Nov 1999 Smart Battery Charger Is Programmed via the SMBus

DESIGN FEATURES
Smart Battery Charger Is
Programmed via the SMBus
by Mark Gurries
Introduction
Smart Batteries are becoming prevalent in the laptop computer world
because they offer an industry-standard, high accuracy “gas gauge”
system. These batteries conform to a
set of specifications that define the
operation of all of the components in
a Smart Battery powered System
(SBS). The battery has an embedded
controller that tracks information
related to battery charging and use.
This information is provided to the
system via a serial, 2-wire SMBus
interface, a variant of the I2C™ bus in
wide use today. The battery can be
queried for information on remaining
capacity, total capacity, time remaining at current rate of discharge,
discharge current, terminal voltage
and so on. Since most Smart Batteries can become a master on the bus,
the battery can control the Smart
Battery Charger for optimal charging.
The LTC1759 Smart Battery Charger
IC is designed to be controlled by this
type of Smart Battery. In addition, a
safety signal provided by the battery
indicates whether the battery is
present in the system and warns of
possible thermal problems or battery
faults if other systems fail. The
emphasis of the SBS is on safety, ease
of use and compatibility.
There are two types of Smart Battery Chargers (SBCs) allowed by the
SBS specifications. A Level 2 charger,
such as the LTC1759, is a slave on
the SMBus and responds to commands from the battery to control
charging. A Level 3 charger can be
either a slave or a master on the
SMBus, since it can query the battery
to determine charging information.
The SBC is independent of batterychemistry type. It provides charging
current and charging voltage in
response to commands from the battery. Charge termination is sent by
the battery as either zero current or
I2C is a trademark of Philips Electronics N.V.
6
zero voltage or as “terminate charge”
alarm. Charging will also terminate if
the safety signal indicates that the
battery is not present or the battery is
too hot to charge safely.
The LTC1759 is a complete Level 2
Smart Battery Charger. It is able to
autonomously charge a Smart Battery by receiving and interpreting
commands over its built-in SMBus
interface. The LTC1759 adheres to all
the safety requirements of the Smart
Battery Charger Specification, including 3-minute timers that protect from
SMBus communication failures and
overcharging of Li-Ion batteries during wake-up mode—features absent
from some competing solutions. Hardware-programmable current and
voltage limits provide an additional
level of protection that cannot be
altered by errant software.
The LTC1759 manages all the complexities of a Smart Battery Charger
System. This is appealing to those
who wish to support Smart Batteries
without getting involved in all the
details. SBC compliance, safety,
output voltage accuracy, SMBus
accelerators and LTC’s patented wall
adapter current limiting are just a few
of the features that make this an
outstanding part.
LTC1759 Smart Battery
Charger Features
The LTC1759 merges the intelligence
of a Smart Battery Charger with a
constant-current (CC), constant-voltage (CV), current mode switching
battery charger circuit. The LTC1759
incorporates the following features:
❏ 0.5% output voltage accuracy at
room temperature, 1% over
temperature range
❏ 5% output current regulation
❏ An external, resistor-programmable voltage limit, with four
ranges that support stacking of
the popular 4.2V battery cell
❏ An SMBus programmable output
voltage, from 2.465V to 21V in
either 16mV or 32mV granularity,
depending upon the programmed
voltage range (10-bit resolution)
❏ An external, resistor-programmable current limit with four
limits: 1A, 2A, 4A and 8A
❏ An SMBus programmable output
current with 10-bit resolution
over all ranges
❏ LTC’s patented programmable AC
wall adapter current limiting to
maximize charge rate
❏ Low VIN-to-VOUT operation
(dropout < 0.5V)
❏ 95% efficiency
❏ Compliant with Smart Battery
Charger Specification Rev. 1.0,
Level 2
❏ Low power consumption when AC
is not present, while remaining
compliant with all Smart Battery
Charger requirements for status
and interrupts
❏ Built-in SMBus accelerators
(similar to the LTC1694)
❏ New 36-pin narrow SSOP
package (0.209˝ wide)
Circuit Description
The LTC1759 is composed of a synchronous, current mode, PWM
step-down (buck) switcher controller,
a charger controller, two 10-bit DACs
to control charger parameters, a
thermistor Safety Signal decoder,
hardware voltage and current limit
decoders and an SMBus controller
block (refer to Figure 1).
