Sep 2003 Feature-Rich Battery Charger that Manages Both Battery Charging and Bus Voltage Regulation

DESIGN FEATURES
Feature-Rich Battery Charger that
Manages Both Battery Charging and
by John Shannon
Bus Voltage Regulation
Introduction
Until now power management in
portable devices has required a mix
of major components to fulfill the basic functions of battery charging and
generation of system supply voltages.
A typical solution requires at least two
major devices (and associated external components): one charger IC for
charging the battery and another IC
to supply a regulated system bus voltage from a constantly changing battery
voltage. The LTC1980 is a single-device
solution that manages both battery
charging and generation of the regulated system bus voltage.
way through the converter, a fact that
is exploited to charge or discharge the
battery, depending on the power needs
of the system.
The battery charger portion of the
LTC1980 is a full-featured, constant
current, constant voltage, Li-Ion
charger with timer termination. The
LTC1980 can be set up for either 1- or
2-cell, and 4.1V or 4.2V chemistries.
This switch mode charger maintains
high efficiency over a wide range of
input voltages. The flyback topology
allows any input voltage to generate
any output voltage, unlike buck or
boost topology chargers that require
the input voltage to be always higher
or always lower than the battery voltage.
Powerful Features
The LTC1980, in simple terms, controls the power flow between the AC
adapter, a battery and the system bus.
The basic LTC1980 circuit is a synchronous flyback converter. In such
a configuration, power can flow either
+
4.1V
Li-Ion
BATTERY
C1
68µF
VBAT
+
BH511-1014
D1*
1N5819
3.3V
+
5.1Ω
•
1nF
If the AC adapter is present and has
sufficient voltage then the LTC1980
VREG
•
5.1Ω
Charging (AC Power Present)
enters charge mode. In charge mode
power flows from the adapter to both
the system bus and the battery. The
charger uses a constant current, constant voltage algorithm that is suitable
for Li-Ion cells. Deeply discharged
batteries are trickle charged with a
low current until the battery voltage
exceeds the trickle charge threshold,
at which point full current charging
commences. The switch mode operation of the charger typically keeps
efficiency above 80%, which results
in less heat generation compared to
a linear charger. Adapter power also
flows directly to the system bus via a
linear regulator. The efficiency of the
linear converter is simply the ratio of
the system bus voltage to the adapter
voltage, so losses are minimized if the
adapter voltage is close to the desired
system bus voltage.
C4
68µF
OPTIONAL PASS
TRANSISTOR
FOR LDO FDC636P
VLDO 3.1V
1nF
1/2 FDC6401N
1/2 FDC6401N
C6
470µF
R5
154k
50mΩ
RSENSE
14
BGTDR
18
VBAT
23
OVP
3
REGFB
22
CAOUT
PROG
R9
10k
1
R10
110k
R11
1M
12
20
PGND
21
GND ISENSE
11
7
6
5
2
VC
C11
1nF
TIMER
4
C7
0.27µF
24
C8
0.1µF
WA
15
REG
16
MODE
9
BATT1
10
BATT2
SS
17
R6
100k
8
RGTDR VREG LDODRV LDOFB
LTC1980
PROGT
DCOUT
VBIAS1
C9
1µF
200pF
R8
169k
WALL
ADAPTER
ACIN
SYSTEM LOAD
DC/DC
CONVERTERS
VOUT
R15
300k
R7
100k
VBIAS2
13
C10
0.33µF
19
R12
100k
R14
100k
*OPTIONAL DIODE FOR
SHORTED WALL ADAPTER
TERMINAL PROTECTION.
C12
82pF
R13
806k
Figure 1. Typical application for single Li-Ion cell
28
Linear Technology Magazine • September 2003
DESIGN FEATURES
Discharging (Battery Mode)
When the adapter input falls, so that
the system bus voltage requirements
can no longer be met, the LTC1980
switches to the regulator mode. In this
mode the LTC1980 no longer functions
as a battery charger. It instead acts
as a battery discharger. Power flows
“backwards” from the battery to the
linear regulator. The output voltage
of the flyback, which is input to the
linear regulator, should be as low as
possible in order to maximize efficiency
and battery run time. The efficiency
of the battery to system bus voltage
conversion can be as high as 88%.
