UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
1
www.ti.com
D
Precision 4.1 V Reference (1%)
D
High-Efficiency Battery Charger Solution
D
High-Side or Low-Side Switch-Mode
Current-Sensing
D
Average-Current-Mode Control from Trickle
to Overcharge
D
Resistor-Programmable Charge Currents
D
Internal State Logic Provides Four Charge
States
D
Programmable Overcharge Time
D
CHG Pin Initiates Charging
D
Output-Status Bits Report Charge State
description
The UCC3956 family of switch-mode lithium-ion battery-charger controllers accurately control lithium-ion battery
charging with a highly-efficient average-current-control loop. This chip is designed to work as a stand-alone charger
controller for a single-cell or multiple-cell battery pack. This chip combines charge-state logic and average-current
PWM control circuitry with a 14-bit counter to program the overcharge time. The charge-state logic indicates current-
or voltage-control depending on the charge state. The chip includes undervoltage lockout (UVLO) circuitry to ensure
sufficient supply voltage is present before output switching starts. Additional circuit blocks include a
differential-current-sense amplifier, a 1% voltage reference, voltage- and current-error amplifiers, PWM latch,
charge-state decode bits, and a 500-mA output driver.
absolute maximum ratings over operating free-air temperature range (unless otherwise noted)
Input voltage (VDD, OUT)
20 V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output current sink
Continuous
120 mA
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Peak
600 mA
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output current source
Continuous
120 mA
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Peak
600 mA
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CS+, CS-
Voltage
0.5 to VDD
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current with CS+, CS- less than -0.5
50 mA
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Remaining pin voltages
-0.3 V to 6 V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage temperature range, T
stg
-65
C to 150
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operating virtual junction temperature range, T
J
-55
C to 150
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lead temperature (soldering, 10 seconds)
300
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ESD Rating (human body model, HBM)
500V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under "recommended operating conditions" is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
Unless otherwise indicated, voltages are reference to ground and currents are positive into and negative out of the specified terminals. Consult
Packaging Information section of the Portable Products Databook (TI Literature No. SLUD001) for thermal limitations and considerations of
packages. All voltages are referenced to GND.
Copyright
2000, Texas Instruments Incorporated
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
CHGENB
IMIN
CS-
CS+
CHG
STAT1
STAT0
REF
VDD
OUT
VA+
VA-
VAO
IBAT
CA-
CAO
CTO
RSET
COSC
GND
N, DW PACKAGES
(TOP VIEW)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
2
www.ti.com
block diagram
UDG-96197-1
AVAILABLE OPTIONS
TA
PACKAGED DEVICES
TA
(N)
(DW)
0
C to 70
C
UCC3956N
UCC3956DW
The DW package is available taped and reeled. Add TR suffix to device type (e.g.
UCC3956DWTR) to order quantities of 3000 devices per reel.
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
3
www.ti.com
electrical characteristics over recommended operating free-air temperature range,
T
A
= 0
_
C to 70
_
C for UCC3956, COSC = 500 pF, RSET = 70 k
, CTO = 169 nF, VDD = 12 V, T
A
= T
J,
(unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
Current Sense Amplifier (CSA) Section
DC gain
CS = 0,
CS+ = 50 mV, CS+ = 250 mV
4.9
5.0
5.1
V/V
DC gain
CS+ = 0,
CS = 50 mV, CS = 250 mV
4.9
5.0
5.1
V/V
Current sense amplifier output (CAO)
CS+ = CS = 0 V
1.99
2.05
2.11
V
Common mode rejection ratio
(CMRR)
VCM = 1.1 V to 18 V,
VDD = 18 V
50
65
dB
Low-level output voltage (VOL)
CS+ = 0.2 V,
CS = 0.5 V,
IO = 1 mA
0.2
1
V
High-level output voltage (VOH)
CS+ = 0.5 V,
CS = 0.2 V,
IO = 500 mA
3.7
4.1
4.4
V
Output source current
IBAT = 3 V,
VID = 700 mV
500
A
Output sink current
IBAT = 1 V,
VID = 700 mV
500
A
3dB bandwidth
VCM = 0 V,
CS+ - CS = 100 mV,
See Note 2
0.1
3.0
MHz
Current Error Amplifier (CEA) Section
Bias current
8 V < VDD < 18 V,
CHGENB = REF
0.1
0.5
A
Current error amplifier voltage
8 V < VDD < 18 V,
CAO = CA
1.99
2.05
2.11
V
Open-loop voltage gain (AVOL)
60
90
dB
Gain bandwidth
TJ = 25
C,
F = 100 kHz
1
3
MHz
Low-level output voltage (VOL)
IO = 250
A,
CA = 3 V
0.2
1.0
V
High-level output voltage (VOH)
IO = 1 mA,
CA = 2 V
3.7
4.1
4.4
V
ICA, ITRCK_CONTROL
VCHGENB = GND
8
10
12
A
Voltage Error Amplifier (VEA)
Bias current
Total bias current;
Regulating level
0.5
3.0
A
Input offset voltage
8 V < VDD < 18 V,
VCM = 4.1 V
10
mV
Open-loop voltage gain (AVOL)
60
90
dB
Gain bandwidth
TJ = 25
C,
F = 100 kHz
0.75
3.00
MHz
Low-level output voltage (VOL)
IO = 500
A,
VA = 3.8 V
0.2
1.0
V
High-level output voltage (VOH)
IO = 500
A,
VA = 4.4 V
3.8
4.1
4.3
V
Voltage amplifier output leakage
VCHGENB = GND,
STAT0 = 0,
STAT1 = 0,
VAO = 2.05 V
1
1
A
Pulse Width Modulator Section
Maximum duty cycle
CAO = 0.5 V
92
96
100
%
Modulator gain
CAO = 1.7 V to 2.1 V
57
64
71
%/V
PWM Oscillator (OSC) Section
Frequency
7 V < VDD < 18 V
90
100
110
kHz
Overcharge Timer (OCT) Section
Frequency
7 V < VDD < 18 V
See Note 1
4.65
5.00
5.35
Hz
Reference Section
Initial accuracy
TJ = 25
C
4.06
4.10
4.14
V
Accuracy
0 < TJ < 70
C, VDD = 8 V to 18 V
4.05
4.10
4.15
V
Accuracy
40
C < TJ < 85
C, VDD = 8 V to 18 V
4.03
4.10
4.17
V
Load regulation
0 < IO < 2 mA
3
15
mV
Short circuit I
REF = 0 V
8
20
30
mA
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
4
www.ti.com
electrical characteristics over recommended operating free-air temperature range,
T
A
= 0
_
C to 70
_
C for UCC3956, COSC = 500 pF, RSET = 70 k
, CTO = 169 nF, VDD = 12 V, T
A
= T
J
,
(unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
Charge Enable Comparator (CEC) Section
Threshold voltage
1.90
2.05
2.15
V
Input bias current
0.5
0.2
A
Voltage Sense Comparator (VSC) Section
Threshold voltage
Volts below VA+
50
125
200
mV
Charge current comparator (CIC) Section
Threshold voltage
CS+ = CS = 0, function of IBAT = 2.05 V
2.00
2.05
2.10
V/V
Input bias current
Total bias current; regulating level
0.5
0.2
A
Output Stage Section
VOL
IOUT = 10 mA
0.1
0.3
V
VOH, volts below VDD
IOUT = 10 mA
0.1
0.5
V
Rise time
COUT = 1 nF
30
70
ns
Fall time
COUT = 1 nF
30
70
ns
STAT0 and STAT1 Open Drain Outputs Section
Maximum sink current
VOUT = 12 V
15
30
mA
VOL
IOUT = 1 mA
0.1
0.2
V
Charge control (CHG) Section
Threshold voltage
1.5
1.8
2.1
V
Charge-pin pull-down current
VCHG = 1 V
8
23
40
A
Undervoltage Lockout Current Section
Turn-on threshold
6.00
6.50
7.0
V
Hysteresis
75
150
400
mV
Input current
Quiescent current
5
8
mA
Undervoltage lockout current
VDD = 5 V
0.25
0.75
mA
NOTE 1: 14-bit tuner functionally tested at 500 kHz.
NOTE 2: Ensured by design, not production tested.
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
5
www.ti.com
pin descriptions
CA: The inverting input to the current-error amplifier.
CAO: The output of the current-error amplifier and inverting input of the PWM comparator. This pin is driven high
during shutdown.