The Smart Battery or system
controller programs both constantcurrent (CC) and constant-voltage (CV)
limit values though commands over
the SMBus interface. The buck converter uses N-channel MOSFETs for
switches, allowing low cost, high
efficiency operation. It also provides
reverse battery discharge protection
and ultralow dropout operation. A
Linear Technology Magazine • November 1999
DESIGN FEATURES
VCC
8V
UV 7
8 INFET
+
+
–
–
0.2V
+
BAT1 31
6.7V
–
VCC 32
1.2V
1 BOOST
–
SDB 5
+
PWM
LOGIC
1.3V
ONE
SHOT
SYNC 4
2 TGATE
SHDN
200kHz
OSC
3 SW
8.9V
34 GBIAS
S
35 BGATE
Q
R
36 PGND
SLOPE COMP
33 BOOSTC
–
+
B1
+
C1–
+
CA2–
VC 27
CLN 10
VREF
1k
CA1–
28 PROG
75k
–
+
VA
+
–CL1
AC_PRESENT
COMP1 11
21 DCDIV
22 DCIN
1V
10µA
–
PWR_FAIL
INTB 13
+
20k
812.5k
23 BAT2
65k
SDA 14
72k
10-BIT
VOLTAGE
DAC
CHGEN 12
SCL 15
612k
26 VSET
CHARGER
CONTROLLER
RNR 20
VREF
290k
–
VDD
THERM 19
29 SENSE
BAT1
+
CLP 9
92mV
+
30 SPIN
+
AGND 6
THERMISTOR
DECODER
SMBus
CONTROLLER
LIMIT
DECODER
13
10-BIT
CURRENT
DAC
DGND 18
24 ILIMIT
25 VLIMIT
17 ISET
16 VDD
Figure 1. LTC1759 block diagram
thermistor safety-detection circuit
isused to detect the presence of a battery and determine whether the temperature of the battery allows safe
charging to occur. Linear Technology’s
patented input current limiting feature is implemented, allowing the
fastest battery charge times without
overloading the wall adapter.
When a constant current value is
received via an SMBus transmission,
it is scaled and limited to a value
below that programmed by the RILIMIT
resistor. This modified value programs
the current DAC, setting the DC charging current. The current DAC is a
Linear Technology Magazine • November 1999
10-bit delta-sigma DAC that sinks
current from the PROG pin when
charging current is desired (refer to
Figure 2). Amplifier CA1 senses the
voltage drop across RSENSE and forces
this voltage across RS2 (200Ω); the
current through RS2 is sent through a
current mirror as a pull-up current
on the PROG pin. The matching of
current through RS2 with current from
the PROG pin by CA2 implements
constant-current operation. Since the
delta-sigma DAC output is a series of
pulses, a smoothing capacitor is
needed to filter the pulses into DC.
When a constant-voltage value is
received via an SMBus transmission,
the value is scaled, adjusted to cancel
offset and limited to a value below
that programmed by the RVLIMIT resistor. This modified value programs the
voltage DAC, setting the DC charging
voltage. The voltage DAC drives the
bottom of an internal voltage divider
network. The top of the voltage divider
is connected directly to the battery
output though the BAT2 pin. A voltage error amplifier, VA, compares the
divided battery voltage on the VSET
pin with an internal, precision reference voltage. The output of the VA
amp is configured as a current source
that can drive the PROG pin. The
PROG pin is a current summing node
for both current and voltage feedback
loops. The VA loop steals control of
the current feedback loop when the
battery voltage exceeds the programmed voltage, forcing the charging
current down to the level required to
maintain the programmed voltage.
