April 2010 - 6mm × 6mm DC/DC Controller for High Current DCR Sensing Applications

6mm × 6mm DC/DC Controller for High Current
DCR Sensing Applications
Eric Gu, Theo Phillips, Mike Shriver and Kerry Holliday
The LTC3855 is a versatile 2-phase synchronous buck controller IC with on-chip drivers,
remote output voltage sensing and inductor temperature sensing. These features are
ideal for high current applications where cycle-by-cycle current is measured across
the inductor (DCR sensing). Either channel is suitable for inputs up to 38V and outputs
up to 12.5V, further increasing the controller’s versatility. The LTC3855 is based on
the popular LTC3850, described in the October 2007 issue of Linear Technology.
MONITORING THE TEMPERATURE
“lossless” method is less accurate than
using a sense resistor, in large part because
as the inductor heats up, its resistance
increases with a temperature coefficient of resistivity (TCR) of 3930ppm/°C.
Therefore as the temperature rises, the
current limit decreases. When DCR sensing
When current is sensed at the inductor,
either a sense resistor is placed in series
with the inductor, or an R-C network
across the inductor is used to infer the
current information across the inductor’s DC resistance (DCR sensing). This
is used, the current limit for the LTC3855
is determined by the peak sense voltage as
measured across the inductor’s DCR. The
LTC3855 includes a temperature sensing
scheme designed to compensate for the
TCR of copper by effectively raising the
peak sense voltage at high temperature.
Figure 1. A 1.2V, 50A, 2-phase converter. The two channels operate 180º out-of-phase to minimize output ripple and component sizes.
10k
73.2k
22.1k
22.1k
1%
TG1
ITH1
BOOST1
VFB1
PGND1
SGND
EXTVCC
100k
TG2
SW2
20k
1%
PGOOD
2.2Ω
M2
RJK0330DPB
×2
4.7µF
1µF
COUT1
100µF
6.3V
×4
0.1µF
CMDSH-3
M3
RJK0305DPB
×2
M4
RJK0330DPB
×2
L2
0.33µH
3.92k
30.1k
L1, L2: VISHAY IHLP5050FD-01, 0.33µH
COUT1: MURATA GRM31CR60J107ME39L
COUT2: SANYO 2R5TPE330M9
RNTC: MURATA NCP18WB473J03RB
28 | April 2010 : LT Journal of Analog Innovation
VIN
4.5V TO
14V
270µF
16V
3.92k
BOOST2
NC
PGOOD2
PGOOD1
DIFFP
ILIM2
PGND2
ILIM1
BG2
SENSE2–
RUN2
SENSE2+
DIFFOUT
3.92k
0.1µF
+
INTVCC
TK/SS2
DIFFN
20k
1%
30.1k
L1
0.33µH
CMDSH-3
VIN
LTC3855
ITH2
2200pF
0.1µF
BG1
VFB2
220pF
M1
RJK0305DPB
×2
SW1
CLKOUT
PHSASMD
FREQ
ITEMP2
RUN1
ITEMP1
MODE/PLLIN
TK/SS1
0.1µF
22µF
×4
300kHz
SENSE1+
RNTC
47k
(PLACE BETWEEN L1 & L2)
0.1µF
SENSE1–
10k
1%
+
COUT2
330µF
2.5V
×4
VOUT
1.2V
50A
design ideas
80
90
70
IL1
10A/DIV
65
60
IL2
10A/DIV
55
0
20
40
60
80 100
INDUCTOR TEMP (°C)
120
140
85
80
75
ILOAD
20A/DIV
50
45
EFFICIENCY (%)
DC CURRENT LIMIT (A)
75
95
VOUT
(AC COUPLED)
100mV/DIV
COMPENSATED
UNCOMPENSATED
IOUT (MAX)
VIN = 12V
ILOAD = 30A–50A
50µs/DIV
70
MODE = CCM
fSW = 300kHz
VIN = 12V
0
10
40
20
30
LOAD CURRENT (A)
50
60
Figure 2. The converter of Figure 1 can deliver the
current whether hot or cold, with its calculated
worst-case current limit remaining above the target
50A, even well above room temperature.