CS, CS+: The inverting and non-inverting inputs to the current-sense amplifier. This amplifier has a fixed-gain of
5.
CHG: A rising-edge triggered-input pin that indicates charging. Once the internal 14-bit timer has timed out, the chip
enters its shutdown charge state. At this point, CHG is pulled low by an internal buffer. Another low-to-high transition
is required to reset the timer and restart charging.
CHGENB: The input to a comparator that detects when the battery voltage is low and places the charger in trickle
charge. The charge-enable comparator forces the output of the voltage-error amplifier to a high-impedance state
while forcing a fixed 10-
A current into the CA to set the trickle charge.
COSC: The oscillator ramp pin which has a capacitor (COSC) to ground. The ramp oscillates between 0.8 V to 3.2 V
and the frequency is determined by:
Frequency
+
3.475
(COSC
)
20 pF)
RSET
A rising edge on CHG initiates the oscillator.
CTO: The slow oscillator ramp pin, which is used to generate a clock signal for the 14-bit timer to program the
overcharge time. A capacitor (tied to ground) is charged and discharged with equal currents at a frequency
programmed between 0.75 Hz and 5 Hz. The ramp oscillates between 0.1 V and 3.0 V and the frequency is
determined by:
Frequency
+
0.06
(CTO
RSET)
The oscillator operates only while in overcharge.
GND: The reference point for the internal reference, all thresholds, and the return for the remainder of the device.
IBAT: The output of the current-sense amplifier.
IMIN: The minimum-charge-current programming pin is provided to program an optional-charge termination in
addition to the programmable timer.
OUT: The output of the PWM driver.
REF: The 4.1-V precision reference, which should be bypassed with a 0.1-
F capacitor.
RSET: This pin programs the charge current for the oscillator ramp. The oscillator-charge current is determined by:
I
COSC
+
1.37 V
RSET
The trickle-control current (I
TRCK_CONTROL
) is determined by:
I
TCK_CONTROL
+
0.68 V
RSET
(1)
(2)
(3)
(4)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
6
www.ti.com
pin descriptions (continued)
STAT0, STAT1: CMOS open-drain binary-output decode pins indicating the four different charge states. The
maximum high-voltage sense comparator.
VA: The inverting input to the voltage-error amplifier that is used as a battery-sense input. It is also the input to the
voltage-sense comparator. The bulk-charge state is completed and overcharge state is initiated when VA reaches
95% of VA+.
VA+: The non-inverting input to the voltage-error amplifier that is used as the battery-charge reference voltage.
VAO: The output of the voltage-error amplifier. The upper-output clamp of this amplifier is 4.1 V.
VDD: The input voltage of the chip. This chip is operational between 6 V and 18 V and should be bypassed with a
0.1-
F capacitor.
Table 1. Charge State Decode Chart
STAT1
STAT0
TEST CONDITION
Trickle Charge
0
0
CHGENB < 2.05 V
Bulk Charge
0
1
VA < 95% VA+,
CHGENB > 2.05V
Overcharge
1
0
VA > 95% VA+,
VIBAT < VIMIN
Overcharge (Top Off)
1
1
VIBAT > VIMIN
APPLICATION INFORMATION
The UCC3956 contains all the necessary control functions for implementing an efficient-switch-mode lithium-ion
battery charger. lithium-ion batteries are rapidly becoming the battery of choice for rechargeable portable and laptop
products. When compared to NiCd, NiMH, and lead-acid batteries, lithium-ion offers less weight and volume for the
same energy. Lithium-ion batteries do not suffer from the memory effect found in NiCd batteries. This effect, caused
by not completely discharging and charging a battery, reduces battery capacity over several charge cycles. Because
lithium-ion batteries have a high average cell voltage of around 3.6 V, they can often replace two to three nickel-based
cells.
The advantages that lithium-ion batteries offer, come at the cost of a wide operating voltage. Near zero capacity, the
cell has a typical voltage of 2.5 V. A fully-charged cell has a typical voltage of 4.1 V. Unlike many so called smart or
universal chargers, the UCC3956 is optimized for lithium-ion characteristics. In order to restore capacity quickly, the
chip features both constant-current and constant-voltage modes of operation. A programmable overcharge time,
provided by the UCC3956 timer, allows the charger to predictably restore 100% capacity to the battery.
charger operation
When CHG is transitioned from a low- to high-logic level, the chip cycles through several charge states. If the battery
voltage is severely depleted, the charger begins in a low-current trickle-charge state. When the battery voltage is
above a user-set threshold, the charger initiates a constant-current bulk-charge state. Once the battery reaches 95%
of it's final voltage, the charger enters an overcharge state. During the overcharge state, the converter transitions
from a constant-current to a constant-voltage mode of operation. Figure 2 shows typical current, voltage, and
capacity levels of a lithium-ion battery during a complete charge cycle.