Since the ∆Σ DAC output is in the
form of a series of pulses, a smoothing network is needed to filter the
pulses into DC at the VSET pin. The
capacitors C5 and C4 form a capacitance divider that provides some
filtering of the feedback voltage from
the battery while filtering the DAC
pulses.
The LTC1759 requires two power
supplies. The PWM circuitry runs
directly off the wall adapter supply
through the VCC pin, whereas the
logic functions run independently
from the VDD supply. This allows the
PWM circuitry to go into 40µA micropower shutdown mode when AC power
is removed, allowing the logic and
SMBus activity to remain alive, as
required by Intel’s ACPI standards.
This separate supply also allows the
logic and SMBus to run at 3V or 5V
depending on the system designer’s
needs. To minimize power draw of the
LTC1759 logic, the logic circuits are
driven by a clock circuit that shuts
down when there is no activity and
wakes up to service SMBus activity or
to generate interrupts. Once the
request is serviced, the LTC1759 goes
back to sleep.
7
DESIGN FEATURES
R1
15.8k
R2
1k
RCL, 0.033Ω
Q1
AC
ADAPTER
INPUT
≥17VDC
+
C15
VDD
22µF
50V 3.3V OR 5V
Al
C9
C14
0.1µF
0.1µF
7
16
4
5
12
RVLIMIT, 33k
25
RILIMIT, 33k
24
18
RSET, 3.83k
C11, 1µF
17
28
C13, 0.33µF
R4, 1.5k
C12, 0.68µF
R7, 1k
27
11
6
20
19
VDD
RWEAK
475k
14
RNR
10k
RUR
1k
15
13
LTC1759
UV
DCIN
VDD
DCDIV
SYNC
INFET
SDB
VCC
CHGEN
CLP
VLIMIT
CLN
ILIMIT
TGATE
DGND
BOOSTC
ISET
GBIAS
PROG
BOOST
VC
SW
COMP1
AGND
BGATE
SPIN
RNR
SENSE
THERM
BAT1
SDA
BAT2
SCL
VSET
INTB
PGND
22
21
C2
0.47µF
8
R3
499Ω
SYSTEM
POWER
C1
1µF
32
C16
22µF
9
10
2
C5, 2.2µF
C4, 0.1µF
Q2
33
34
1
D2
D2
C6
0.68µF
L1
15µH
RSENSE
0.025Ω
+
Q3
3
D1
C3
22µF
SMART
BATTERY
35
30
29
RS1, 200Ω
31
RS2, 200Ω
23
R6
68Ω
26
36
C8
0.047µF
C7
0.015µF
INTB
SCL
SDA
D1: MBRS130LT3
D2: FMMD7000
L1: SUMIDA CDRH127-150
SMBus
TO
HOST
Q1: Si3457DV
Q2, Q3: Si3456DV
Figure 2. A complete 4A Smart Battery Charger
Shutdown of the PWM through the
CHGEN–SDB pin combination occurs
when the AC power is lost or the
battery is removed. The LTC1759
detects the AC loss through the DCDIV
pin. This threshold is usually set just
below the lowest valid voltage of the
wall adapter. AC power status may be
read by the system over the SMBus.
The UV pin is only used to put the
PWM circuitry into micropower shutdown and is connected directly to the
wall adapter supply.
Inductor selection is not critical
with the design, since the loop
response of the charger is intentionally set to be very slow. Almost any
value will work, with a practical lower
limit of about 15µH. Lower inductance will create higher ripple
currents, requiring a lower ESR
capacitor on the output. It will also
cause cosmetically ugly discontinuous switching operation to occur at
higher currents than necessary.
