U-504 Using the bq2000/T to Control Fast Charge

U-504 Using the bq2000/T to Control Fast Charge
Using the bq2000/T
to Control Fast Charge
4.
Introduction
The bq2000/T are programmable, monolithic ICs for
fast-charge management of nickel cadmium (NiCd), nickel
metal-hydride (NiMH), and lithium-ion (Li-Ion) batteries in
single- or multi-chemistry applications. This application
note discusses simple ways to select all necessary components to implement various switch-mode topologies. It also
discusses how to configure the bq2000/T inputs to accommodate different application concerns. Please review the
bq2000 and bq2000T data sheets before using this application note.
V SLP is the sleep-mode threshold and approximately 1.0V below Vcc. If the BAT input is driven
to this threshold, power to the IC is turned off and
the MOD output is driven low. This threshold is internally provided for implementations in which the
IC must remain connected to the battery when
charge power is removed. Under this sleep mode,
the IC draws 1µA or less.
The operation of these four thresholds determines the following design guidelines for configuring the BAT input
To charge a fixed number of nickel-based cells, the BAT
input is configured for the voltage of a single cell. Thus
for an N-cell pack the resistor ratio of the divider is
(N-1):1. See Figure 1. This configuration assures a 3mV
PVD sensitivity for fast charge, which is an excellent termination criterion for NiMH cells and good for most
NiCd cells. Of course the total divider network represents a load on the battery when power is not present
and is sized accordingly. Also, the BAT input must not be
driven with more than 20µA in the absence of power.
(This is the permissible limit for the substrate diode that
clamps this input to Vcc.)
Basic Charge-Control Operation
Charge Initiation
The bq2000/T initiates a charge on either (1) power-up or
(2) excursion at the BAT input from above a VRCH threshold to below it. The VRCH threshold is below the threshold of voltage regulation, VMCV, and therefore does not
initiate charge of a “full” Li-Ion battery. This feature is especially useful if cell polarization is taken into account
because the battery voltage decays to a lower value following fast charge.
Configuring the BAT Input
BAT+
2
The BAT input to the bq2000/T is the input to an A/D
converter with a resolution of about 3mV. A/D measurement is performed only when the timing oscillator and
current regulator have been switched off. The BAT input
has the following four voltage thresholds:
1.
2.
3.
VSS
bq2000
4
VLBAT (approximately 1.0V) is the minimum qualified input voltage to initiate full current to the battery at the start of fast charge. Below this level, the
bq2000/T follows its pulse-trickle algorithm. This
arrangement enables it to “wakeup” a pack protector in a Li-Ion pack or trickle up a deeply discharged nickel-based chemistry pack.
(N-1) * R
BAT
R
0.1µF
100kΩ
2000fig1.eps
VRCH (1.9V) is the battery replacement threshold.
As described above, an excursion through this
threshold triggers battery replacement and
reinitiates of fast charge.
Figure 1. Battery Voltage Divider for
Nickle Chemistry—Single Pack
VMCV (2.0V) is the threshold of voltage regulation.
Above this level, the MOD output is forced low, regardless of the condition of the SNS input, resulting
in a pulsed current regulation similar in operation
to a bang-bang type voltage regulator. When the
BAT input voltage exceeds 2.0V more than 85% of
the time, charge is terminated.
Charging multi-cell packs of nickel-based chemistry and
some high-capacity NiCd packs often requires compressing the batteries’ signal voltage into the range of the A/D
converter to accommodate the cell range or to require a
steeper negative slope on the battery as a criterion of ter-
SLUA064B–DECEMBER 2004
1
Using the bq2000/T to Control Fast Charge
4.
mination. This compression is accomplished by a single
additional resistor in the divider chain from the battery,
R1. See Figure 2. This extra resistor adds offset to the
battery-divider voltage, allowing a larger voltage excursion on the battery for a smaller excursion at the BAT input. The fixed in-circuit voltage to which this additional
resistor is tied is Vcc.