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
7
www.ti.com
APPLICATION INFORMATION
charger operation (continued)
A block diagram of the UCC3956 is shown on the first page of the data sheet, while Figure 1 shows a typical application
circuit for a buck-derived switch-mode charger. The UCC3956 can be used for charging a single cell or multiple cells
in series. If more than three cells are stacked in series, however, a level-shifting gate-drive may be needed to operate
the buck switch. The application circuit charges a 1200-mAh 2-cell stack at a 1C rate.
setting the oscillator frequency
The frequency of operation for the converter is set by picking values for RSET and COSC.
f
OSC
+
3.475
(COSC
)
20pF)
RSET
The UCC3956 is capable of operating at frequencies higher than 200 kHz. However, the actual operating frequency
of the buck converter is ultimately determined by the usual tradeoffs of size, cost and efficiency. The application circuit
frequency is set at 100 kHz with COSC = 180 pF and RSET = 162 k
.
7
5
6
13
14
12
11
17
15
16
9
1
10
20
19
3
18
4
8
2
STAT0
VDD
STAT1
CHG
RSET
CTO
GND
COSC
IBAT
CAO
CA-
OUT
CHGENB
VA+
VA-
VAO
CS-
CS+
REF
IMIN
RS4
11k
10 k
0.1
F
RF4
15.0 k
CF4
1 nF
CF3
100 pF
RG1
20 k
RG2 38.3 k
0.1
F
22 k
10 k
RS3 10 k
RS2
2.21 k
0.10
F
CF1
2.2 nF
RS1 12.2 k
2 CELL
1200 mAH
0.18
0.18
180 pF
0.15
F
162 k
CHG
150 mH
2.7 k
2.7 k
VIN
10 V to 18 V
IBULK = 1.2 A
ITRICKLE = 90 mA
fOCS = 100 kHz
Timeout = 120 minutes
UDG-00127
RSENSE (See Note 1)
RSENSE (See Note 1)
Figure 1. Typical Charge Cycle Levels
NOTE: 1.
Either high- or low-side current sensing possible.
(5)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
8
www.ti.com
APPLICATION INFORMATION
trickle charge state
When the battery's voltage is below a predetermined threshold, the battery is either deeply discharged or has shorted
cells. The trickle-charge state offers a low-charging current to bring the battery up above zero capacity. In the case
of shorted cells, the trickle-charge state prevents the charger from delivering high currents during this fault condition.
Stacking several cells makes the detection of a shorted cell more difficult.
For lithium-ion batteries, the trickle-charge threshold is typically set to a value around 2.5 V per cell (this corresponds
to near zero capacity). When the cell voltage is below the threshold, only a trickle current is applied to the battery.
The threshold is established by programming CHGENB to 2.05 V when the battery (or stack) voltage is at the
threshold. Referring to the application circuit of Figure 1, the trickle-charge voltage threshold is determined by:
V
TRICKLE_THRESHOLD
+
RS1
)
RS2
)
RS3
RS3
2.05
With a trickle threshold of 5 V (for two cells) and setting RS3 to 10 k
, (RS1+ RS2) should be approximately 14.4 k
.
The applications circuit's trickle-charge current is set to about 7.5% of the bulk-charge current. The current value is
set by picking the appropriate value for RG1. Referring to the block diagram and Figure 1, during trickle charge a fixed
current (0.68/RSET) flows out of the current amplifier's inverting input and into RG1. The voltage amplifier output is
disabled during trickle charge and acts as a high-impedance node. The resulting voltage at the output of the current
sense amplifier sets the trickle charge current.
I
TRICKLE
+
RG1
7.5
RSET
RSENSE
In the application circuit the sense resistor is 0.18
and RSET is 162 k
, for a trickle current of about 90 mA a 20-k
resistor is selected for RG1.
