
the LTC3838/LTC3839 delays the turn-on of
the top FET until inductor current drops
design features
The controlled on-time architecture yields minimum on-times
as low as 30ns and makes high frequency switching practical
in a high input voltage, low output voltage converter, while
maintaining high efficiency at heavy load currents.
CIN2
22µF
×4
2.2Ω
90
LTC3839
VIN
10Ω
SENSE1–
1nF
10Ω
1nF
SENSE1+
0.1µF
SENSE2+
BOOST1
VOUT
RS1
0.004Ω
MT1
L1
0.3µH
1µF
MB1
TG2
SW1
SW2
DRVCC1
INTVCC
BG2
VOUT
3.3V
25A
8
6
80
FORCED
CONTINUOUS
MODE
70
60
RS2
0.004Ω
DISCONTINUOUS
MODE
50
LOSS
FORCED
CM
0.1
4
2
LOSS
DCM
0
100
1
10
LOAD CURRENT (A)
MB2
PGND
45.3k
100
VOUTSENSE+
10k
VOUTSENSE–
PGOOD
0.01µF
PHASMD
TRACK/SS
MODE/PLLIN
150pF
ITH
DTR
18.7k
LTspice IV
CIN1: SANYO 16SVP180MX
CIN2: MURATA GRM32ER61C226KE20L
COUT1, COUT2: MURATA GRM31CR60J107ME39L
DB1, DB2: CENTRAL CMDSH-3
L1, L2: WÜRTH 7443340030
MT1, MT2: INFINEON BSC050NE2LS
MB1, MB2: INFINEON BSC032NE2LS
VRNG
CLKOUT
SGND
RUN
10
VIN = 5V
8
FORCED
CONTINUOUS
MODE
80
6
4
70
60
50
RT
circuits.linear.com/547
90
DISCONTINUOUS
MODE
LOSS
FORCED CM
0.1
1
10
LOAD CURRENT (A)
POWER LOSS (W)
PGOOD
33.2k
L2
0.3µH
DRVCC2
EXTVCC
BG1
100k
MT2
DB2
4.7µF
10Ω
0.1µF
BOOST2
TG1
DB1
2.2Ω
COUT1
100µF
×6
10Ω
SENSE2–
10
VIN = 12V
POWER LOSS (W)
1µF
EFFICIENCY (%)
CIN1
180µF
100
+
EFFICIENCY (%)
VIN
4.5V TO 14V
2
LOSS
DCM
0
100
Figure 3. A 2MHz, 3.3V/25A step-down converter. The LTC3838/LTC3839 can operate at switching frequencies above the AM radio band (fSW > 1.8MHz). The high
switching frequency permits the use of inductors of very small footprint, so that the entire circuit can fit within a 0.9in2 area with both sides populated. The peak
efficiency is 95%, and full load efficiency well above 90% at 25A, even at a frequency of 2MHz.
to desired value, preventing overcharging
the output capacitor. Once the transient
condition subsides, the switching frequency quickly returns to the programmed
nominal or external clock frequency.
Meanwhile, the on-time is adjusted
(hence controlled on-time) so that the
switching frequency is constant during
steady-state operation, synchronized to
its internal programmable or an external
clock, to mimic a fixed frequency controller with predictable switching noise.
HIGH AND WIDE STEP-DOWN RATIO,
SWITCHING FREQUENCY
The LTC3838/LTC3839’s 30ns minimum
on-time (60ns effective on-time with
dead-time delays) enables low duty cycles
for high VIN to low VOUT applications,
even while the part operates at high
frequency. The 90ns minimum off-time
helps achieve high duty cycle operation
and avoid output dropout when VIN is
only slightly above the regulated VOUT.
The LTC3838 and LTC3839 are capable of
a full decade programmable switching
frequency from 200kHz to 2MHz. They
can be synchronized to external clocks
of ±30% of the programmed frequency.
April 2012 : LT Journal of Analog Innovation | 21
In addition to the LTC3838/LTC3839’s architectural advantages,
the proprietary detect transient release (DTR) feature
improves the transient performance in low output voltage
applications. This enables these parts to maintain accuracy
and respond to load transients faster than other topologies.
LTC3838/LTC3839
EA
VREF
VFB
SW
5V/DIV
INTVCC
1/2 INTVCC
+
–
ITH
+
–
DTR
LOAD
RELEASE
DETECTION
TO LOGIC
CONTROL
DTR
1V/DIV
CITH2
(OPTIONAL)
BOTTOM MOSFET GATE
TURNS BACK ON, INDUCTOR
CURRENT (IL) GOES NEGATIVE
IL
10A/DIV
INTVCC
CITH1
BG
5V/DIV
RITH2
RITH1
DTR DETECTS LOAD RELEASE,
TURNS OFF THE BOTTOM MOSFET GATE
FOR FASTER INDUCTOR CURRENT (IL) DECAY
5µs/DIV
Figure 4. Transient detection is done through the detect-transient (DTR) pin, which is DC-biased slightly above ½ INTVCC, and AC-coupled to ITH pin through the
compensation capacitor CITH1. The equivalent compensation resistance RITH = RITH1 || RITH2 .
NOVEL TRANSIENT DETECTION
REDUCES LOAD-RELEASE VOUT
OVERSHOOT
As the output voltage becomes lower
and the VIN -to-VOUT step-down ratio
increases, a major challenge is to limit
the overshoot in VOUT during a fast
load current drop. An innovative feature of the LTC3838/LTC3839 is to detect
“load-release” transients indirectly by
monitoring the ITH negative slew rate.