Figure 3. VOUT is stable in the face of a of 30A to
50A load step for the converter of Figure 1, and the
inductor current sharing is fast and precise.
Figure 4. Efficiency for the converter of Figure 1.
DIFFERENTIAL SENSING
resistance for linearization), the current limit can be maintained above
the nominal operating current, even
at elevated temperatures (Figure 2).
OTHER IMPORTANT FEATURES
At high load current, an offset can
develop between the power ground, where
VOUT is sensed, and the IC’s local ground.
To overcome this load regulation error,
the LTC3855 includes a unity gain differential amplifier for remote output voltage
sensing. Inputs DIFFP and DIFFN are tied
to the point of load, and the difference
between them is expressed with respect
to local ground from the DIFFOUT pin.
Measurement error is limited to the
input offset voltage of the differential
amplifier, which is no more than 2mV.
SINGLE OUTPUT CONVERTER
WITH REMOTE OUTPUT VOLTAGE
SENSING AND INDUCTOR DCR
COMPENSATION
Figure 1 shows a high current DCR application with temperature sensing. The
nominal peak current limit is determined
by the sense voltage (30mV, set by grounding the ILIM pins) across the DC resistance
of the inductor (typically 0.83mΩ), or
36A per phase. This sense voltage can be
raised by biasing the ITEMP pins below
500mV. Since each ITEMP pin sources
10µA, peak sense voltage can be increased
by inserting a resistance of less than
25k from ITEMP to ground. By using an
inexpensive NTC thermistor placed near
the inductors (with series and parallel
The circuit also maintains precise regulation by differentially sensing the output
voltage. The measurement is not contaminated by the difference between power
ground and local ground. As a result,
output voltage typically changes less
than 0.2% from no load to full load.
MULTIPHASE OPERATION
The LTC3855 can be configured for dual
outputs, or for one output with both
power stages tied together, as shown in
Figure 1. In the single output configuration, both channels’ compensation (ITH),
feedback (VFB), enable (RUN) and track/
soft-start (TRK/SS) pins are tied together,
and both PGOOD1 and PGOOD2 will
indicate the power good status of the
output voltage. By doubling the effective
switching frequency, the single output
configuration minimizes the required
input and output capacitance and voltage ripple, and allows for fast transient
response (Figure 3). The LTC3855 provides
inherently fast cycle-by-cycle current
sharing due to its peak current mode
architecture plus tight DC current sharing.
A precise 10µA flows out of the FREQ pin,
allowing the user to set the switching
frequency with a single resistor to ground.
The frequency can be set anywhere from
250kHz to 770kHz. If an external frequency source is available, a phase-locked
loop enables the LTC3855 to sync with
frequencies in the same range. A minimum on-time of 100ns allows low duty
cycle operation even at high frequencies.
If the external sync signal is momentarily
interrupted, the LTC3855 reverts to the
frequency set by the external resistor.
Its internal phase-locked loop filter is
prebiased to this frequency. An internal
switch automatically changes over to the
sync signal when a clock train is detected.
Since the PLL filter barely has to charge or
discharge during this transition, synchronization is achieved in a minimum
number of cycles, without large swings in
switching frequency or output voltage.
The LTC3855 is also useful for designs
using three or more phases. Its CLKOUT pin
can drive the MODE/PLLIN pins of additional regulators. The PHASMD pin tailors the phase delays to interleave
all the switch waveforms.
(continued on page 35)
April 2010 : LT Journal of Analog Innovation | 29
product briefs
The LTC3108 is a highly integrated
DC/DC converter ideal for harvesting and managing surplus energy from
extremely low input voltage sources
such as TEG (thermoelectric generators),
thermopiles and small solar cells. The
step-up topology operates from input
voltages as low as 20mV. Using a small
step-up transformer, the LTC3108 provides
a complete power management solution
for wireless sensing and data acquisition.
