LINEAR TECHNOLOGY LTC3851 Technical data

L DESIGN FEATURES

MODE/PLLIN

FREQ/PLLFLTR

RUN

TK/SS

I

TH

V

FB

SENSE

–

SENSE

+

V

IN

TG

BOOST

SW

INTV

CC

BG

GND

I

LIM

LTC3851

0.1µF

0.047µF

0.1µF

330pF

2200pF

R

FREQ

82.5k

15k

V

OUT

3.3V
15A

47pF

+

4.7µF

+

C

OUT

330µF
s2

C

IN

22µF

HAT2170H

HAT2170H

C

OUT

: SANYO 6TPE330MIL
CIN: SANYO 63HVH22M
L1: VISHAY IHLP5050-EZERR68M01

CMDSH05-4

154k
1%

48.7k
1%

V

IN

4.5V TO 32V

L1

0.68µH

3.01k

0.1µF

30.1k

R

FREQ

(k)

10

100

OSCILLATOR FREQUENCY (kHz)

500

250

750

1000

36 60 160 1000

Synchronous Buck Controller in
3mm × 3mm QFN Fits Automotive
and Industrial Applications with
4V–38V Input Capability

Introduction

The LTC3851 is a versatile synchro­nous step-down switching regulator
controller that is available in a space
saving 16-lead 3mm × 3mm QFN or
convenient narrow SSOP packages. Its
wide input range of 4V to 38V makes it
well-suited for regulating power from a
variety of sources, including automo­tive batteries, 24V industrial supplies
and unregulated wall transformers.
The strong onboard drivers allow the
use of high power external MOSFETs
to produce output currents up to 20A
with output voltages ranging from

0.8V to 5.5V.
The constant frequency peak cur-

rent mode control architecture provides
excellent line and load regulation along
with load current sharing capability
and dependable cycle-by-cycle current
limiting. OPTI-LOOP® compensation
simplifies loop stability design and
provides well-behaved regulation over
a broad range of operating conditions.

The LTC3851 has ±1% output voltage
tolerance over temperature. The part’s
low minimum on-time (90ns, typical)
allows for low duty cycle operation
even with switching frequencies as
high as 750kHz.

Two Current Sensing Options

The LTC3851 features a high input
impedance current sense comparator.
This allows the use of the inductor’s
DC resistance (DCR) as the current
sense element in conjunction with an
RC filter. DCR current sensing allows
the designer to eliminate the need for a
discrete sense resistor, thereby maxi­mizing efficiency and lowering solution
cost. Alternately, higher current sense
accuracy may be achieved by connect­ing the SENSE+ and SENSE– pins to a
precision sense resistor in series with
the inductor. The LTC3851 offers the
choice of three pin-selectable maxi­mum current sense thresholds (30mV,

by Mark Mercer

Figure 1. Relationship between oscillator
frequency and resistor connected between
FREQ/PLLFLTR and GND

50mV and 75mV) to accommodate a
wide range of DCR values and output
current levels.

As with all constant frequency,
peak current mode control switching
regulators, the LTC3851 utilizes slope
compensation to prevent sub-harmon­ic oscillations at high duty cycles. This

16

Figure 2. High efficiency 3.3V/15A power supply with DCR sensing

Linear Technology Magazine • September 2008

DESIGN FEATURES L

EFFICIENCY (%)

LOAD CURRENT (A)

1000.01

100

0

0.1 1 10

10

20

30

40

50

60

70

80

90

Burst Mode

OPERATION

CONTINUOUS
CONDUCTION MODE

PULSE-

SKIPPING

MODE

VIN = 12V
V

OUT

= 3.3V

MODE/PLLIN

FREQ/PLLFLTR

RUN

TK/SS

I

TH

V

FB

SENSE

–

SENSE

+

V

IN

TG

BOOST

SW

INTV

CC

BG

GND

I

LIM

LTC3851

0.1µF

0.1µF

100pF

1000pF

0.01µF

PLLIN

350kHz

1000pF

10k

10Ω

10Ω

7.5k

R

SENSE

0.002Ω

V

OUT

1.5V
15A

1000pF

+

4.7µF

+

C

OUT

330µF
s2

C

IN

180µF

33pF

RJK0305DPB

RJK0301DPB

C

OUT

: SANYO 2R5TPE330M9

L1: SUMIDA CEP125-OR6MC

CMDSH-3

42.2k
1%

48.7k
1%

V

IN

6V TO 14V

L1

0.68µH

is accomplished internally by adding
a compensating ramp to the inductor
current signal. Normally, this results
in a >40% reduction of maximum
inductor peak current at high duty
cycles. However, the LTC3851 uses
a novel scheme that allows the maxi­mum peak inductor current to remain
stable throughout all duty cycles.

Versatility

During heavy load operation, the
LTC3851 operates in constant fre­quency, continuous conduction mode.
At light loads, it can be configured
to operate in high efficiency Burst
Mode® operation, constant frequency
pulse-skipping mode or forced con­tinuous conduction mode. Burst Mode
operation offers the highest efficiency
because energy is transferred from the
input to the output in pulse trains
of one to several cycles. During the
intervening period between pulse
trains, the top and bottom MOSFETs
are turned off and only the output
capacitor provides current to the load.
Forced continuous conduction mode
results in the lowest output voltage
ripple, but is the least efficient at light
loads. Pulse-skipping mode offers
a compromise—lower output ripple
than Burst Mode operation and more
efficiency than continuous conduc­tion mode.

Figure 3. Efficiency vs load current with three
modes of operation for the circuit of Figure 2

The switching frequency of the
LTC3851 may be programmed from
250kHz to 750kHz by the resistor,
R

, connected to the FREQ/PLL-

FREQ

FLTR pin. This provides the flexibility
needed to optimize efficiency. Figure 1
shows a plot of the switching frequency
vs R

. Additionally, the switching

FREQ

frequency may be synchronized to
an external clock. While doing so,
the LTC3851 will operate in forced
continuous conduction mode.

The output voltage can be ramped
during start-up by means of an ad­justable soft-start function, or it can
track an external ramp signal. Track
and soft-start control are combined in
a single pin, TK/SS. Whenever TK/SS
is less than 0.64V, the LTC3851 op­erates in pulse-skipping mode. This

feature allows for starting up into a
pre-biased load. When TK/SS is be­tween 0.64V and 0.74V, the regulator
operates in forced continuous mode
to ensure a smooth transition from
start-up to steady state. Once TK/SS
exceeds 0.74V, the mode of opera­tion is determined by the state of the
MODE/PLLIN pin.

The RUN pin enables or disables

the LTC3851. This pin has a precision

1.22V turn-on threshold which is use­ful for power supply sequencing. The
turn-off threshold is 1.10V. There is
an internal 2µA pull-up current source
on the RUN pin.

The LTC3851’s fault protection
features include foldback current
limiting, output overvoltage detection
and input undervoltage detection. If
an overload event causes the output
to fall to less than 40% of the target
regulation value, then the LTC3851
folds back the maximum current sense
threshold. This reduces the on-time in
order to minimize power dissipation in
the top MOSFET. If the output voltage
is more than 10% above the target
regulation value, the bottom MOSFET
turns on until the output falls back
into regulation. If the input voltage is
allowed to fall low enough such than
the output of the internal linear regu­lator falls below 3.2V, then switching
operation is disabled. This feature

continued on page 36

Linear Technology Magazine • September 2008

Figure 4. High efficiency 1.5V/15A power supply synchronized to 350kHz

17