The detection is done through the detecttransient (DTR) pin that is coupled to
ITH pin through the compensation capacitor. At steady state, the DTR pin remains
slightly higher than the detection threshold (half of the voltage on INTVCC pin)
with a voltage divider of the compensation resistors from INTVCC to SGND.
In the event of a sudden drop of load
current, the output voltage overshoots
and ITH slews down quickly. If the
DTR pin drops below half of INTVCC , the
22 | April 2012 : LT Journal of Analog Innovation
LTC3838/LTC3839 temporarily turns off the
bottom MOSFET, and the inductor current flows through the body diode of the
bottom MOSFET. This increases the reverse
voltage drop across the inductor, allowing
the inductor current to drop to zero faster,
lowering the VOUT overshoot by reducing
overcharging of the output capacitor.
Once the inductor current reaches
zero, the bottom MOSFET turns back
on to pull the inductor current to
negative, discharging the output
capacitor to recover regulation.
Figure 5. Load-release detect transient (DTR) feature significantly reduces VOUT overshoot and time to recover
regulation. (Shades are obtained with infinite persistence on oscilloscope triggered at load release steps.)
VSW
3V/DIV
VSW
3V/DIV
VOUT
50mV/DIV
AC-COUPLED
VOUT
50mV/DIV
AC-COUPLED
ITH
1V/DIV
ITH
1V/DIV
IL
10A/DIV
IL
10A/DIV
5µs/DIV
FIGURE 1 CIRCUIT, CHANNEL 1 MODIFIED:
• RFB2 = 0Ω, VRNG2 = SGND, CITH1 = 120pF, CITH2 = 0pF,
• FROM DTR1 PIN: RITH1/2 = 46.4k TO SGND, 42.2k TO INTVCC
VIN = 5V, LOAD RELEASE = 15A TO 5A, VOUT = 0.6V
5µs/DIV
• CONNECTION FROM RITH1/2 AND CITH1
TO DTR1 PIN REMOVED
• DTR1 PIN TIED TO INTVCC
design features
The LTC3838 and LTC3839 are
based on and have all features of the
single-channel controller LTC3833.
For a full discussion of the features
shared with LTC3833, refer to the cover
article, “Fast, Accurate Step-Down DC/
DC Controller Converts 24V Directly
to 1.8V at 2MHz” in the LT Journal
of Analog Innovation, October 2011
(Volume 21 Number 3). Download
at cds.linear.com/docs/LT%20
Journal/LTJournal-V21N3-2011-10.pdf
For More Information
VIN
4.5V TO 14V
+
CIN2
22µF
×4
CIN1
180µF
2.2Ω
1µF
VIN
LTC3839
SENSE1–
SENSE2–
SENSE1+
SENSE2+
BOOST1
BOOST2
0.1µF
0.1µF
2.55k
L1
0.33µH
VOUT
1.2V
50A
MT1
TG1
+
COUT2
330µF
×2
4.7µF
MB1
BG2
PGOOD
0.01µF
The LTC3838 and LTC3839 are high performance, feature-rich, 2-phase, synchronous
step-down DC/DC controllers that excel
at meeting the performance demands of
high current, low voltage loads, in either
dual or single output applications.
Their controlled on-time architecture
retains the fast response and low on-time
of traditional constant on-time controllers, and allows for constant frequency
and external clock synchronization.
Other unique features include
novel remote output sensing, which
VOUTSENSE–
PGOOD
PHASMD
TRACK/SS
47pF
CONCLUSION
MB2
COUT3 +
330µF
×2
COUT4
100µF
×2
VOUTSENSE+
10k
47.5k
VOUT
DRVCC2
EXTVCC
PGND
10k
Figure 6. The LTC3839 in a single 1.2V/50A
output, 2-phase, 300kHz, DCR sense, step-down
converter, with the detect transient load-release
(DTR) feature enabled for VOUT overshoot
reduction. The LTC3838 can also be used here.
The LTC3838/LTC3839 is ideal for powering low
voltage, high current, fast slew rate loads such as
with a microprocessor.
L2
0.33µH
SW2
DRVCC1
INTVCC
BG1
100k
2.55k
MT2
DB2
SW1
1µF
0.1µF
TG2
DB1
2.2Ω
COUT1
100µF
×2
0.1µF
470pF
41.2k
137k
ITH
MODE/PLLIN
DTR
VRNG
CIN1: SANYO 16SVP180MX
CIN2: MURATA GRM32ER61C226KE20L
COUT1, COUT4: MURATA GRM31CR60J107ME39L
COUT2, COUT3: SANYO 2R5TPE330M9
DB1, DB2: CENTRAL SEMI CMDSH-4ETR
L1, L2: VISHAY IHLP5050CEERR33M01
MT1, MT2: INFINEON BSC050NE2LS
MB1, MB2: INFINEON BSC010NE2LS
CLKOUT
RT
SGND
RUN
allows for a ±500mV remote ground,
and load-release transient detection for overshoot reduction.
In addition, LTC3838 and LTC3839
include popular features, such as:
•external VCC power pin for loss
reduction in the controller
•continuously programmable range
of current limits for flexibility with
either RSENSE or inductor DCR sensing
•selectable light load operating modes:
discontinuous operation (similar to
Burst Mode® operation) for higher
efficiency, or forced continuous
operation for constant frequency
•overvoltage protection and
current limit foldback
•soft-start/rail tracking, PGOOD,
and RUN pins for each output.
The LTC3838 is offered in 38-pin
QFN (5mm × 7mm) and TSSOP packages. The LTC3839 is offered in a 32-pin
QFN (5mm × 5mm). All packages
have exposed pads for enhanced
thermal performance. n
April 2012 : LT Journal of Analog Innovation | 23
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