RBAT = (VEXCURSION - 1) ∗ REQ
where RBAT is the resistor connected between BAT+ and
the offset point. See Figure 2.
The design procedure for the varying pack size is as follows:
1.
Determine the difference between the end-of-charge
voltage for the maximum number of cells and the
start-of-charge voltage for the minimum number of
cells. This signal, which must be compressed into
the A/D voltage window of 1–2V, is VEXCURSION. For
a 1V window, the gain of the divider network is simply 1/VEXCURSION.
2.
Determine the offset, VOFF, from one of the endpoint equations by substituting the gain calculated
in step 1. The minimum condition is expressed by
the equation
For high-capacity packs, the goal is to increase the percell negative voltage excursion, which will serve as a criterion of fast-charge termination. Note: Throughout this
application note, the expression PVD is often equated
with -∆V. Although some think of these as different termination criteria, they are actually two ways of saying
the same thing. For a voltage to qualify as a “peak”, a
subsequent measurement must be less than this “peak”
measurement by a discernable amount. Since no circuit
can anticipate a subsequent reading, the peak voltage detection occurs upon measuring a subsequent voltage that
is discernibly less than the maximum voltage measured
to that point in time. In the case of the bq2000/T this sensitivity is -3.8mV, leading to the parallel drawn between
PVD and -∆V, as the later is usually just a less sensitive
PVD. The desensitization is accomplished by compressing
the total battery voltage excursion during charge into a
fraction of the A/D voltage window at the BAT input to
the bq2000/T. This design procedure is as follows:
1
 + VOFF = 1
(VBAT - VOFF) ∗ 

 VEXCURSION 
Substituting the minimum battery voltage for
VBAT and Solving for VOFF gives the voltage that
would appear at the BAT input in the absence of
the battery.
3.
1.
Determine the multiplier of the peak voltage sensitivity desired for the battery. A typical desired -∆V
value for high-capacity cells is −15mV The multiplier is then (−15mV/−3.8mV) or about 4. The gain
is then determined to be 1/(4 ∗ N), where N is the
number of cells.
2.
The offset is determined from the goal of having the
maximum battery voltage correspond to the maximum input voltage. In this example, the resulting
offset is (6 ∗ N)/(4 ∗ N-1), where N is the number of
cells. This offset is established at BAT by choosing
a resistive divider from VCC to GND. These resistors correspond to resistors R1 and R2 as shown in
Figure 2.
3.
A third resistor, RBAT, is chosen to establish the
proper gain with respect to the parallel combination of resistors R1 and R2 represented by REQ. In
the example used in step 1, the gain was 1/4N. This
is achieved by setting RBAT = REQ ∗ (4N-1).
Choose a suitable resistor divider from Vcc to GND
to establish this offset voltage.
VCC
BAT+
2
VCC
VSS
bq2000
4
R1
RBAT
BAT
VOFF = R2 * VCC
R1 + R2
REQ =
R1 * R2
R1 + R2
R2
0.1µF
Implement the gain function by determining the
equivalent resistance of the parallel combination of
the offset setting resistors, REQ, and setting
100kΩ
2000fig2.eps
Finally, Li-Ion batteries can be charged at a constant current only until their characteristic regulation voltage is
reached. The bq2000/T accommodates this restriction by
regulating the voltage at the BAT input to 2V. The battery resistive divider should ensure that the BAT input
reaches its regulation voltage when the battery reaches
its characteristic regulation voltage. See Figure 3.
Figure 2. Battery Voltage Divider for
Nickle Chemistry—Multi-Pack or
High-Capacity NiCd
2
Using the bq2000/T to Control Fast Charge
BAT+
BAT+
2
VSS
(VREG-2)R
bq2000
4
bq2000
(VREG-2) * R
BAT
NIMH
2(VREG-2)R/(2N-VREG)
BAT
2*R
100K
GND
0.1µF
2R
0.1µF
BAT-
100kΩ
2000fig3.eps
Fig4.eps
Figure 4. Voltage Regulation
Battery Divider
Figure 3. Voltage Regulation Battery Divider for Multi-chemistry Applications
Configuring the SNS Input
The safest way to design for multi-chemistry packs is to
add a mechanical connection to the nickel-based chemistry packs, constituting an additional battery negative
connection. The default Li-Ion resistive divider is attenuated by a resistor in the charger, which connects to this
point when a nickel battery is installed, but which floats
when a lithium battery is installed. This fail-safe mechanism is normally required by cell ratios of 1 lithium to 3
nickel. See Figure 3.