Output capacitor selection is not
ESR critical but must be able to handle
all of the ripple current from the
charger. Do not count on the battery
to carry the ripple current because
the effective impedance as seen by
8
the charger can be much greater than
the ESR of the capacitor. Many battery
packs have built-in series-protection
MOSFETs that raise the ESR of the
battery. There may also be optional
power-routing MOSFETs in series
with the battery in multiple-battery
configurations, further increasing the
battery ESR. From the charger point
of view, the output capacitor ESR can
be as high as 1Ω, allowing a wide
range of capacitor options. When
using a resistive or electronic load,
some instability may occur. This can
be fixed by adding a temporary 300Ω
resistor in series with the PROG pin
capacitor or putting a 10µF capacitor
on the output. Avoid using ceramic
capacitors in the output because they
tend to make noise when the switcher
goes discontinuous and starts to drop
cycles at audible frequencies under
very light load currents—use tantalums instead. Input capacitance
selection is driven by the input ripple
current of the charger, which is usually 1/2 of the maximum output
current. For a 4A charger, a 22µF,
50V ceramic is recommended, since
this part can typically handle 2A of
ripple current. It also takes up the
least amount of space and can cost
less than other capacitor options.
Current protection, from battery to
wall adapter, is provided by a
P-channel MOSFET (Q1). A voltage
comparator monitors the voltage
across the MOSFET and will turn it
off when the wall adapter drops to
less than 200mV above the battery
voltage. Although an inexpensive
diode could be used instead of this
MOSFET, the MOSFET only adds
100mV to the already low 0.4V dropout mode of operation without
producing extra heat. During startup without a battery, the MOSFET
parasitic diode is used to allow wall
adapter power to reach the VCC pin
and power up the PWM control
circuitry.
Primary compensation is done on
the PROG pin; however, DAC pulse
filter requirements determine the
effective value of the capacitor. Pulse
ripple current must be less than 20mV
or loop jitter will occur, giving the
appearance of loop instability at light
charging currents. The V C pin
capacitor’s primary function is to provide soft-start support. There must
always be a resistor of 1.5k in series
Linear Technology Magazine • November 1999
DESIGN FEATURES
RVLIMIT
Nominal Charging Voltage (VOUT) Range
Granularity
0
2465 < VOUT < 8432mV
16mV
10k
2465 < VOUT < 12,640mV
16mV
33k
2465 < VOUT < 16,864mV
32mV
Both SCL and SDA have dynamic
pull-up circuits that improve the rise
time on systems with significant
capacitance on the two SMBus signals. The dynamic pull-up circuitry
detects a rising edge on SDA or SCL
and applies 2mA–5mA pull-up to VDD
for approximately 1µs (Figure 3). This
action allows the bus to meet SMBus
rise-time requirements with as much
as 150pF on each SMBus signal. The
improved rise time will benefit all of
the devices that use the SMBus line,
especially devices that use the I2C
logic levels.
100k
2465 < VOUT < 21,056mV
32mV
AC Adapter Current Limiting
Open or tied to VDD
2465 < VOUT < 32,768mV
32mV
Wall adapters are typically AC/DC
converters with 20V output at 3A–4A
of load current. When a notebook is
running, all of the available current
from the wall adapter may be
consumed by the system, leaving no
power for charging the battery. However, as soon as the system’s power
requirements drop below the wall
adapter’s current limit, battery charging can resume. In order to recharge
the battery in the shortest time possible, the recharging should start as
soon as there is any current leftover
from the system. The ideal situation
is when the sum of battery charging
current and the system current is
just below the wall adapter’s current
limit. The LTC1759 incorporates a
patented battery charger input current-limiting function that allows the
charger current to be automatically
Table 1. ILIMIT trip points and ranges
RILIMIT
Nominal Charging Current Range
Granularity
0Ω
0 < I < 1023mA
1mA
10k
0 < I < 2046mA
2mA
33k
0 < I < 4092mA
4mA
Open (>250k) or shorted to VDD
0 < I < 8184mA
8mA
Table 2. VLIMIT trip points and ranges
with the VC pin capacitor to allow
proper shutdown.
From a thermal standpoint, the
output voltage remains approximately
0.5% accurate over the battery temperature charging range. This higher
precision allows a higher charge
capacity in the battery, and, more
importantly, will cause fewer problems with voltage-based charge
termination circuitry in the battery.
