LINEAR TECHNOLOGY LT3480 Technical data

L DESIGN FEATURES

LT3480

GND

C8
100pF

C6
680pF

C3

2.2µF
50V

C7

0.1µF
50V

C1

0.47µF

L1*

10µH

D1

DFLS240L-7

C4
22µF
10V

R5
20k

R2

110k

R6

100k

R1

56.2k

R3
590k

E5

E1

V

OUT

5V, 2A

E7

R4
100k

PGOOD

BD

* L1: SUMIDA CDR7D43MNNP-100NC

BOOST

SW

V

IN

V

IN

6.3V TO 38V
TRANSIENT

TO 60V

0V

5V

RUN/SS

RT

V

C

FB

PGOOD

SYNC

1

2

3

4

5

10

11

9

8

7

6

Tiny High Efficiency 2A Buck
Regulator Directly Accepts
Automotive, Industrial and
Other Wide Ranging Inputs

Introduction

Automotive batteries, industrial power
supplies, distributed supplies and
wall transformers are all sources of
wide-ranging, high voltage inputs. The
easiest way to step down these sources
is with a high voltage monolithic step­down regulator that can directly accept
a wide input range and produce a
well-regulated output. The LT3480 is a
new step-down regulator that accepts
input from up to 38V (60V transient)
while providing excellent line and load
regulation and dynamic response. The
LT3480 offers high efficiency solutions
over wide load range and keeps the
output ripple low during Burst Mode®
operation.

LT3480 Features

Available in either a 10-pin MSOP
or a 3mm × 3mm DFN package, the
LT3480 offers an integrated 3.5A
power switch and external compensa­tion for design flexibility. The LT3480
employs a constant frequency, current
mode architecture. The switching fre­quency can be set between 250kHz and

2.4MHz by using a resistor tied from
the RT pin to ground. This allows a
trade off between component size and
efficiency. The switching frequency
can be synchronized to an external
clock for noise sensitive applications.
An external resistor divider programs

The LT3480 is a new
step-down regulator that
accepts input from up to

38V (60V transient).

the output voltage to any value above
the part’s 0.8V reference.

The LT3480 offers soft-start via a
resistor and capacitor on the RUN/SS
pin, thus reducing maximum inrush
currents during start-up. The LT3480
can withstand a shorted output. A
cycle-by-cycle internal current limit
protects the circuit in overload and
limits output power; when the output
voltage is pulled to ground by a hard
short, the LT3480 reduces its oper­ating frequency to limit dissipation

and peak switch current. This lower
frequency allows the inductor current
to safely discharge, thus preventing
current runaway.

diode is integrated into the IC to mini­mize solution size and cost. When the
output voltage above 2.5V, the anode
of the boost diode can be connected to
output. For output voltages lower than

2.5V, the boost diode can be tied to a
separate rail or to the input (<28V). For
systems that rely on a well-regulated
power source, the LT3480 provides
a power good flag that signals when
V
output voltage.

Modes of Operation:
Low Ripple Burst and
Forced Continuous

Two modes of operation can be se­lected through the SYNC pin. Applying
a logic low to the SYNC pin enables
low ripple Burst Mode operation,
which maintains high efficiency at
light load while keeping the output
voltage ripple low. During Burst Mode

by Kevin Huang

The high side bootstrapping boost

reaches 90% of the programmed

OUT

18

Figure 1. A 600kHz 6.3V–38V input DC/DC Converter using the LT3480 delivers 2A at 5V output.

Linear Technology Magazine • March 2007

DESIGN FEATURES L

LOAD CURRENT (mA)

0

EFFICIENCY (%)

50

500 1000 1500 2000

60

100

90

80

70

VIN = 12V

5µs/DIV

V

OUT

10mV/DIV

I

L

0.2A/DIV

200µs/DIV

V

OUT

2V/DIV

I

L

2A/DIV

LT3480

GND

C8
100pF

C6
680pF

C3

2.2µF
50V

C7

0.1µF

C1

0.47µF

L1*

2.2µH

D1

DFLS240L-7

C4
10µF
10V

R5
20k

R2

110k

R6

100k

R1

11.5k

R3
590k

E5

E1

V

OUT

5V, 2A

E7

R4
100k

PGOOD

BD

* L1: SUMIDA CDRH4D22/HP-2R2NC

BOOST

SW

V

IN

V

IN

9V TO 22V

60V TRANSIENT

0V

5V

RUN/SS

RT

V

C

FB

PGOOD

SYNC

1

2

3

4

5

10

11

9

8

7

6

Figure 2. Efficiency for circuit in Figure 1

operation, the LT3480 delivers single
cycle bursts of current to the output
capacitor followed by sleep periods
when the output power is delivered to
the load by the output capacitor. Be­tween bursts, all circuitry associated
with controlling the output switch is
shut down, reducing the input supply
current and BD quiescent current to
30µA and 80µA respectively. As the
load current decreases to a no load
condition, the percentage of time
that LT3480 operates in sleep mode
increases and the average input cur­rent is greatly reduced, resulting in
high efficiency. The LT3480 has a very
low (less than 1µA) shutdown current
which significantly extends battery
life in applications that spend long
periods of time in sleep or shutdown
mode. For applications that require
constant frequency operation even at
no load, the LT3480 can be put into

Figure 3. LT3480 Burst Mode operation at 10mA load

Figure 4. Soft-start of LT3480

forced continuous mode operation by
tying the SYNC pin above 2.5V.