If the SNS input is within ±50mV of the bq2000/T ground
(VSS), the MOD output is fully enabled. If the SNS input
is greater than 50mV (V SNSHI ) or less than -50mV
(VSNSLO) compared to the ground of the bq2000/T, then
the MOD output is driven low. Since battery and temperature voltages are always measured with respect to
ground, the user can take advantage of this feature to implement both high-side and low-side current sense regulators. If no control of MOD is desired, the SNS pin can
be grounded and the MOD output used only to switch an
external current source on or off. The control feature
makes it possible to configure the SNS input to provide
either linear or switching regulation of charge current.
Furthermore, frequency is most dependent on filter components and hysteresis-setting capacitance and only
slightly dependent on power component values. This
characteristic makes it possible to “slave” the circuit to
an external oscillator of fixed frequency.
NiMH batteries rarely exceed 1.6V per cell at charge
rates consistent with lithium cells. Therefore, the single
divider network as shown in Figure 4 is acceptable for
cell ratios of 2 lithium to 5 NiMH, as long as the maximum charge voltage of the nickel-based pack is below
that of the Li-Ion pack. This design is not recommended
for NiCd batteries, however, as they will often achieve
voltages in excess of 1.8V/cell, especially toward the end
of cycle life. If the nickel-based chemistry cells do reach
the regulation voltage during charge, they usually terminate only for maximum time or temperature with the
bq2000. The bq2000T is the safest choice for the 2:5 ratio,
because it relies on ∆T/∆t as the primary means to terminate nickel-based chemistries. Ratios of 2 lithium to 4
nickel normally fit comfortably within the charge window
for their useful lives, for either voltage-based or
temperature-based termination.
Low-Side Sensing
For low-side sensing applications, the sense resistor is
placed in the path between the battery’s negative terminal and power supply ground. See Figure 5. The battery’s
negative terminal becomes the signal ground for the
bq2000/T. Signals and power applied to the bq2000/T are
capacitively decoupled and referenced to this point. The
resistor selected should have a value equal to 50mV divided by the desired charge current. Thus, at a current of
1A, the sense resistor is 50mΩ. Some applications may
3
Using the bq2000/T to Control Fast Charge
operating frequency. The hysteresis capacitor is
connected from the MOD output to SNS input directly;
however, the voltage hysteresis is attenuated by the ratio
between the two capacitors. Since the MOD output
swings 5V, the SNS input moves by the ratio 5V ∗
CHYS/(CHYS + CF). This injected hysteresis is of 5–25mV
for best results. Below is a simplified design procedure:
CHYS
Rf
1
Cf
2
MOD
SNS
1.
Select the desired maximum operating frequency F.
This is the frequency of operation when the output
voltage is exactly half the input voltage. At duty cycles of 75% or 25%, the frequency is 75% of this
value.
2.
Choose CHYS = 4.7pF to minimize the capacitive
load on the MOD output.
3.
Choose
VSS
BATbq2000
RSNS
Power ground
Signal ground
2000CS.eps
RF ∗ CF =
1
.
(4 ∗ F)
This reduces exponential functions to constants in
the equations relating all the values. Choose CF
for the desired hysteresis level, and then calculate
RF from the above relation.
Figure 5. Current-Sensing Circuit
find the resulting sense resistor too small to be practical.
These are easily accommodated by making the sense voltage presented to the bq2000/T a suitable fraction of the
sense current. In the 1A example, the sense resistor can
be made to be 0.1Ω provided the voltage across the sense
resistor is divided between two equal resistors at its presentation to the SNS input. The application however,
must also be able to tolerate the higher voltage on the
sense resistor.