SMB Alert
The SBS standards allow for the option
of an open-collector interrupt line to
notify the host when a critical power
event has occurred. This feature is
called SMBALERT#. The LTC1759
implements this feature by asserting
the INTB line low when AC power is
lost or restored and when a battery is
physically installed or removed. INTB
is cleared when the host reads the
LTC1759 status register or performs
a successful read of the SMBALERT#
Response address of the LTC1759.
Setting Safe Voltage
and Current Ranges
The LTC1759 voltage/current ranges
are programmed with two external
resistors, RVLIMIT and RILIMIT, as shown
in Tables 1 and 2. These limits prevent
communication errors or errant software from causing the charger to
damage the battery. At the same time,
the variable granularity allows for
better control of voltage and current
Linear Technology Magazine • November 1999
in the lower ranges. The voltage limits
are ROM mask programmable.
SMBus Acceleration
Unlike the I2C bus, which allows the
use of variable pull-up currents on
the bus signals, the SMBus pull-up
current is specified as a maximum of
350µA. In larger systems, the capacitance load on the SMBus can cause
rise-time violations (TRISE > 1µs),
which could result in a communication failure. This is especially the case
when I2C devices are mixed with
SMBus-compliant devices on the same
bus. The thresholds of the I2C bus
receivers are generally higher than
their SMBus cousins and are more
sensitive to slow rise times.
WITH
ACCELERATOR
WITHOUT
ACCELERATOR
Figure 3. SMBus accelerator operation (RPULLUP = 15k, CL =150pF, VDD = 5V)
9
DESIGN FEATURES
reduced to avoid overloading the wall
adapter, yet still charge the battery
with the maximum available current.
Table 3. Safety signal resistance ranges
Safety Signal Resistance
ChargerStatus Bits
SAFETY_UR = 1
SAFETY_HOT = 1
BATTERY_PRESENT = 1
SAFETY_HOT = 1
BATTERY_PRESENT = 1
All Safety Bits Clear
BATTERY_PRESENT = 1
SAFETY_COLD = 1
BATTERY_PRESENT = 1
SAFETY_OR = 1
SAFETY_COLD = 1
BATTERY_PRESENT = 0
0Ω–500Ω
500Ω–3k
3k–30k
30k–100k
>100k
VDD
VDD
RWEAK
475k
1%
Description
Improved
Safety Signal Sensing
Underrange
Hot
Ideal
Cold
Overrange
LTC1759
VDD
RNR_SELB
RUR_SELB
RNR
VDD
RNR
10k RUR
1% 1k
1% THERM
+
SAFETY_COLD
–COLD
RTHERM
+
TOTAL
PARASITIC
CAPACITANCE
MUST BE LESS
THAN 75pF
–UR
+
SAFETY_UR
THERM
LATCH
SAFETY_HOT
–HOT
+
HYSTERESIS
SAFETY_OR
–OR
Figure 4. LTC1759 safety-signal-monitoring circuitry
TESTING RTHERM = 33k
WITH RNR = 10k
TESTING RTHERM = 33k
WITH RWEAK = 475k
Figure 5. Testing a cold thermistor
10
RTHERM IS NOT
TESTED WITH
RUR SINCE IT
TESTED COLD
The Safety Signal in most Smart Batteries is a resistor or thermistor to the
battery’s negative terminal. The SBC
must sense the resistance of the Safety
Signal to ground and determine if the
battery is connected and whether it is
safe to charge. The SBC must report
the status of the Safety Signal during
an SMBus read of the ChargerStatus()
register. Table 3 shows the five ranges
of resistance and what the ChargerStatus() bits must indicate.
The LTC1759 monitors the safety
signal using a state machine to control the thermistor sensing scheme of
Figure 4. This approach allows the
LTC1759 to conserve power while
supporting battery-presence detection and safety signal reporting when
AC is not present. It also provides
high noise immunity at the underrange-to-hot trip point.
