Dual, Fast, Step-Down Controller’s External Reference Input Enables Dynamic Voltage Scaling from 0.4V to 5.5V and 0.3% Total Combined Regulation Accuracy

design features
Dual, Fast, Step-Down Controller’s External Reference Input
Enables Dynamic Voltage Scaling from 0.4V to 5.5V and
0.3% Total Combined Regulation Accuracy
Shuo Chen and Terry Groom
Low voltage, high current
systems require accurate
differential regulation. It is
not uncommon for power
supply rails at or below
0.9V to demand 25A or
more, with fast transients
that look like intermittent
electrical shorts to the
power supply rail. Such
systems typically require
power supply regulation
accuracy of less than 1%
regulated DC and 3% in the
face of input transients.
Increasingly, core processors and other
large scale digital ICs (such as ASICs and
FPGAs) require dynamic voltage scaling—either multiple fixed levels, or a
continuously adjusted reference voltage
using a servo loop—to deliver power
based on the processor demand. The
goal is that the system can keep the
applied power supply at the minimum
voltage necessary for proper operation
based on processing demand to conserve
Figure 1. Channel 2 of the LTC3838-2
regulates to an external reference;
channel 1 to an internal reference.
VOUT1 and VOUT2 allow remote grounds
up to ±500mV and ±200mV, respectively.
VREF2 is also differentially sensed, but
no separate pin for the remote ground
of external reference is required.
PART NUMBER
LTC3838-1/-2 Ch.1
and LTC3838-1 Ch.2
LTC3838-2 Ch.2
(e.g., with ±0.1%
Linear Technology
Voltage References)
REFERENCE
VOLTAGE
OUTPUT
VOLTAGE
TOTAL COMBINED ACCURACY
(GROUND, LINE, LOAD & TEMP)*
0.6V Internal
0.6V to 5.5V
0.6V External
0.6V to 5.5V
< ±0.67% (-40ºC ≤ T A ≤125ºC)
1.5V External
1.5V to 5.5V
< ±0.4% (-40ºC ≤ T A ≤125ºC)
2.5V External
2.5V to 5.5V
< ±0.3%
< ±0.75% (0ºC ≤ T A ≤85ºC)
< ±1% (-40ºC ≤ TA ≤125ºC)
*external resistor divider error not included
Table 1. Output voltage regulation accuracy over remote power ground deviation (up to ±200mV), input voltage
(4.5V to 38V), output current, and temperature
energy. One example is LSI’s adaptive
voltage scaling & optimization (AVSO).
stage. The problem is that soft-start and
many common fault control features such
a overvoltage protection might be sacrificed, depending on the technique used.
The LTC3838-2 is designed to meet
the extreme accuracy requirements
through precision differential output
sensing, and offer dynamic output
voltage scaling using the differential
external reference voltage input.
DUAL DIFFERENTIAL V OUT
ACCURACY THAT MATTERS
To obtain superior regulation accuracy,
power supply designers sometimes bypass
a controller’s internal error amplifier and
instead use a discrete precision reference
and external op amps to control the power
The LTC3838-2 avoids this trade-off by
allowing the use of an external reference
for accuracy while preserving valuable
fault and protection features. With a precision voltage reference (such as LTC6652)
from Linear Technology, or a DAC for programmability, the channel 2 output of the
LTC3838-2 can be tightly regulated from
0.4V to 5.5V in applications with currents
up to 25A per channel. At a very low 0.6V,
the LTC3838-2 is able to achieve a total
VOUT2+
VOUT1+
RFB2
COUT1
LTC3838-2
RDFB2
VOUTSENSE1+ VDFB2+
RDFB1
RFB1
VOUT1–
REMOTELY-SENSED
POWER GROUND 1,
±500mV MAX vs SGND
VOUTSENSE1– VDFB2–
EXTVREF2
VREF2+
+
–
VREF2–
RDFB3 =
RDFB1//RDFB2
TO PROGRAM VOUT2 = VREF2,
COUT2 REMOVE RDFB1AND USE
RDFB3 = RDFB2
VOUT2–
REMOTELY SENSED
POWER GROUND 2,
±200mV MAX vs SGND
REMOTELY SENSED
EXTERNAL REFERENCE GROUND
April 2013 : LT Journal of Analog Innovation | 15
For differential external reference sensing, the LTC3838-2 has only one pin for
external reference input. Channel 2 features a unique feedback amplifier configuration,
which eliminates the need for a separate pin to sense the external reference’s
remote ground. Instead, one additional resistor, equal to the parallel of the two
feedback resistors, is used to connect to the remote ground externally.
combined accuracy of ±4mV, or ±0.67%,
over all operating conditions including line, load, extreme temperature and
remote ground deviation up to ±200mV.