4.
Calculate the inductor value L from the relationship below:
L =
VIN ∗ RSNS ∗ RF ∗ C2 F ∗ VCC
CHYS ∗ (57.87V)
The constant in the denominator results from the
dimensionless exponential functions. Consider the example below:
As suggested from the above, the voltage on the sense resistor is presented to the SNS input through a resistance
that forms part of a filter. The rest of the filter is a capacitor that decouples the SNS input from the Vss of the
bq2000/T. When the user is content with a sense voltage
of 50mV, the SNS input is connected to the power supply
ground at the grounded terminal of the sense resistor
through a resistor sized for the desired operating frequency. See Figure 5. An exact expression for the sizes of
the capacitive and resistive components involves the simultaneous solution of two very complex equations. A few
simplifying assumptions, however, can allow most users
to bound their operating frequencies to within about 5%.
Battery is 5-NiMH cells.
VIN = 12V I = 1A ∴RSNS = 0.05Ω
FMAX = 100KHz CHYS = 4.7pF
Choose CF = 2200pF for a hysteresis of 10.7mV
(approximately 20% of the 50mV signal). RF = 1.1K
from 3 above.
Then L = 58µH. The actual current ripple at this
frequency is 25%, 5% of this is due to the phase
delay through the filter. Since it is desirable to keep
the ripple current large to keep the inductor small,
the assumption made in 3 above is a good rule of
thumb. For a smaller ripple current, increase the
size of the capacitive filter staying at or below
4700pF. For a larger ripple, diminish the size of CF
staying at or above 1000pF. The inductor changes
in value accordingly.
The six values that determine operating frequency are input voltage VIN, inductance L, sense resistor RSNS, filter
resistance RF, filter capacitance CF, and hysteresis capacitor CHYS. Input voltage is given. The sense resistor converts the current waveform in the inductor to a voltage
waveform for presentation to the SNS input. Low detection voltage, while requiring low-valued resistors to sense
current, makes possible the sharing of resistors between
a charger and gas gauge IC. The hysteresis capacitor and
SNS input filter components have the most influence on
4
Using the bq2000/T to Control Fast Charge
RSNS
ISOURCE
VCC
BAT+
CHYS
RSNS
BAT+
TC7S04
MOD
1
Cf
bq2000
VCC
2
V:I
SNS
VSS
BATbq2000
Power ground
SNS
GND
CF
RF
RF
BAT-
2000CSHS.eps
Fig6.eps
Figure 7. Current-Sensing Circuit—
High Side
Figure 6. Voltage-to-Current Converter
Design for High-Side Sensing
High-Side Sensing
Synchronizing the bq2000/T to
a Fixed Oscillator
High-side sense requires that the sense resistor be placed
in the circuit between the switching inductor and the
battery positive terminal. From there, the signal must be
translated down to the SNS input to the bq2000/T. While
this translation may seem difficult at first, the AC signal
integrity needs to be good only about the regulation point.
This limitation makes a three transistor voltage mirror
possible. See Figure 6. One transistor of NPN polarity
provides constant-current bias to a diode-connected PNP
transistor with the sense resistor in its emitter. A second
PNP transistor with a suitably large emitter resistor is
connected in parallel with the diode connected PNP and
sense resistor, but with its collector open. This collector
drives the filter resistor RF to the regulation voltage
above ground. In this case, the VSS of the bq2000/T is itself connected to power ground, and all signals and power
are decoupled to power ground. The filter capacitor CF is
connected in parallel with RF in this case. The last remaining difficulty is providing the hysteresis signal from
MOD as in this case the signal output from MOD must be
inverted to properly drive the SNS input. Most
buck-mode switching regulators have such an inversion
in the circuit that can be conveniently capacitively coupled to the SNS input. Failing that, it is possible to add a
single inverter to the circuit such as a TC7S04F. See Figure 7. Such a single-gate circuit is necessary in synchronized battery-charger designs.