The Linear Regulator
A low dropout regulator, using an
external P-FET as the pass element,
regulates the system bus voltage. The
linear regulator takes its power from
the output of the AC adapter. Dissipation in the linear regulator is lowest
when the AC adapter voltage is near the
system bus voltage. When the system
is in battery discharging mode, the
voltage input to the linear regulator is
the output of the synchronous flyback
converter. This voltage should be set
to be only a percent or two above the
required output voltage (allowing for
LT1803 and LT6220, continued from page 27
which is very useful on limited supplies. Put another way, in order to get
100kΩ sensitivity and still handle a
1mA signal level without resorting to
gain reduction, the circuit would need
a 100V negative voltage supply.
The operation of the circuit is quite
simple. At low photodiode currents
(below 10µA) the output and inverting input of the op amp are no more
than 1V below ground. The LT1634
in parallel with R3 and Q2 keep a
constant current though Q2 of about
20µA. R4 maintains quiescent current through the LT1634 and pulls
Q2’s emitter above ground, so Q1 is
reverse biased and no current flows
through R2. So for small signals, the
only feedback path is R1 (and C1) and
the circuit is a simple transimpedance
amplifier with 100kΩ gain.
As the signal level increases though,
the output of the op amp goes more
Linear Technology Magazine • September 2003
VADAPTER
2V/DIV
IBAT
500mA/DIV
VADAPTER
2V/DIV
BATTERY CHARGING
BATTERY CHARGING
BATTERY
DISCHARGING
0V
0V
IBAT
500mA/DIV
BATTERY
DISCHARGING
200µs/DIV
200µs/DIV
Figure 2. Adapter voltage and battery current
(adapter removal)
Figure 3. Adapter voltage and battery current
(adapter insertion)
IR drops in the pass element). This
prevents saturating the gate drive
to the pass element and will aid in
transient recovery.
Figure 1 shows a typical application circuit for charging a single 4.1V
Li-Ion cell. The adapter voltage can
vary from 4V to 9V, demonstrating one
key advantage of the flyback topology.
Figures 2 and 3 show battery current
and adapter voltage during the transition from battery charging (adapter
present) to regulator mode (battery
discharging). The load on the linear
regulator is 200mA, supported either
by the battery or the adapter. When
the adapter is present the battery is
charged at about 650mA. Once the
wall adapter is removed the battery
is discharged as power flows back
through the synchronous flyback
converter to support the 200mA load
on the linear regulator.
negative. At 12.5µA of photodiode current, the 100kΩ gain dictates that the
LT6220 output is about 1.25V below
ground. At that point, however, the
emitter of Q2 is at ground, and the
base of Q1 is one Vbe below ground.
Thus, Q1 turns on and photodiode
current starts to flow through R2. The
transimpedance gain is therefore now
reduced to R1||R2, or about 3.1kΩ.
The circuit response is shown in Figure
11. Note the smooth transition between
the two operating gains, as well as the
linearity of both regions.
reduced supply current, lower input
offset voltage, lower input bias current, and higher DC gain than other
devices with comparable bandwidth,
which is critical in circuits having high
input impedance, such as active filters, or in circuits having precision
requirements, such as current sensing
amplifiers. The LT1803 and LT6220
series are offered in a variety of small
packages including a 3mm × 3mm
dual fine pitch leadless package with
the standard dual op amp pinout and
also in the SOT-23 package for a single
amplifier. The combination of speed,
DC accuracy and low power makes the
LT1803 series and the LT6220 series
a preferred choice for battery powered,
low voltage signal conditioning.
Conclusion
The LT1803 series and LT6220 series
deliver exceptional performance, and
the rail-to-rail inputs and outputs of
these devices maximize signal dynamic range while simplifying design
for single supply systems. The LT1803
series and the LT6220 series feature
Conclusion
The LTC1980 manages both battery
charging and system voltage regulation, which is typically the work of
two separate devices and their corresponding external circuitry. This
feature combined with the fact that the
design of the LTC1980 also allows for
battery voltages either above or below
the adapter voltage, greatly simplifies
the task of integrating a battery and
adapter into a portable device.
Notes
1 A DC bias on op amp B’s + input could set the
output restore to some other reference voltage.
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