TRACKING DYNAMIC DIFFERENTIAL
EXTERNAL REFERENCE
by scaling feedback with respect to a
fixed lower reference voltage, where
the percentage error does not change.
For example, with an external reference of 2.5V, the total relative tolerance
is less than ±0.3%. The LTC3838-2’s dual
channels can be configured to singleoutput applications using channel 2’s
external reference at such accuracy.
Relative accuracy improves as the reference increases because the absolute
error is a smaller percentage out of a
larger reference voltage. This contrasts
with programming the output voltage
For differential external reference sensing, the LTC3838-2 has only one pin for
external reference input. Channel 2
features a unique feedback amplifier
configuration, which eliminates the need
for a separate pin to sense the external
reference’s remote ground. Instead, one
Figure 2. A LTC3838-2, 300kHz, 2-phase single-output step-down converter with inductor-DCR sense. This application converts a 4.5V to 14V input to a dynamic 0.4V to
2.5V output at 50A.
VIN
4.5V TO 14V
+
CIN2
22µF
×4
CIN1
180µF
2.2Ω
1µF
LTspice IV
0.1µF
4.02k
L1
0.4µH
MT1
+
COUT2
330µF
×2
SENSE1–
SENSE2–
SENSE1+
SENSE2+
BOOST1
BOOST2
0.1µF
TG1
DB2
4.7µF
MB1
DRVCC2
EXTVCC
DRVCC1
INTVCC
BG1
BG2
EXTVREF2
VOUTSENSE1+
VDFB2+
VREF2+
0.4V TO 2.5V 10k
10k
VREF2–
VOUTSENSE1–
PGOOD1
VDFB2–
PGOOD2
TRACK/SS1 TRACK/SS2
ITH1
16 | April 2013 : LT Journal of Analog Innovation
137k
DTR1
RT
SGND
RUN1
L2
0.4µH
VOUT
0.4V TO 2.5V
50A
SW2
PGND
CIN1: SANYO 16SVP180MX
CIN2: MURATA GRM32ER61C226KE20L
COUT1, COUT4: MURATA GRM31CR60J107ME39L
COUT2, COUT3: SANYO 2R5TPE330M9
DB1, DB2: CENTRAL SEMI CMDSH-4ETR
L1, L2: VISHAY IHLP5050FDERR40M01
MT1, MT2: INFINEON BSC050NE2LS
MB1, MB2: INFINEON BSC010NE2LS
4.02k
MT2
TG2
SW1
1µF
16.2k
0.1µF
DB1
2.2Ω
COUT1
100µF
×2
LTC3838-2
0.1µF
16.2k
circuits.linear.com/624
VIN
ITH2
DTR2
PHASMD
MODE/PLLIN
CLKOUT
RUN2
100k
PGOOD
0.01µF
100pF
1000pF
7.5k
MB2
COUT3 +
330µF
×2
COUT4
100µF
×2
design features
In addition to regulation accuracy, the LTC3838-2 offers widebandwidth tracking to a dynamic external reference. Tracking
bandwidth is important in applications like dynamic voltage
scaling because bandwidth determines how quickly the supply
can respond to changes to the programmed external reference.
additional resistor equal to the parallel of
the two feedback resistors is used to connect to the remote ground externally. See
the LTC3838-2 data sheet for an explanation of how this configuration works.
LT3838-1 CONTROLLER: INTERNAL
REFERENCE ON BOTH CHANNELS
quickly the supply can respond to changes
to the programmed external reference.
The LTC3838-1 shares the same functions
as LTC3838-2, except channel 2 of the
LTC3838-1 uses a 0.6V internal reference.