The 2.2V LDO powers an external microprocessor, while the main output is
programmed to one of four fixed voltages
to power a wireless transmitter or sensors. The power good indicator signals
that the main output voltage is within
regulation. A second output can be
(LTC3855 continued from page 29)
The MOSFET drivers and control circuits
are powered by INTVCC, which by default
is powered through an internal low
dropout regulator from the main input
supply, VIN. If lower power dissipation
in the IC is desired, a 5V supply can be
connected to EXTVCC. When a supply is
detected on EXTVCC, the LTC3855 switches
INTVCC over to EXTVCC, with a drop of
just 50mV. The strong gate drivers with
optimized dead time provide high efficiency. The full load efficiency is 86.7%
and the peak efficiency is 89.4% (Figure 4).
The LTC3855 features a RUN and TRACK/SS pin
for each channel. RUN enables the output
and INTVCC, while TRACK/SS acts as a softstart or allows the outputs to track an
external reference. If a multiphase output is desired, all RUN and TRACK/SS pins
are typically tied to one another.
Peak current limiting is used in this
application, with the peak sense voltage set by the three-state ILIM pin. A
high speed rail-to-rail differential current
enabled by the host. A storage capacitor
provides power when the input voltage source is unavailable. Extremely low
quiescent current and high efficiency
design ensure the fastest possible charge
times of the output reservoir capacitor.
POWERFUL SYNCHRONOUS
N-CHANNEL MOSFET DRIVER IN A
2MM × 3MM DFN
The LTC4449 is high speed synchronous MOSFET driver designed to
maximize efficiency and extend the
operating voltage range in a wide variety of DC/DC converter topologies,
from buck to boost to buck-boost.
The LTC4449’s rail-to-rail driver outputs operate over a range of 4V to
6.5V and can sink up to 4.5A and source
up to 3.2A of current, allowing it to
easily drive high gate capacitance and/
or multiple MOSFETs in parallel for high
sense comparator looks across the current sense element (here the inductor’s
DC resistance, implied from the associated R-C network). If a short circuit
occurs, current limit foldback reduces
the peak current to protect the power
components. Foldback is disabled during start-up, for predictable tracking.
CONCLUSION
The LTC3855 is ideal for converters using
inductor DCR sensing to provide high current outputs. Its temperature compensation and remote output voltage sensing
ensure predictable behavior from light
load to high current. From inputs up to
38V it can regulate two separate outputs
from 0.6V to 12.5V, and can be configured
for higher currents by tying its channels
together, or by paralleling additional
LTC3855 power stages. At low duty cycles,
the short minimum on-time ensures constant frequency operation, and peak current limit remains constant even as duty
cycle changes. The LTC3855 incorporates
these features and more into 6mm × 6mm
QFN or 38-lead TSSOP packages. n
current applications. The high side driver
can withstand voltages up to 38V.
Adaptive shoot-through protection circuitry is integrated to prevent
MOSFET cross-conduction current.
With 14ns propagation delays and
4ns to 8ns transition times driving 3nF loads, the LTC4449 minimizes
power loss due to switching losses and
dead time body diode conduction.
The LTC4449 features a three-state
PWM input for power stage control and
shutdown that is compatible with all controllers that employ a three-state output
feature. The LTC4449 also has a separate
supply input for the input logic to match
the signal swing of the controller IC.
Undervoltage lockout detectors monitor
both the driver and logic supplies and disable operation if the voltage is too low. n
(LT3029 continued from page 35)
CONCLUSION
The LT3029 is a dual 500mA/500mA monolithic LDO with a wide input voltage range
and low noise. The two channels are fully
independent, allowing for flexible power
management. It is ideal for battery-powered systems because of its low quiescent
current, small package and integration of battery protection features. n
100
START-UP TIME (ms)
ULTRALOW VOLTAGE STEP-UP
CONVERTER AND POWER MANAGER
FOR ENERGY HARVESTING
10
1
0.1
0.01
10
1k
100
BYPASS CAPACITANCE (pF)
10k
Figure 2. Start-up time vs bypass capacitor value
April 2010 : LT Journal of Analog Innovation | 35
Similar pages