The state machine sequentially
switches RWEAK, RNR and RUR to pullup against the battery’s internal
thermistor. The resulting voltage is
monitored by the comparators and
used to determine the thermistor’s
operating range. The state machine is
able to sample the safety signal with
all three resistors in 100µs. This
allows the thermistor to be read during an SMBus read requesting Safety
Signal status and then shut down to
conserve power. A system using a
fixed 10k pullup for all ranges will
waste current when AC is not present.
RWEAK is used to continuously monitor battery presence; it uses very little
current and allows detection of the
insertion or removal of a battery
regardless of whether or not AC is
present. RNR is used to determine if
the safety range is cold or ideal. RUR is
used to determine if the safety range
is hot or underrange. The testing of
RTHERM is shown for a cold and underrange thermistor in Figures 5 and 6,
respectively. When AC is present, the
state machine continuously tests the
continued on page 18
Linear Technology Magazine • November 1999
DESIGN FEATURES
VIN = 6V
VIN 6V–4V STEP
5V/DIV
VSW
5V/DIV
VSW
5V/DIV
IL
0.5A/DIV
IL
500mA/DIV
VO
50mV/DIV
VO
0.1V/DIV
0.5ms/DIV
2µs/DIV
Figure 13. Transient response of the circuit
in Figure 10 with step input (4V–6V)
Figure 12. Continuous conduction mode switching
waveforms in step-down mode; VIN = 6V, VO= 5V
Conclusion
90
VIN = 4.8V
The LT1306 is a complete synchronous boost DC/DC converter offering
a set of features that few competing
devices are able to match. The unique
rectifier design results in a boost/
step-down converter that disconnects
the load in shutdown and controls
input current during startup.
85
EFFICIENCY (%)
The continuous-conduction mode
switch-node voltage and the inductor
current for step-down operations (Figure 12) are contrasted with those of
boost operation in Figure 11. Note
that in step-down mode, when the
rectifier is conducting, the switch
voltage exceeds VIN. Input step (from
4V to 6V) transient response is illustrated in Figure 13. The converter
efficiency is plotted in Figure␣ 14.
80
VIN = 3.6V
75
70
VIN = 6V
65
VO = 5V
60
1
Authors can be contacted
at (408) 432-1900
10
100
1000
LOAD CURRENT (mA)
10,000
Figure 14. Efficiency of Figure 10’s circuit
Smart Battery, continued from page 10
thermistor every 100µs. When AC is
not present, RNR and RUR thermistor
testing occurs only when a battery is
first inserted or removed or during a
transmission requesting Safety Signal status.
The underrange detection scheme
is a very important feature of the
LTC1759. As can be seen from Figure
6, the RUR/RTHERM trip point of 0.333
• VDD (1V) is well above the 0.047 •
VDD (140mV) threshold of a system
using a 10k pull-up for all ranges. A
system using a 10k pull-up would not
be able to resolve the important
underrange-to-hot transition point
with a modest 100mV of ground offset
between the battery and thermistordetection circuitry. Such offsets are
anticipated when charging at normal
current levels.
Conclusion
The LTC1759 complies with the Smart
Battery Charger standard published
by the Smart Battery System organi18
TESTING RTHERM = 420Ω
WITH RUR = 1k
TESTING RTHERM = 420Ω
WITH RNR = 10k
TESTING RTHERM = 420Ω
WITH RWEAK = 475k
Figure 6. Testing an underrange thermistor
zation, in which Linear Technology is
a promoter and voting member. The
charger controller also complies with
Intel’s ACPI standard by being able to
respond to system commands even
when there is not AC wall adapter
power. The charger offers the widest
current and voltage range of opera-
tion compared to competitive parts.
Feature for feature, it also offers the
highest integration possible today with
a Smart Battery Charger. The
LTC1759 achieves significant cost
savings, performance and safety
advantages over other Smart Battery
Chargers currently available.
Linear Technology Magazine • November 1999