The converter is typically designed to run in discontinuous-conduction-mode during trickle charge. This allows a
reasonably small value of inductance to be used. The average current mode of the UCC3956 provides improved
discontinuous-duty-cycle control, when compared to peak-current mode implementations.
In Figure 2, the trickle-charge state corresponds to the time interval between t0 (when CHG is transitioned from low
to high) and t1. During the trickle-charge state, STAT0 and STAT1 are logic-level lows. At time t1 the trickle threshold
is met, and the charger transitions to the bulk-charge state. In many instances, the battery voltage is initially above
the trickle threshold. In this case, the trickle-charge state is not needed.
(6)
(7)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
9
www.ti.com
APPLICATION INFORMATION
80%
100%
0%
t0
t1
t2
t3
t4
t5
95% of V FINAL
VFINAL
ITRICKLE
IOC
IBULK
TRICKLE
CHARGE
BULK CHARGE
OVERCHARGE
TOPOFF
Battery
Capacity
Battery V
oltage
Battery Current
V
TRICKLE_THRESHOLD
Time
Figure 2. Typical Charge Cycle Levels
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
10
www.ti.com
APPLICATION INFORMATION
bulk-charge state
As the name implies, the bulk-charge state is responsible for restoring a majority of the charge back into the battery.
The bulk-charge current is determined by the C-rate and the capacity of the battery. In the application circuit, two
stacked 1200-mAh batteries are charged at a 1C rate. This requires 1.2 A of current during bulk charge. In this case,
a fully-discharged battery takes about 60 minutes to reach approximately 80% capacity. Battery packs with a high
ESR typically have a shorter bulk period, due to the voltage drop generated by the bulk current and the ESR of the
battery.
Both the voltage-sense amplifier and current-sense amplifier are enabled during bulk charge. The voltage amplifier
is saturated in this state as the battery voltage is slowly rising, but is not yet high enough to drive the voltage amplifier
into regulation. The voltage amplifier output is clamped at a nominal voltage of 4.1 V. The current-sense amplifier
is configured so that its output voltage increases with decreasing RSENSE current. RSENSE should be sized such
that the output voltage of the current-sense amplifier (V
IBAT
) is within specification during bulk charge.
V
IBAT BULK
+
2.05
*
5
IBULK
RSENSE
With 1.2 A of bulk current and the current sense amplifier output set at 1 V, a sense resistor of 0.18
is required. As
always, power dissipation and converter efficiency must be considered when choosing RSENSE.
Referring to the feedback diagram of Figure 3, the output of the voltage-sense amplifier and current-sense amplifier
are summed at the inverting input of the current amplifier. Assuming that the current-sense amplifier is within
regulation, the required value of RG2 can be calculated. The application circuit uses a value of 38.3 k
for RG2,
setting the bulk current to 1.2 A.
RG2
+
2.05
RG1
5
IBULK
RSENSE
Referring to Figure 2, the bulk-charge state corresponds to the interval between t1 and t2. The step-in voltage at time
t1 is caused by bulk current flowing into the battery ESR and sense resistor. In the bulk-charge state, STAT0 is a
logic-level high and STAT1 is a logic-level low.
UDG-96263-1
Figure 3. Typical Charge Cycle Levels
(8)
(9)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
11
www.ti.com
APPLICATION INFORMATION
overcharge state
The overcharge state of the converter starts when the battery reaches 95% of its final voltage (time t2 of Figure 2).
The overcharge state is initiated when the voltage at the inverting input of the voltage amplifier is 95% of the
non-inverting-input voltage. Using 95% rather than 100% of the final battery voltage assures that the overcharge timer
is always set before the battery current tapers off. At the beginning of overcharge state, STAT0 indicates a logic-level
low and STAT1 indicates a logic-level high.
In the application circuit of Figure 1, the voltage at which overcharge is initiated is set by resistors RS1, RS2 and RS3.
These resistors are also used to set the trickle-charge threshold. A 0.1-
F decoupling capacitor is added to this node
as a filter. The battery (or stack) voltage that initiates the overcharge state is:
VOC_THRESHOLD
+
0.95
RS1
)
RS2
)
RS3
RS2
)
RS3
4.1
For a single-cell stack, RS1 should be 0
. This results in a final battery voltage of 4.1 V. It is important not to charge
a lithium-ion battery above 4.2 V. When charging a battery stack, RS1 should be selected to properly set the
final-stack voltage. In the application circuit, RS1 is selected to be 12.21 k
and RS2 is selected to be 2.21 k
. This
sets the overcharge level at 8.2 V, while setting the trickle-charge threshold to about 5 V.
