June 2006 A Complete 500mA Linear Charger and 300mA Synchronous Buck Converter in a Tiny 3mm x 3mm DFN Package

DESIGN IDEAS L
A Complete 500mA Linear Charger
and 300mA Synchronous Buck
Converter in a Tiny 3mm × 3mm
by Ashish Kirtania
DFN Package
Introduction
The LTC4080 is a full-featured, singlecell 4.2V Li-Ion battery charger with
an integrated synchronous buck
DC/DC converter designed primarily
for handheld applications. Its tiny
3mm × 3mm DFN package and low
external component count provide
space-savings in today’s crowded
circuit boards. The high operating
frequency (2.25MHz) of the switching
regulator minimizes overall solution
footprint further by allowing the use of
tiny, low profile inductors and ceramic
capacitors. To extend battery life, the
buck regulator offers high efficiency
burst mode operation in which the
DESIGN IDEAS
A Complete 500mA Linear Charger and
300mA Synchronous Buck Converter in
a Tiny 3mm × 3mm DFN Package ......33
Ashish Kirtania
Triple Amps Handle High-Res
Workstation Video from Single
Supplies ............................................35
Jon Munson
500mA Output Current Low Noise
Dual Mode Charge Pump ..................36
Yang Wen
R3
510Ω
D1
VCC
(3.75V
to 5.5V)
VCC
R4, 510Ω
ACPR
D2
CIN
4.7µF
LTC4080
SW
EN_BUCK
FB
MODE
GND
Julian Zhu
New Dual Input USB/AC Linear
Li-Ion Battery Chargers .....................45
Alfonso Centuori
Linear Technology Magazine • June 2006
CPL
10pF
PROG
R1
715k
R2
806k
4.2V
Li-Ion
BATTERY
VOUT
(1.5V/300mA)
COUT
4.7µF
Figure 1. Full featured Li-Ion charger with thermal management
and efficient buck regulator in a compact, single IC solution
regulator typically consumes only
20µA at no load.
ing the PROG pin voltage (VPROG)and
applying the following equation:
Battery Charger Features
ICHRG =
The LTC4080 battery charger uses
a unique constant-current, constant-voltage, constant-temperature
algorithm with programmable charge
current up to 500mA and a final float
voltage of 4.2V±0.5%. The maximum
charge current is programmed using
a single external resistor (RPROG) from
the PROG pin to ground. The charge
current (ICHRG) out of the BAT pin can
be determined at any time by monitor100
80
EFFICIENCY (%)
60
40
20
0
0.01
1000
EFFICIENCY
(Burst)
EFFICIENCY
(PWM)
VPROG • 400
RPROG
In typical operation, the charge
cycle begins in constant current mode.
When the battery approaches the final
float voltage of 4.2V, the charge current starts to decrease as the battery
charger switches to constant-voltage
mode. When the charge current drops
to 10% of the full-scale charge current,
commonly referred to as the C/10
point, the open-drain charge status
pin, CHRG, assumes a high impedance state.
An internal thermal regulator reduces the programmed charge current
100
POWER
LOSS
10
(PWM)
POWER LOSS
(Burst)
0
POWER LOSS (mW)
Step-Up/Step-Down Charge Pump DC/DC
Converter Provides up to 150mA in a
Tiny 2mm × 2mm DFN Package .........43
L1, 1OµH*
+
*COILCRAFT LPO1704-103M
Mitchell Lee
Joseph Duncan
CBAT
4.7µF
RPROG
806Ω
Ideal Diodes Combine Battery Stacks
(Minimize Heat and Voltage Loss) ......40
High Efficiency, Low Input Voltage,
Synchronous Buck Controller Drives
up to 15A Load Current .....................41
CHRG
EN_CHRG
Tiny Buck-Boost Converter for
Low Current Applications..................38
Eddy Wells
500mA
BAT
VBAT = 3.8V
0.1
VOUT = 1.5V
L = 10µH
C = 4.7µF
0.01
0.1
1
10
100
1000
LOAD CURRENT (mA)
Figure 2. Efficiency of the
buck regulator in Figure 1
VOUT
20mV/DIV
AC COUPLED
ILOAD
250mA/DIV
I=0
50µs/DIV
Figure 3. Transient response of the buck
regulator to a 0.5mA–200mA load step
33
L DESIGN IDEAS
if the die temperature attempts to rise
above a preset value of approximately
115°C. This feature not only protects
the LTC4080 and external components
from excessive temperature, it can
also reduce the charge time by allowing the user to set a higher maximum
charge current—essentially taking
into account typical, instead of worst
case, ambient temperatures for a given
application.
