LINEAR TECHNOLOGY LTC3853 Technical data

L DESIGN IDEAS

TG1 TG2

BOOST1,2,3

SW1

SW2

BG1 BG2

SENSE1

+

SENSE2

+

SENSE1

–

SENSE2

–

V

FB1

V

FB2

I

TH3

I

TH1

I

TH2

V

IN

INTV

CC

DRV

CC1,2

LTC3853EUH

MODE

20k
1%

FREQ

ILIM

PGND

PGOOD1,2

SGND

RUN2

TK/SS2 TK/SS3TK/SS1

V

FB3

SENSE3

–

SENSE3

+

SW3

TG3

BG3

PGOOD3

EXV

CC

RUN1

10Ω

10Ω

4.7µF

10k 1%

INTV

CC

V

OUT1,2

1.8V
10A

3.16k
1%

20k
1%

R

SENSE2

8mΩ
5%

V

OUT3

12V
5A

V

IN

14V TO 24V

C

ITH1

1.5nF

R

ITH1

825Ω

1%

R

ITH3

6.49k
1%

470pF

C

OUT1

470µF

2.5V

C

OUT1

: SANYO 2R5TPE470M9

C

OUT3

: SANYO 16TQC47M
L1, L2, L3: VISHAY IHLP2525CZ-11
M1, M2, M3: Si4816BDY

C

ITH1a

220pF

22µF
50V
X5R

C

SS1

0.22µF

C

SS3

0.1µF

C

ITH3

1nF

470pF

C

ITH3a

220p

F

470pF

10Ω

L2

1.5µH

L1

1.5µH

L3

4.7µH

C

B1,2,3

0.1µF

SW1,2,3

D

B1,2,3

M2M1 M3

10Ω

+

C

OUT3

2s
47µF
16V

+

10Ω

280k

1%

10Ω

24.9k
1%

R

SENSE1

8mΩ
5%

R

SENSE3

8mΩ
5%

10µF
X5R

5V

200k

1µF

+

10µF

X5R

100k

100k

RUN3

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.

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

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

single output. A minimum on-time of
<90 nanoseconds allows low duty cycle
operation even at high frequencies—at
24VIN to 1.8V

, this 500kHz regula-

OUT

tor 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
INTV

with a typical voltage drop of

CC

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

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

40

Linear Technology Magazine • June 2008

POWER LOSS (mW)

EFFICIENCY (%)

LOAD CURRENT (A)

100.1

100

0

10k

100

1

1k

10

20

30

40

50

60

70

80

90

VIN = 24V

V

OUT3

= 12V

2+1 MODE

EFFICIENCY

POWER LOSS

CONTINUOUS MODE

PULSE-SKIPPING MODE

BURST MODE OPERATION

V

OUT1,2

200mV/DIV

I

L2

2A/DIV

I

L1

2A/DIV

25µs/DIV

VIN = 14V
V

OUT1,2

(NOM) = 1.8V

LOAD STEP ON V

OUT1,2

= 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.

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

DESIGN IDEAS L

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

LT3570, continued from page 29

DSL Modem

Figure 4 shows an application for a
DSL modem or set-top box. The sup­ply voltage for V
adapter that can range from 8V to 30V.

IN2

comes from a wall

This voltage is stepped down to 5V at
100mA for V
the power to drive both the boost
regulator and LDO controller. V
is set to 8V at 200mA and V

Linear Technology Magazine • June 2008

, which then supplies

OUT2

OUT3

is set

OUT1

to 3.3V at 500mA. Figure 5 shows the
load step response of V
with a 200mA load step on V

OUT1

and V

OUT1

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

OUT2

.

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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