LINEAR TECHNOLOGY LT3653 Technical data

L DESIGN FEATURES

V

BUS

OV

SENS

D0–D2

CHRG

NTC

USB

WALL

ADAPTER

HIGH VOLTAGE INPUT

7.5V TO 30V
TRANSIENT

TO 60V

SYSTEM
LOAD

M1
(OPTIONAL)

L1 = TDK, VLCF5020T-4R7NIR7-1
L2 = COILCRAFT, LPS4018-332MLC
M1 = VISHAY, Si 2333DS
D1 = DIODES INC., DFLS240

SEE THE LTC4098 DATASHEET FOR MORE INFORMATION
ON CONFIGURING THE NTC BATTERY TEMPERATURE
QUALIFICATION OR REDUCED IDEAL DIODE IMPEDANCE.

SINGLE-CELL
Li-Ion

C5

0.1µF

L2

3.3µH

L1

4.7µH

LTC4098

GND

SW

V

OUT

BATSENS

PROGCLPROG

WALLV

C

LT3653

HVOKGNDV

C

I

LIM

R3
1k

R2

3.01k

R4

6.04k

R1

27.4k

C4
10µF

6.3V

D1

TO µC
TO µC

C1

4.7µF
50V

C2
22µF

6.3V

C3

0.1µF
10V

V

IN

V

OUT

I

SENSE

SW

BOOST

OV

GATE

BAT

IDGATE

1.2A Monolithic Buck Regulator
Shrinks Supply Size and Cost with
Programmable Output Current Limit

Introduction

Power supplies are often overqualified
for their job. This is because power
ICs often specify a current limit that
is more than twice the rated output
current of the device. The power supply
components are sized to handle the
maximum current that the IC can de­liver, even if loads are unlikely to draw
that current during normal operation.
The components are bigger and more
expensive than they need to be.

set an accurate maximum output cur-
rent on the supply once the real world
load is known. Accurately setting the
maximum output current reduces the
required current rating of the regula­tor’s power path components, thus
replacing big, expensive components
with smaller, less expensive ones.
A limit on the regulator’s maximum

Figure 1. Charging a single cell Li-ion battery from either a USB input or high voltage input. This
solution offers a seamless, highly efficient, low part count approach to dual input charging and
PowerPath™ control of a Li-ion battery-powered application. If additional integration is required
for more system supplies, the LT3653 can be used in a similar fashion with the LTC3576 PMIC.

20

There is, however, an alternative:

output limits the maximum power
dissipation of both the supply and
the load, thus reducing the potential
for localized heating. Monitoring and
controlling the output current also
makes for a robust solution, which is
able to withstand harsh overload and
short circuit conditions.

The LT3653 and LT3663 are mono­lithic step-down switching regulators
that have an accurate output current
limit programmable from 400mA to

1.2A. The LT3663 is a general pur­pose high voltage step-down regulator
while the LT3653 is designed for use
with Linear Technology Bat-Track™
enabled battery chargers and power
management ICs (PMICs). The maxi­mum input voltages of 30V (LT3653)
or 36V (LT3663) with 60V transient
ride through capability are well suited

by Tom Sheehan

to automotive, industrial, distributed
supply, and wall transformer applica­tions.

Programmable
Output Current Limit

Monolithic switching regulators typi­cally limit the peak switch current to
protect the internal switch from being
damaged during an overload or short
circuit event. The maximum switch
peak current limit is typically more
than two times the maximum output
current rating of the part. While the
peak switch current limit prevents
overstressing the IC, it does not keep
the entire application from overheat­ing during an overload condition. For
example, a regulator with an output
current rating of 1A is typically capable
of providing over 2A at the output.
During an output overload condition,
the power dissipation of the regula­tor could more than double, making
thermal management more difficult.
The LT3653 and LT3663 reduce local­ized hot spots by controlling the total
power dissipation of the application
with a programmable, accurate cur­rent limit.

Conservative design principles call
for power path components that are
rated for worst-case currents. In the
above example, where a 1A part is
capable of delivering 2A, the power
path components must be sized for
greater than 2A, because during an
output short circuit or overload the
inductor and diode can conduct up
to 2A. In contrast, the PowerPath
components in LT3653 and LT3663
applications are sized based on the
programmed maximum output cur­rent limit. Therefore, an application
with a 750mA output current limit
requires only 750mA rated compo­nents. This allows for smaller, lower

Linear Technology Magazine • March 2009

DESIGN FEATURES L

LT3663

V

IN

V

IN

RUN

I

LIM

BOOST

SW

GND

0.1µF

2.2µF

22µF59k28.7k

11k

6.8µH

DIODES,
INC.
DFLS240

I

SENSE

V

OUT

V

OUT

FB

ON OFF

OUTPUT CURRENT (A)

OUTPUT VOLTAGE (V)

6

5

4

0

1

2

3

0 0.2 0.4 0.6 0.8 1 1.41.2

R

ILIM

= 28.7k

cost devices and a smaller overall
application footprint.