Like its predecessors, the LTC3838 and
LTC3839, both the LTC3838-1 and -2 use the
controlled on-time, valley current mode
architecture, which offers superior regulation during fast load transients without
the typical switching period response delay
of fixed frequency controllers, while still
capable of constant frequency switching
locked to an external 200kHz to 2MHz
clock. They retain all features of the
LTC3838, including the proprietary detect
transient release (DTR), which improves
the transient performance in low output
voltage applications. Like the LTC3838,
both LTC3838-1 and -2 include a full set
of popular features, such as an external
VCC power pin, RSENSE or inductor-DCR current sensing, selectable light load operating
modes, overvoltage protection and current
Figure 3 shows Bode plots from a 350kHz
LTC3838-2 step-down converter compensated to an aggressive bandwidth close
to 1/3 of the switching frequency without sacrificing stability. This allows the
LTC3838-2 to track an external sine wave
of 3.5kHz or 1/100 switching frequency
at full power, without any noticeable
distortion even at the sine wave’s very
high bandwidth start and stop instants
(Figure 4). Careful attention should be
paid to the bandwidth requirements for
any dynamic system. The wide-bandwidth
external-reference-tacking capability, in
addition to high speed transient performance, makes the LTC3838-2 ideally suited
for the most dynamic supply applications.
Figure 2 shows a typical LTC3838-2
application with external reference
input. This 2-phase converter is capable
of producing 50A over a wide ranging
output from 0.4V to 2.5V. For example,
at 1.5V this application can achieve 0.4%
total combined accuracy for all operating conditions. The high accuracy and
superior transient performance make
the LTC3838-2 well suited for the most
demanding processor applications.
In addition to regulation accuracy, the
LTC3838-2 offers wide-bandwidth tracking to a dynamic external reference.
Tracking bandwidth is important in
applications like dynamic voltage scaling because bandwidth determines how
Figure 3. Loop gain and closed-loop Bode plots taken with an OMICRON Lab network analyzer
on VOUT2 of a 350kHz LTC3838-2 step-down converter with external reference (EXTVREF2).
60
90
50
75
PHASE
45
30
10
15
0
–15
–20
–30
10
PHASE
GAIN
VOUT2
1V/DIV
–5
–60
–10
–120
0
–10
–30
0
0
PHASE (deg)
GAIN (dB)
60
20
GAIN
EXTVREF2
1V/DIV
PHASE (deg)
30
60
5
GAIN (dB)
40
Figure 4. The LTC3838-2 tracks a 1V peak-to-peak,
3.5kHz sine wave external reference.
100
FREQUENCY (kHz)
–45
1000
–15
1
10
100
FREQUENCY (kHz)
–180
1000
SW2
10V/DIV
PGOOD2
5V/DIV
100µs/DIV
April 2013 : LT Journal of Analog Innovation | 17
The LTC3838-1/-2 is the ideal choice for power in
applications requiring fast transient performance,
dual accurate differential output regulation, and
external references for increased VOUT accuracy
and programmability down to 0.4V.
limit foldback, soft-start/rail tracking, and
PGOOD and RUN pins for each channel.
In addition to the differential remote output sensing on both channels, a significant
improvement of the LTC3838-1/-2 over the
original LTC3838 is the maximum current
sense threshold voltage (i.e., current limit)
accuracy. Unlike the LTC3838, which has
a continuously variable and two fixed
current limit ranges (VRNG), the LTC3838-2
has a fixed VRNG of 30mV (typical) and
its tolerance over temperature is ±20%,
POWER VIN
3.3V TO 14V
which is much improved. The LTC3838-1
has the same 30mV and an additional
60mV (typical) VRNG setting whose tolerance is also significantly tighter. Refer to
Table 2 for the comparison on the current
limit tolerances and VRNG controls of the
LTC3838-series 2-channel controllers.
VBIAS SUPPLY
5V TO 5.5V
+
CIN1
180µF
LTspice IV
circuits.linear.com/625
CIN2
22µF
×4
2.2Ω
1µF
VIN
L1
0.47µH
M1
SENSE2–
SENSE1+
SENSE2+
BOOST1
BOOST2
TG1
COUT2
330µF
×2
4.7µF
DRVCC1
INTVCC
DRVCC2
EXTVCC
0.01µF
VOUTSENSE1–
VDFB2–
PGOOD2
470pF
137k
ITH1
DTR1
VRNG
RT
SGND
RUN1
ITH2
DTR2
PHASMD
MODE/PLLIN
CLKOUT
RUN2
+
COUT4
100µF
×2
20k //10k
SGND
PGOOD2
100k
0.01µF
TRACK/SS1 TRACK/SS2
47pF
23.2k
20k
PGOOD1
VOUT2
0.9V
20A
10k
VDFB2+
10k
PGOOD1
COUT3
330µF
×2
BG2
VOUTSENSE1+
100k
RS2
L2
0.47µH 0.0015Ω
SW2
BG1
10k
18 | April 2013 : LT Journal of Analog Innovation
M2
DB2
SW1
1µF
100Ω
0.1µF
TG2
PGND
Figure 5. When an external 5V rail is commonly available
to bias up the controller, this LTC3838-1 application
converts a dynamic 3.3V to 14V power input to 20A dual
outputs of 1.2V and 0.9V.
