LINEAR TECHNOLOGY LTC360X Technical data

L DESIGN IDEAS

V

OUT

1.2V
8A

C

OUT

100µF
s2

L1

0.5µH

V

IN

5V TO 18V

C

IN

10µF

25V

3s

+

C1

0.22µF

C

SS

0.1µF

4.7µF

6.3V

V

IN

0.47µF
25V

7.68k
1%

1500pF

R

ON

187k

1%

30.1k
1%

30.1k
1%

100k

LTC3608

V

FB

I

ON

FCB

I

TH

V

RNG

PGOOD

SGND

BOOST

RUN/SS

V

ON

INTV

CC

SW

PV

IN

0.1µF

100pF

CMDSH-3

1Ω

KEEP POWER GROUND AND SIGNAL GROUND SEPARATE.
CONNECT AT ONE POINT.

= SGND

= PGND

PGND

SV

IN

0.01µF

EXTV

CC

C1: TAIYO YUDEN JMK316BJ226ML-T
CIN: TAIYO YUDEN TMK432BJ106MM
C

OUT

: TDK C4532X5R107M

L1: SUMIDA CDEP85NP-R50MC-125

32V
with Integrated FETs Deliver up to 12A
from Sub-1mm Height Packages

Introduction

Monolithic buck regulators are easy
to hook up and they make it possible
to squeeze an entire DC/DC converter
into very tight spaces. Although mono­lithics are an easy fit, they aren’t the
perfect fit for every application. For
instance, they typically lack the capa­bility to efficiently convert high input
voltages (>12V) to low voltages at high
output currents (>4A), thus leaving the
job to a traditional controller IC and
external MOSFETs.

A new family of devices, though,
offers the advantages of monolithics
with the low duty cycle and high ef­ficiency of discrete components. The
LTC3608, LTC3609, LTC3610 and
LTC3611 are synchronous buck con­verters that bring high power density
and simplified design to point-of-load
applications. With a maximum input of

32

32

Synchronous Buck Regulators

IN

by Stephanie Dai and Theo Phillips

are packaged in thermally enhanced
packages less than 1mm in height. A
typical application of the LTC3608 is
shown in Figure 1.

Features

The LTC3608, LTC3609, LTC3610 and
LTC3611 integrate high performance
synchronous buck controllers with
super-low R
to produce compact high efficiency
converters (Figure 2). Two package
sizes are available, each having a
high voltage or high current option
(Table 1). Each device features a sub­100ns on-time, allowing very low duty
cycle operation and high switching
frequency. The current-mode control
architecture of these parts simplifies
tuning of loop stability and allows
excellent transient response with a
variety of output capacitor types, in­cluding all-ceramic output capacitor
applications.

The LTC3610 can operate in forced
continuous mode, which provides the
lowest possible output ripple and EMI,
or discontinuous mode, which has
better light load efficiency because
inductor current is not allowed to
reverse.

Current into the ION pin sets the
on-time—a resistor RON from VIN to the
ION pin reduces on-time as V
thus limiting changes in switching
frequency. Furthermore, response to a
load step can be very fast since the loop
does not have to wait for an oscillator
pulse before the top switch is turned
on and current begins increasing.

The current limit, which is inferred
from the maximum allowable sense
voltage across the on-resistance of
the bottom FET, can be adjusted by
applying a voltage to the V
Maximum load current limits for each Figure 1. Typical application of the LTC3608

Linear Technology Magazine • September 2008

DS(ON)

A new family of devices

offers the advantages of

monolithic DC/DC converters

with the low duty cycle and

high efficiency of discrete

components. The LTC3608,

LTC3609, LTC3610 and

LTC3611 are synchronous

buck converters that bring

high power density and

simplified design to point-of-

load applications.

32V they utilize current-mode control
up to a 2MHz switching frequency,
deliver up to 12A of load current, and

DMOS MOSFETs

rises,

IN

pin.