The battery voltage at the beginning of the overcharge state may not correspond to the voltage amplifier coming out
of saturation. Therefore, bulk current may continue in the battery during the initial portion of the overcharge state (see
Figure 2). When the voltage amplifier comes into regulation, the amplifier's output voltage begins to decrease. The
current-sense amplifier's output voltage needs to increase in order for the current amplifier's inverting input to remain
at 2.05 V. This translates into a decreasing battery current. The battery current continues to decrease as the battery
approaches 100% capacity.
Although the bulk-charge state restores a majority of the capacity to the battery, the overcharge state typically takes
a majority of the charge-cycle time. The bulk-charge state usually takes one-third of the total charge time, while the
overcharge state takes the remaining two-thirds. Different methods are used to terminate the charge of lithium-ion
batteries. Many chargers use a current threshold to terminate charge. While this method is simple to implement, the
current tail near the end of charge is often quite flat (see Figure 2). To make matters worse, the current level versus
battery capacity may differ from cell to cell. This makes it difficult to accurately terminate at 100% capacity. In order
to avoid the possibility of overcharging the battery, the design may require termination at a higher current level (before
100% capacity is reached). A more predictable method of charge termination is to use a fixed overcharge time.
The UCC3956 provides current-level detection as well as a timer. In a typical design, the current-level detection is
used to give an indication of near-full charge. As shown in Figure 2 this occurs at time t4. This indication is useful since
the time to charge from t4 to t5 may be quite long. Since lithium-ion batteries have no memory effect, there is little
reason to have the user wait for the battery to be 100% charged. If the battery is not taken from the charger at time
t4, the charger continues charging. The timer expires and the charge cycle terminates at time t5.
A typical value used to indicate near-full charge is one-tenth of the bulk current value. This current level is established
by setting the appropriate voltage on IMIN. IMIN is tied to an internal comparator along with the output of the current
sense amplifier. When the current sense amplifier voltage becomes greater than the voltage on IMIN, the internal
state machine indicates near full charge by setting STAT0 and STAT1 to logic level highs. In the application circuit
of Figure 1, resistors RS4 and RS5 determine the voltage at IMIN. With RS4 at 11 k
and RS5 at 10 k
, near full
charge is indicated at 120 mA.
V
IMIN
+
4.1
RS5
(RS4
)
RS5)
I
NEAR_FULL
+
2.05
*
V
IMIN
5
R
SENSE
(10)
(11)
(12)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
12
www.ti.com
The UCC3956 timer has a 14-bit counter that allows long overcharge times with reasonable component values. As
previously stated, the charger continues charging the battery until the timer expires (unless the battery is pulled from
the charger). As seen in Figure 2, the timer starts at time t2 and expires at time t5. The frequency of the timer can
be determined as follows:
f
TIMER
+
0.06
RSET
CTO
With a 14-bit counter the time-out period in minutes becomes:
TIMEOUT
+
4550
CTO
RSET
In the application circuit, a value of 0.15
F is used for CTO to give 120 minutes of overcharge (more than twice the
bulk-charge time). When the timer expires, CHG is pulled low by an internal buffer and the charge cycle terminates.
If tied to a bidirectional port, CHG can be read by a microprocessor.
inductor sizing
For good efficiency, the inductor should be sized to give continuous current in the bulk-charge state. For a buck
converter, duty-cycle in continuous mode is given by:
D
+
V
BATTERY
)
V
SCHOTTKY
V
INPUT
)
V
SCHOTTKY
Allowing a 25% ripple in the bulk current yields a reasonable value of inductance. The inductor value can be
calculated as follows:
L
+
4
V
INPUT
*
V
BAT
D
IBULK
f
OSC
A 150-
H inductor is used in the application circuit.
current control loop
The UCC3956 features an outer-voltage loop and an inner-average current loop. The virtues of average-current
mode control are well documented in Reference [1]. A simplified block diagram of the feedback elements is provided
in Figure 3. The network for the current amplifier can be as simple as a single capacitor, providing a dominant-pole
response, which may be adequate for a battery-charger application. The current-amplifier network of shown in Figure
3 provides improved transient performance. The component values for CF3, CF4, and RF4 are selected to give a
constant gain from approximately f
OSC
/10 to f
OSC
. At frequencies below f
OSC
/10, the network gain increases at 20
dB/decade, giving a high dc gain. The network attenuates at 20 dB/decade above the switching frequency, giving
noise immunity.