6.3V–38V to 5V, 2A DC/DC
Converter with All Ceramic
Capacitors

Figure 1 shows the LT3480 produc­ing 5V at 2A from an input of 6.3V to
38V with 60V transient. The circuit is
programmed for a 600kHz switching

frequency. Figure 2 shows the circuit
efficiency at 12V input. The efficiency
peaks at 90% and remains high across
the entire load range. The SYNC pin
is tied to the ground to enable Burst
Mode operation and achieve high effi­ciency at light load. Figure 3 shows the
inductor current and output voltage
ripple under single pulse Burst Mode
operation at 10mA load. The output

Figure 5. High operating frequency allows the use of small inductors and capacitors.
This 2MHz, 9V–22V input DC/DC converter using the LT3480 delivers 2A at 5V output.

Linear Technology Magazine • March 2007

19

L DESIGN FEATURES

LT3480

GND

C8
100pF

C6
680pF

C10
22µF
10V

C3

2.2µF
50V

C7

0.1µF

C9
10µF
10V

C1

0.47µF

L1A*
10µH

D1

DFLS240L-7

C4
22µF
10V

R5
20k

R2
110k

R6

100k

R1

60.4k

R3
590k

E1

V

OUT1

5V, 1A

V

OUT2

–5V, 0.5A

R4
100k

PGOOD

BD

* L1: COOPER ELECTRONIC DRQ74-100

BOOST

SW

V

IN

V

IN

6.3V TO 38V

0V

5V

RUN/SS

RT

V

C

FB

PGOOD

SYNC

1

2

3

4

5

10

11

9

8

7

6

L1B*
10µH

D2

DFLS240L

LT3685

GND

C8
100pF

C6
680pF

C3

2.2µF
50V

C7

0.1µF

C1

0.47µF

L1*

2.2µH

D1

DFLS240L-7

C4
10µF
10V

R5
20k

R2

110k

R6

100k

R1

11.5k

R3
590k

E5

E1

V

OUT

5V, 2A

E7

R4
100k

PGOOD

BD

* L1: SUMIDA CDRH4D22/HP-2R2NC

BOOST

SW

V

IN

V

IN

9V TO 22V

60V TRANSIENT

0V

5V

RUN/SS

RT

V

C

FB

PGOOD

SYNC

1

2

3

4

5

10

11

9

8

7

6

Figure 6. A 2MHz 9V–22V input DC/DC converter using the LT3685 delivers 2A at 5V output.

voltage ripple V

is less than 10mV

P–P

as a result of low ripple Burst Mode
operation.

An external signal can drive the
RUN/SS pin through a resistor and
capacitor to program the LT3480’s
soft-start, reducing maximum inrush
current during start-up. Figure 4
shows the start-up waveform.

2MHz, 9V–22V to 5V, 2A
DC/DC Converter with All
Ceramic Capacitors

Figure 5 shows a step-down DC/DC
converter using all ceramic capacitors.
This circuit provides a regulated 5V
output at up to 2A from an input of
9V to 22V. The high 2MHz switching
frequency allows the use of small
inductor and capacitors.

In typical automotive battery­voltage applications, high voltage
line transients, such as during a
load-dump condition, must be accom­modated. The circuit shown in Figure 5
can operate through intermittent high
voltage excursions to 60V. This con­verter is an ideal choice for operation
near an AM radio receiver because it
operates above the broadcast band
and the switching noise can be filtered
in a predictable manner. The SYNC
pin is tied to output to disable Burst
Mode operation in order to eliminate
AM band interference. The efficiency
of this circuit reaches 85%.

The LT3685, similar to the LT3480
without Burst Mode operation, is also

20

a good candidate for this application.
Figure 6 shows the circuit using the
LT3685 for this application.

negative output tracks the positive
output within 5%. For a more complete
description of this circuit, see Linear
Technology Design Note 100.

Dual Output Converter

Dual output supplies are required
for many applications. The circuit in
Figure 7 uses an LT3480 to generate
both positive and negative 5V supplies.
The two inductors shown are actually
two windings on a coupled inductor.

The load current on the positive output
should be larger than the load on the
negative output. With this restriction
satisfied, the voltage magnitude of the

Figure 7. A ±5V dual output DC/DC converter. As long as the load on the negative channel is less
than the load on the positive channel, the voltage magnitude of the negative output tracks the
positive output within 5%.

Conclusion

The wide input range, low quiescent
current, small size and robustness
of the LT3480 make it an easy fit in
automotive, industrial and distributed
power applications. It is highly efficient
over the entire load range. Its unique
low ripple Burst Mode operation helps
to save battery power life while main­taining low output ripple.

Linear Technology Magazine • March 2007

L