June 2008 - 6mm × 6mm DC/DC Controller Regulates Three Outputs; or Combine Two Outputs for Twice the Current

L DESIGN IDEAS
6mm × 6mm DC/DC Controller
Regulates Three Outputs; or Combine
Two Outputs for Twice the Current
by Theo Phillips
Introduction
The LTC3853 is a versatile 3-phase
synchronous buck controller with
on-chip drivers in a 6mm × 6mm
QFN package. While each channel
can independently deliver currents
in excess of 15A, two of the channels
can be combined for twice the current,
with their relative operating phase
automatically optimized to reduce
output ripple. Channels 1 and 2 can
be programmed for outputs from 0.8V
to 5V, while channel 3 can support
outputs from 0.8V to 13.2V.
and minimizing the input capacitance
requirement.
Dual Output Converter
with 2 + 1 Operation
Figure 1 shows a 2-output converter
working from a 14V to 24V input.
Channels 1 and 2 feed the same 1.8V
output, while channel 3 controls a
second 12V output. This 2 + 1 mode
requires just one RUN pin (RUN1) to
enable both channels 1 and 2. The
error amp of channel 2 is disabled
and both channels share channel 1’s
feedback divider. Post package trimming of the current sense comparators
provides excellent current sharing
between channels 1 and 2, as the load
step of Figure 2 shows. Channels 1 and
2 run 180° out of phase to minimize
the output ripple on their 2-phase
Multiphase Operation
The LTC3853 can be configured for
three single phase outputs, or for
two outputs with channels 1 and 2
tied together. In a 3-output setup, the
switches operate 120° out of phase,
reducing the input RMS ripple current
100k
100k
L1
1.5µH
5V
10Ω
RSENSE1
8mΩ
10Ω
5%
VOUT1,2
1.8V
10A
24.9k
1%
10µF
X5R
20k
1%
CITH1
1.5nF
COUT1
470µF
2.5V
RITH1
825Ω
1%
TG2
SENSE2+
SENSE1–
SENSE2–
VFB2
ITH3
VFB1
ITH1
ITH2
VFB3
RUN2
SGND
TK/SS1
CSS1
0.22µF
SW1,2,3
CSS3
0.1µF
4.7µF
M3
L3
4.7µH
200k
10k 1%
10Ω
INTVCC
470pF 10Ω
RUN1
RUN3
TG3
SW3
BG3
SENSE3+
SENSE3–
TK/SS2 TK/SS3
VIN
14V TO 24V
22µF
50V
X5R
L2
1.5µH
SW2
SENSE1+
+
M2
CB1,2,3 0.1µF
BOOST1,2,3
BG1 LTC3853EUH BG2
EXVCC
PGND
MODE
ILIM
FREQ
470pF
CITH1a
220pF
+
TG1
PGOOD1,2
PGOOD3
SW1
1µF
DB1,2,3
VIN DRVCC1,2 INTVCC
M1
single output. A minimum on-time of
<90 nanoseconds allows low duty cycle
operation even at high frequencies—at
24VIN to 1.8VOUT, this 500kHz regulator never misses a pulse.
The EXTVCC pin can be connected
to a 5V supply to power the internal
MOSFET drivers and control circuits.
An internal switch connects EXTVCC to
INTVCC with a typical voltage drop of
just 50mV. If EXTVCC is not connected,
the LTC3853’s internal regulator uses
the main input supply, VIN, to provide
5V to the internal circuitry and drivers
at INTVCC. In either case, the switching
frequency is set with a divider from
the predictable 5V at INTVCC, with
1.2V at the FREQ pin corresponding
to free-running 500kHz. If an external
frequency source is available, a phase
locked loop enables the LTC3853 to
RSENSE2
8mΩ
5%
10Ω
280k
1%
CITH3
1nF
20k
1%
3.16k
1%
RITH3
6.49k
1%
RSENSE3
8mΩ
5%
VOUT3
12V
5A
470pF
10Ω
CITH3a
220pF
10µF
X5R
+
COUT3
2s
47µF
16V
COUT1: SANYO 2R5TPE470M9
COUT3: SANYO 16TQC47M
L1, L2, L3: VISHAY IHLP2525CZ-11
M1, M2, M3: Si4816BDY
Figure 1. For applications that need up to 30A of current, channels 1 and 2 of the LTC3853 can be combined to share the load for a single output.
The two channels operate 180º out of phase to minimize output ripple. Channel 3’s high common mode range allows it to provide 12V.