100Ω
DB1
2.2Ω
+
SENSE1–
1nF
0.1µF
RS1
0.0015Ω
LTC3838-1
1nF
100Ω
COUT1
100µF
×2
Figure 5 shows the VIN pin connected
via diode-OR to the VBIAS 5V rail and to
power VIN, 3.3V–14V, rail. This allows
the power VIN rail to dynamically switch
between a higher voltage and a minimum of 3.3V. When operating with the
power VIN supply below 5.5V, this application requires the VBIAS supply to be
present at EXTVCC in order to maintain
The LTC3838-1/-2 controllers require a
minimum VIN pin voltage of 4.5V, but
this does not limit the power input
to 4.5V. For example, many digital
systems have an available regulated
100Ω
VOUT1
1.2V
20A
5V rail, which can be used to bias the
VIN pin and gate drivers, and to efficiently step down inputs less than 4.5V.
47pF
470pF
17.4k
CIN1: SANYO 16SVP180MX
CIN2: MURATA GRM32ER61C226KE20L
COUT1, COUT4: MURATA GRM31CR60J107ME39L
COUT2, COUT3: SANYO 2R5TPE330M9
DB1, DB2: CENTRAL SEMI CMDSH-4ETR
L1, L2: WÜRTH 7443330047
M1, M2: INFINEON BSC0911ND
design features
Using an external reference, the LTC3838-2 can achieve
total accuracy levels as low as 0.3% under all operating
conditions. The external reference feature is designed to
accommodate dynamic voltage scaling and track fast
external reference inputs with minimum distortion.
Table 2. Maximum Current Sense Threshold Voltage Specifications and Range Controls
PART
V RNG = SGND
V RNG = INTV CC
V RNG CONTROL
V RNG PIN(s)
LTC3838 and LTC3839
21mV to 40mV
39mV to 61mV
30mV–200mV continuous & 30mV/50mV fixed
each per channel
LTC3838-1
24mV to 36mV
54mV to 69mV
30mV/60mV fixed
single
LTC3838-2
24mV to 36mV
30mV fixed only
no
DRVCC , INTVCC and VIN pin voltages
needed for the IC to function properly.
The EXTVCC supply is optional when the
power VIN supply is at or above 5.5V.
Note that the power input voltage range
of this application cannot be generalized
for other frequencies and output voltages, and each application that needs a
power input voltage different from the
VIN pin voltage should be tested individually for margin of range in which
the switching nodes (SW1, SW2) phaselock to the clock output (CLKOUT).
SUMMARY
The LTC3838-1/-2 is the ideal choice for
power in applications requiring fast
transient performance, dual accurate differential output regulation, and external
references for increased VOUT accuracy and
programmability down to 0.4V. Compared
to the original LTC3838, the LTC3838-1/
LTC3838-2 offers differential output sensing on both channels, improved current
limit accuracy, and the choice of internal/
external reference. Using an external reference, the LTC3838-2 can achieve accuracy
levels as low as 0.3% under all operating
conditions. The external reference feature is designed to accommodate dynamic voltage scaling
and track fast external reference
inputs with minimum distortion.
The LTC3838-1 and -2 are offered
in 38-pin QFN (5mm × 7mm)
packages with exposed pads for
enhanced thermal performance.
Visit www.linear.com/LTC3838-1
and /LTC3838-2 for data sheets,
demo boards, a variety of applications designs, and for more
information about how:
•a 30ns minimum on-time enables
high step-down ratios, e.g.,
from 38V to 0.8V at 350kHz
•2MHz switching frequency
enables applications with
tiny power components
For More Information…
THE LTC3838/LTC3839, PREDECESSOR TO THE
LTC3838-1/-2:
See the article:
• 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 in the
LT Journal of Analog Innovation, April 2012
(Volume 22, Number 1).
THE LTC3833 SINGLE-CHANNEL CONTROLLER
The LTC3838 series of dual controllers 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).
•25A output becomes practical at 2MHz, with 95% peak
efficiency (2V-5V VOUT). n
April 2013 : LT Journal of Analog Innovation | 19
Similar pages