A feedback design that optimizes transient response has the amplified inductor current down-slope approach the
PWM saw-tooth slope [1]. This occurs by designing the total-loop gain to cross unity at one-third to one-sixth of the
switching frequency. The applications circuit is designed to cross unity gain at one-tenth of the switching frequency
(10 kHz), with a 12 V nominal input. The power stage small-signal gain can be approximated by:
G
POWER_STAGE
+
VIN
RSENSE
SL
)
RSENSE
)
ESL
(13)
(14)
(15)
(16)
(17)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
13
www.ti.com
APPLICATION INFORMATION
current control loop (continued)
Referring to Figure 3, the current-sense amplifier provides a gain of 5 db, an inverting stage adds a gain of 1.5 db,
and the modulator has a gain of 0.64 db; adding a fixed gain of 4.8 db to the power stage. The current amplifier's gain
between f
OSC
/10 and f
OSC
is equal to RF4 divided by the parallel combination of RG1 and RG2 times the resistive
divider RG2/(RG1+RG2), simplifying to:
GCA
+
RF4
RG1
RF4 is selected to be 15 k
, resulting in a 10-kHz crossover frequency. Once RF4 is determined, CF3 and CF4 can
be selected to give corner frequencies at f
OSC
/10 and f
OSC
respectively.
CF3
+
1
2
p
f
OSC
RF4
CF4
+
10
2
p
f
OSC
RF4
In the applications circuit, a value of 100 pF is used for CF3 and 1.0 nF is used for CF4. Figure 4 shows the
power-stage gain and feedback-network gain for the current loop. Figure 5 shows the total open-loop gain and phase.
adding the voltage-control loop
The voltage loop begins to regulate during the overcharge period of operation. The output of the voltage amplifier
begins to decrease, demanding less current to the battery. With the current loop closed, the power-stage gain of the
voltage loop is equal to 1
5
R
SENSE
out to the crossover frequency (10 kHz). In order to avoid interactions
with the current loop, the voltage loop will cross unity at 2 kHz. The voltage loop is attenuated by the divider
RG1 (RG1
)
RG2) . A single pole network is added to the voltage amplifier, giving a high gain at dc. Referring to
Figure 3, the voltage-amplifier gain is equal to the impedance of CF1 divided by RS1. A 2.2-nF capacitor gives a
total-crossover frequency near 2 kHz. Figure 6 shows the gain of the power and feedback stages for the voltage loop.
Figure 7 shows the total gain and phase of the voltage loop.
(18)
(19)
(20)
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
14
www.ti.com
TYPICAL CHARACTERISTICS
Figure 4
f - Frequency - MHz
10
100
1 k
10 k
100 k
100M
-60
-40
-20
0
20
40
60
80
Current Loop Power Stage and Feedback Gain
vs
Frequency
Gain -
d
B
Power Stage Gain
Feedback Gain
Figure 5
f - Frequency - MHz
10
100
1 k
10 k
100 k
100M
-80
-60
-40
-20
0
20
40
60
80
100
120
Current Loop Total Gain and Phase
vs
Frequency
Total Phase
Gain - dB
Total Loop Gain
Figure 6
f - Frequency - MHz
10
100
1 k
10 k
100 k
1 M
-60
-40
-20
0
20
40
60
80
Voltage Loop Power Stage Gain
vs
Frequency
Gain -
d
B
Feedback Gain
Power Stage Gain
Figure 7
f - Frequency - MHz
10
100
1 k
10 k
100 k
1 M
-150
-100
-50
0
100
50
Voltage Loop Total Gain and Phase
vs
Frequency
Total Phase
Total Loop Gain
UCC3956
SWITCH MODE LITHIUM ION
BATTERY CHARGE CONTROLLER
SLUS249B - FEBRUARY 1997 - REVISED DECEMBER 2001
15
www.ti.com
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