An internal safety timer sets the
maximum time for a charge cycle,
typically 4.5 hours. When this time
elapses, the charge cycle terminates
and the CHRG pin assumes a high
impedance state even if C/10 has not
been reached yet. A new charge cycle
of 2.25hr automatically starts if the
battery voltage falls below the recharge
threshold (typically 4.1V).
Trickle Charge and Defective
Battery Detection
At the beginning of a charge cycle,
if the battery voltage is below 2.9V,
the charger goes into trickle charge
mode, reducing the charge current to
10% of the programmed value. If the
low voltage condition persists for one
quarter of the charge cycle (1.125hr),
the battery is assumed to be defective,
the charge cycle terminates and the
CHRG output blinks at a frequency of
2Hz with a 75% duty cycle. If, for any
reason, the battery voltage rises above
2.9V, the charge cycle restarts.
Undervoltage Lockout
An internal undervoltage lockout circuit monitors the input voltage and
keeps the battery charger in shutdown
until the input rises above 3.6V and
approximately 80mV above the battery
voltage. The undervoltage condition is
indicated by a high-impedance state
of the open-drain status output pin
ACPR.
Undervoltage Charge
Current Limiting
The battery charger in the LTC4080
includes undervoltage charge current
limiting that prevents full charge current until the input supply voltage
34
reaches approximately 300mV above
the battery voltage. This feature is
particularly useful if the LTC4080
is powered from a supply with long
leads or any relatively high output
impedance.
Buck Converter Features
The buck converter in LTC4080 is
powered from the BAT pin and has a
programmable output voltage (0.8V
to VBAT) providing a maximum load
current of 300mA. It has two modes
of operation, constant frequency mode
and Burst Mode operation, selectable
via the MODE pin. In constant frequency mode, also referred to as PWM
mode, the switching regulator uses
current mode control scheme with
internal compensation and provides
efficiencies up to 91% with very low
ripple. The operating frequency of the
switching regulator is set at 2.25MHz
to minimize possible interference with
the AM band. The switching regulator and the battery charger can run
simultaneously or independently of
each other.
Burst Mode Operation
Burst Mode operation offers higher
efficiency at light loads at the cost of
higher ripple at the output voltage. In
this mode, the inductor current swings
between a maximum value (IPEAK) and
a minimum value (IZERO) irrespective of
the load as long as the FB pin voltage
(VFB) is less than the reference voltage
of 0.8V. Once VFB exceeds 0.8V, the
control logic turns off both switches
along with most of the circuitry and
the regulator draws only about 20µA
from the battery. When the output voltage drops about 2% from its nominal
value, the switching regulator wakes
up and the inductor current starts
ramping again. To minimize the output
voltage ripple, the regulator is limited
to a maximum load current of 55mA
in Burst Mode operation.
Short-Circuit Protection
In the event of a short circuit at the
output or during start-up, the shallow
negative slope (~VOUT/L) of the induc-
tor current may prevent the inductor
from discharging enough to avoid a
cumulative runaway situation over
a number of switching cycles. Even
the hard current limit on the main
PMOS switch is no guarantee against
inductor current runaway because of
current sense blanking. The switching
regulator in the LTC4080 prevents
inductor current runaway by imposing
a current limit on the synchronous
NMOS switch. If the inductor current
through the NMOS switch at the end
of a discharge cycle is not below this
limit, the regulator skips the next
inductor charging cycle.
Buck Undervoltage Lockout
To prevent unreliable operation, when
VBAT is less than 2.7V, an undervoltage
lockout circuit prevents the switching
regulator from turning on. However, if
the regulator is already running and
the battery voltage is dropping, the
undervoltage comparator does not
turn it off until VBAT becomes less
than 2.5V.
Global Thermal Shutdown
The LTC4080 includes a global thermal
shutdown which turns off the entire
part (both battery charger and switching regulator) if the die temperature
exceeds 160°C. The part resumes normal operation once the temperature
drops approximately 14°C.
Conclusion
The LTC4080, with its complete Li-Ion
battery charger and a moderately high
current buck converter in a small 3mm
× 3mm package, offers a very compact
solution with minimum number of
external components. Thermal regulation of the battery charger and the
high efficiency of the converter reduce
charge times and simplify thermal
management. L
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • June 2006