RNG

part are shown in Table 1. Soft-start

LOAD STEP 1A-8A
VIN = 12V
V

OUT

= 1.2V

FCB = 0V

200mV

V

OUT

200mV/DIV

I

L

5A/DIV

LOAD CURRENT (A)

0.01

50

EFFICIENCY (%)

55

65

75

60

70

80

85

90

0.1 1 10

VIN = 12V
FREQ = 550kHz

CCMDCM

and latch off functions are controlled
by the RUN/SS pin, preventing inrush
current and current overshoot during
startup, and providing the option of
latch-off if an under voltage or short
circuit is presented. An open drain
power-good pin monitors the output
and pulls low if the output voltage is
±10% from the regulation point.

Conclusion

The LTC3608, LTC3609, LTC3610
and LTC3611 buck regulators of­fer the efficiency and power output
capability of separate (controller +
discrete) MOSFET solutions with
the ease-of-use and space-saving
advantages of traditional MOSFET­on-the-die monolithics. These parts
also yield higher efficiencies than

Figure 3. Transient response for the typical
LTC3608 application represented in Figure 1
with a load step of 1A to 8A

traditional monolithic solutions.
They conserve power, save space, and
simplify power designs. They reduce
discrete components over control­ler-based solutions, making them a

DESIGN IDEAS L

Figure 2. Efficiencies for a typical
LTC3608 application in discontinuous
conduction mode (DCM) and continuous
conduction mode (CCM)

good fit in everything from low power
portable device applications such as
notebook and palmtop computers
to high-power industrial distributed
power systems.

L

LTC3610 LTC3611 LTC3608 LTC3609

PVIN Max 24V 32V 18V 32V

I

Max 12A 10A 8A 6A

LOAD

Package

R

DS(ON)

Top FET

R

DS(ON)

Bottom FET

LTC4009, continued from page 20

9mm × 9mm × 0.9mm

64-pin

12mΩ 15mΩ 14mΩ 19mΩ

6.5mΩ 9mΩ 8mΩ 12mΩ

LTC4009 family monitors the voltage
across the input blocking diode for
unexpected voltage reversal. Initial
startup, restarts from fault conditions,
and charge current reduction during
input current limit are also carefully
controlled to avoid producing reverse
current.

All members of the family provide
an input current limit flag to tell the
system when the adapter is running
at over 95% of its current capacity.
Finally, each IC features internal
over-temperature protection to pre­vent silicon damage during elevated
thermal operation.

Recovery from all fault conditions is
under full control of the analog feed-

Linear Technology Magazine • September 2008

Table 1. Integrated MOSFET buck regulators

9mm × 9mm × 0.9mm

64-pin

7mm × 8mm × 0.9mm

52-pin

back loops, which guarantees charging
remains suspended until the internal
feedback loops respond coherently and
report the need to supply current to
the load to maintain proper voltage or
current regulation.

Conclusion

The LTC4009 family integrates a full
set of charger building blocks in a small
PCB footprint. The result is a high
power battery charger IC with high
precision and a full set of monitoring
and fault handling features.

The LTC4009 provides adjustable
output voltage control with a simple,
external, user-programmed resistive
voltage divider. As such, it is suitable as
a general purpose charger that works

7mm × 8mm × 0.9mm

52-pin

with multiple battery chemistries and
supercaps. It offers direct control over
the entire charge process, facilitating
implementation of a wide range of
charge termination algorithms with
an external microprocessor.

The LTC4009-1 and LTC4009-2
feature pin-programmable output
voltage for common lithium-ion or
lithium-polymer battery pack configu­rations with one to four series cells.
For these chemistries, the number of
precision external application compo­nents is reduced without sacrificing
accuracy. Both 4.1V/cell (LTC4009-1)
and 4.2V/cell (LTC4009-2) options are
available, allowing the user to balance
capacity and safety per the demands
of the application.

L

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