40
Linear Technology Magazine • June 2008
DESIGN IDEAS L
VOUT1,2
200mV/DIV
25µs/DIV
VIN = 14V
VOUT1,2 (NOM) = 1.8V
LOAD STEP ON VOUT1,2 = 1A TO 6A
Figure 2. Post-package trimming of the
LTC3853’s current sense comparators
provides excellent current sharing between
channels 1 and 2, even during a transient.
sync with frequencies between 250kHz
and 750kHz.
The LTC3853 can be set to operate
in one of three modes under light load
conditions. Burst Mode operation offers the highest light load efficiency by
switching in a “burst” of one to several
pulses replenishing the charge stored
in the output capacitors, followed by a
long sleep period when the load current
is supplied by the output capacitors.
Forced continuous mode offers fixed
frequency operation from no load to
full load, providing the lowest output
voltage ripple at the cost of light load
efficiency. Pulse-skipping mode operates by preventing inductor current
reversal by turning off the synchronous switch as needed. This mode
is a compromise between the other
two modes, offering lower ripple than
Burst Mode operation and better light
load efficiency than forced continuous
mode. Regardless of the mode selected,
the LTC3853 operates in constant frequency mode at higher load currents.
Figure 3 shows the efficiency in each
of the three modes.
LT3570, continued from page 29
DSL Modem
Figure 4 shows an application for a
DSL modem or set-top box. The supply voltage for VIN2 comes from a wall
adapter that can range from 8V to 30V.
This voltage is stepped down to 5V at
100mA for VOUT2, which then supplies
the power to drive both the boost
regulator and LDO controller. VOUT1
is set to 8V at 200mA and VOUT3 is set
Linear Technology Magazine • June 2008
to 3.3V at 500mA. Figure 5 shows the
load step response of VOUT1 and VOUT2
with a 200mA load step on VOUT1.
Conclusion
The LT3570 is a monolithic dual
output switching regulator (buck and
boost) with a NPN LDO controller and
is ideal for a broad variety of applications. Because the LT3570 offers a high
10k
90
EFFICIENCY
80
70
60
POWER LOSS
50
40
1k
VIN = 24V
VOUT3 = 12V
2+1 MODE
30
20
CONTINUOUS MODE
PULSE-SKIPPING MODE
BURST MODE OPERATION
10
0
POWER LOSS (mW)
IL2
2A/DIV
100
EFFICIENCY (%)
IL1
2A/DIV
Each of the LTC3853’s channels
can be enabled with its own RUN
pin, or slewed up or down with its
own TRACK/SS pin. Tracking holds
the feedback voltage to the lesser of
the internal reference voltage or the
voltage on TRACK/SS, which can be
brought up with an external ramp or
with its own 1.2µA internal current
source. With all of the TRACK/SS
pins held low and any output enabled
through its RUN pin, the 5V INTVCC is
still available for ancillary keep-alive
circuits.
Pulse-skipping mode is always
enabled at start-up to prevent sinking current from a pre-biased output
voltage. When the output reaches 80%
of the set value, the part switches over
to forced continuous mode until the
output has entered the POWER GOOD
window, at which point it switches
over to the selected mode of operation. Forced continuous mode reduces
the output ripple as the power good
threshold is crossed, to ensure that
the POWER GOOD indicators make
just one low to high transition.
Three different max current comparator sense thresholds can be set
via the ILIM pin. The current is sensed
using a high speed rail-to-rail differential current sense comparator. The
circuit of Figure 1 uses accurate sense
resistors between the inductors and
the outputs. For reduced power loss at
high load currents, the LTC3853 can
also monitor the parasitic resistance
of the inductor (DCR sensing). Peak
inductor current is limited on a cycleby-cycle basis and is independent
of duty cycle. If load current is high
enough to cause the feedback voltage
0.1
1
LOAD CURRENT (A)
10
100
Figure 3. Efficiency for channel 3 in Figure 1—
in each of the three modes of operation
to drop, current limit fold back protects
the power components by reducing the
current limit. For predictable tracking,
current limit fold back is disabled
during start-up. Input undervoltage
lockout, output overvoltage shutdown
and thermal shutdown also protect
the power components and the IC
from damage.
Conclusion
The LTC3853’s small footprint belies
its versatility and extensive feature
set. From inputs up to 24V it can
regulate three separate outputs, or it
can be configured for higher currents
by tying channels 1 and 2 together.
Either way, the phase relationship
between channels is automatically
optimized to reduce ripple currents.
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 cost-effective LTC3853
incorporates these features, and
more, into a 40-pin 6mm × 6mm QFN
package. L
level of system integration, it greatly
simplifies board design for complex
applications that need multiple voltage supply rails. With the flexibility of
independent supply inputs and adjustable frequency, the user can set a wide
array of custom output voltages. The
LT3570 is a feature rich solution that
satisfies the needs for multiple output
voltages in a compact solution. L
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