In early product development,
system designers usually don’t know
how much current their load will draw.
Once they choose a power supply, they
are committed. However, with the pro­grammable current limit of the LT3653
and LT3663, once the load has been
fully characterized, they can change
the output current limit by changing
an inexpensive 1% resistor.

The output current limit is imple­mented by monitoring and controlling
the average inductor current. When an
overcurrent event occurs, the regulator
disables the power switch. This robust
solution withstands short circuit and
overload conditions throughout the
entire input voltage range.

The LT3653 Plays Well with
Bat-Track Battery Chargers

The LT3653 is a 1.5MHz constant
frequency, current mode control,
step-down regulator designed for use
with Linear Technology’s Bat-Track-

enabled battery charger PowerPath

power managers. The LT3653 steps

down a high voltage input to power the
system load and charge a single-cell
Li-ion battery charger.

Minimizing the voltage across
a linear battery charger increases
efficiency. To accomplish this, a Bat­Track battery charger controls the
LT3653’s VC Pin, overriding the error
amplifier. In this way, the output volt­age of the LT3653 is regulated by the
battery charger to a potential slightly
above the battery voltage, typically
300mV.

Figure 3. A LT3663 application producing
5V at 1.2A from an input of 7.5V to 36V. The
input is capable of handling 60V transients.

Linear Technology Magazine • March 2009

LT3653 programmed current limit, the
regulator’s output voltage decreases to
reduce charge current as the battery
charger enters dropout. If the system
load continues to increase, the battery
charge current first decreases to zero
and then reverses direction to deliver
power to the system load, supple­menting the LT3653. The transitions
between these modes of operation are
seamless to the system load. The out­put current from the LT3653 regulator

Figure 2. The LT3663 output
current limit at 1.2A

Input overvoltage protection allows
the LT3653 to handle 60V input tran­sients. The HVOK pin indicates that
the internal bias supplies are present
and no faults have occurred (i.e., over­temperature and input overvoltage
and undervoltage). The LT3653 in­cludes internal compensation, and
an internal boost diode to minimize
the number of external components.
The LT3653 is available in an 8-lead
2mm × 3mm DFN package with an
exposed pad.

never exceeds the programmed output
current limit.

The LT3663 Directly
Accepts 36V Inputs

The LT3663 is a 1.5MHz constant
frequency, current mode control,
general purpose, monolithic switch­ing regulator suited for automotive
batteries, industrial power sup­plies, distributed supplies, and wall
transformers. The LT3663 includes
a low current shutdown mode, input
overvoltage and undervoltage lockout,
and thermal shutdown. The LT3663 is
available in 8-lead (2mm × 3mm) DFN
package with exposed pad. An 8-lead

Charging a Single Cell Li-Ion
Battery from Either a USB
or High Voltage Input

Figure 1 shows a LT3653 and LTC4098
application charging a single cell Li­ion battery from either a USB input
or high voltage input. This solution
offers a seamless, highly efficient, low
part count approach to dual input
charging and power path control of a
Li-ion battery-powered application. If
additional integration is required for
more system supplies, the LT3653 can
be used in a similar fashion with the
LTC3576 PMIC.

When a high voltage input is ap­plied, the LT3653 HVOK pin signals
the LTC4098 that it is capable of
delivering power. The LTC4098 takes
control of the LT3653’s VC pin and

MSOP package with exposed pad will
be available soon.

The LT3663 can also function as
a constant current, constant voltage
(CC/CV) source to charge a superca­pacitor or other energy storage device.
The IC operates in constant current
mode at the programmed current
limit until the capacitor reaches the
programmed output voltage. It then
operates in a constant voltage mode
to maintain that voltage.

Figure 2 shows the LT3663 out­put current limit at 1.2A. For output
currents below 1.2A the regulator is
in constant voltage mode. When the
output current is increased to 1.2A it
goes into constant current mode. The
output current is maintained at 1.2A
from V

nominal down to 0V.

OUT

regulates the output voltage to just
above the battery voltage. This Bat­Track function optimizes the battery
charger efficiency.

When present, the high voltage
input supplies the battery charge cur­rent and the system load current. If
the total current increases beyond the

7.5V–36V to 5V Buck
Regulator with 1.2A
Output Current Limit

Figure 3 shows a LT3663 application
producing 5V at 1.2A from an input
of 7.5V to 36V. The input is capable

continued on page 29

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