LINEAR TECHNOLOGY LTC400X Technical data

L DESIGN FEATURES

Feature-Packed Charger Handles
All Battery Chemistries and Produces
3A/50W for Fast Charging from a
4mm × 4mm QFN

Introduction

The LTC 4009, L TC400 9 -1 and
LTC4009-2 are a family of high power
battery charger ICs that achieve a small
circuit size and high performance with­out compromising functionality. The
family operates with high efficiency
while packing the most desirable
charging and protection features into
a space-efficient 20-lead 4mm × 4mm
QFN package. When combined with
just a few external components and
termination control, the LTC4009
family facilitates construction of
chargers capable of delivering up to
3A to batteries with output power
levels approaching 50W. These ICs are

Feature LTC4009 LTC4009-1 LTC4009-2 LTC4008

Output Voltage Selection

especially well-suited to implementing
microprocessor-controlled chargers
for all chemistry types, including
smart batteries.

High Performance

The LTC4009 family builds upon the
proven quasi-constant frequency,
constant off-time PWM control ar­chitecture found in other Linear
Technology battery chargers such as
the LTC4006, LTC4007, LTC4008,
and LTC4011. This buck topology
provides continuous switching with
synchronous rectification, even with
no load current.

Table 1. LTC4009 family features

External Resistor

Divider

Pin Programmable

at 4.1V/cell

by James A. McKenzie

Normally the charger operates over
a wide duty cycle range in a manner
similar to a fixed-frequency PWM con­troller running at 550kHz. However,
if the input or output voltage drives
the duty cycle to extremes, below 20%
or above 80%, the LTC4009 smoothly
adjusts the operating frequency
downward to avoid pulse-skipping
that might otherwise begin to occur
at 550kHz. Under very low dropout
conditions requiring high duty cycle
operation, the internal watchdog
timer on the LTC4009 prevents the
charger from switching below 25kHz.
This allows the charger to achieve a

Pin Programmable

at 4.2V/cell

External Resistor

Divider

Output Voltage Accuracy

(Room Temperature)

Maximum Charge Current 3A 3A 3A 4A

Charge Current Accuracy ±5% ±5% ±5% ±5%

Input Current Limit Accuracy ±4% ±4% ±4% ±7%

Input Current Limit/Indicator

External PWM Switching MOSFETs All NFET All NFET All NFET PFET/NFET

Nominal PWM Frequency 550kHz 550kHz 550kHz 300kHz

Shutdown Pin

C/10 Indicator

Charge Current Monitor

Termination Method External External External External

Fault Indicator

Thermistor Interface

INFET Control

18

±0.5% + Divider Error ±0.6% ±0.6% ±0.8% + Divider Error

L L L L

L L L

L L L L

L L L L

Linear Technology Magazine • September 2008

Merged with ACP

L

L

L

DESIGN FEATURES L

CLP

FROM

ADAPTER

15V AT 2A

BULK

CHARGE

0.1µF

5.1k

D2

D1

D3

Q1

294k

10pF

25mΩ

3.01k

0.1µF

2µF

Q2

Q3

D4

L1

6.8µH

33mΩ

3-CELL
Li-Ion
BATTERY
STACK

DCIN

CHRG

0.1µF

22.1k

2.43k

3k

RED

6.04k

26.7k

53.6k

LTC4009

DCDIV

ACP

ICL

SHDN

ITH

PROG

CLN

BOOST

TGATE

SW

INTV

DD

BGATE

TO/FROM

MCU

GND

CSP

CSN

BAT

FBDIV

V

FB

20µF

POWER TO SYSTEM

0.1µF

4.7nF

20µF

3.01k

31.2k

D2, D4, D5: MBR230LSFT1
D3:CMDSH-3
Q1: 2N7002
Q2, Q3: Si7212DN
L1: IHLP-2525CZ-11

+

D5

maximum duty cycle of 98% or higher
without producing frequencies that
could extend down into the audible
range.

With a synchronous rectifier, not
only are high current applications
supported at efficiency levels greater
than 90%, but switching activity is
continuous and independent of the
load current. Maintaining full continu­ous conduction mode in the inductor
at final output voltage, under no-load
conditions, avoids pulse-skipping
which can generate audible noise and
provide poor load regulation.

The input current limit accuracy
is typically ±3% and a maximum of
±4% over the full operating tempera­ture range. Output voltage accuracy
is typically ±0.5% and a maximum of
±0.8% over temperature.

Small PCB Footprint

Besides its small surface mount pack­age size, the LTC4009 family offers
other features that drive down the
total solution size.

For instance, as shown in Figure 1,
the family supports direct drive of both
an N-channel MOSFET power switch
and N-channel MOSFET synchronous

When combined with just

a few external components

and termination control, the

LTC4009 family facilitates

construction of chargers

capable of delivering

up to 3A to batteries
with output power levels
approaching 50W. These

ICs are especially well

suited to implementing
microprocessor-controlled
chargers for all chemistry

types, including smart

batteries.

rectifier. N-channel MOSFETs are
desirable in high current applica­tions because of their lower R
and the LTC4009 family uses a novel
adaptive gate drive that is insensitive
to MOSFET inertial delays to avoid
overlap conduction losses. Many
suppliers now source dual N-channel
MOSFETs in a single space-efficient
package, often with individual drive

DS(ON)

,

capabilities tailored to synchronous
buck PWM switching topologies.

Increasing the switching frequency
to 550kHz and adjusting internal bias
circuits to allow higher charge current
ripple minimize both the inductor size
and output capacitance requirements.
This is particularly important because
these components tend to dominate
the overall solution size due to con­tinual improvements in IC and passive
SMD packaging technology.

The physical layout of a typical
3A application is shown in Figure 3,
requiring only 240mm2 of board
space.

A Rich Tradition

The LTC4009 family builds upon the
general purpose features offered by
the LTC4008 and the output voltage
programming convenience afforded
by the LTC4006. Each member of the
LTC4009 family contains the same
charge current and input current limit
programming features, along with a
full complement of charge monitor­ing, safety and fault management
functions. The LTC4009 has a fully
adjustable output voltage, which is set
with a simple resistor divider. Charge

Linear Technology Magazine • September 2008

Figure 1. A 12.6V, 3A lithium-ion charger

19