Synchronizing the bq2000/T is actually quite simple.
First, design the filter components and choose the inductor for a maximum operating frequency below the desired
frequency for synchronizing the bq2000/T. The big difference between configuring for synchronization or for
high-side or low-side sensing is that the hysteresis capacitor in synchronization is not connected directly to the
MOD output, but rather to the output of either a single
AND gate in the case of low-side sensing or to the output
of a NAND gate in the case of high-side sensing. One of
the inputs to the gate is MOD and the other is the desired synchronizing frequency. Possible single-gate devices are the TC7S00FU and the TC7S08FU. See Figure
8.
Configuring the TS Input
The TS input to the bq2000/T is characterized by three
operational thresholds that determine qualification and
termination conditions. The thresholds on the TS input
are all ratiometric to the power supply. This design allows resistor biasing to VCC and VSS of a single negative
temperature coefficient (NTC) thermistor with consistent
temperature thresholds at any operating voltage as
shown in Figure 11. We recommend a filter capacitor at
this input of no more than 0.01µF, combined with a
100kΩ resistor, to prevent noise terminations at elevated
operating temperatures.
5
Using the bq2000/T to Control Fast Charge
SYNCH
TC7S08
SYNCH
bq2000
TC7CS00
RF
MOD
CHYS
SNS
CF
bq2000
MOD
CHYS
SNS
VSS
CF
Lowside
VSS
RF
Highside
Fig8.eps
Figure 8. Synchronization Circuit for Low- and High-Side Designs
perature threshold, following which it starts fast
charging.
The following are descriptions of the three thresholds:
1.
VLTF set at 0.5 ∗ VCC is the cold temperature/pause
threshold. At voltages above this threshold, the
bq2000/T pulse-trickles and flashes the LED pin to
indicate that the bq2000/T is in the pause or inhibit
mode. This process suspends fast charge in progress
by freezing the contents of elapsed-time counters,
defeating all termination algorithms and resetting
all data-gathering. When restored to a normal operating range, the bq2000/T resumes a suspended
fast charge or top-off in progress but rebuilds discarded historical data before the fast-charge termination algorithms can take effect.
2.
VHTF set at 0.25 ∗ VCC is the maximum starting
temperature threshold. If fast charge starts at input voltages below this threshold on the TS input,
the bq2000/T flashes the LED pin as it does in the
pause mode to indicate that charge is pending.
This threshold has no effect after fast charge has
started.
3.
VTCO set at 0.225 ∗ VCC is the cutoff temperature
for fast charge and top-off. At voltages below this
threshold on the TS input, a fast charge or top-off
in progress terminates and does not resume. This is
considered a “done” condition. Note that this condition is superceded at the start of fast charge by the
starting temperature threshold described above. A
very hot battery placed in the charger causes the
bq2000/T to flash the LED pin to indicate charge
pending, even if the cutoff temperature is exceeded.
The bq2000/T does not pulse-trickle charge an overheated battery until it cools below the cutoff temperature threshold. Then the battery is
pulse-trickled until it cools below the starting tem-
A simple configuration procedure is as follows:
If no temperature limits or inhibit (pause) function are
desired, bias the TS input to a voltage level between 1.25
and 2.0 volts. This biasing can be done with a simple divider network between VCC and VSS. See Figure 9.
If temperature limits are not required, but an inhibit
function is, use the same bias network, but terminate the
second resistor to the inhibit signal rather than to VSS. A
logic high inhibits, while a low enables. See Figure 10.
Finally, if a full-featured design is desired, choose two resistor values to bias a NTC thermistor taking note of its
cold temperature value, RC, and its high temperature
value RH. The resistor from VCC to the thermistor, R1,
and from the thermistor to VSS, R2, can be calculated
from the formulas given below:
R1 =
22 ∗ R H ∗ R C
9 ∗ (R C - R H )
R2 =
22 ∗ R H ∗ R C
(9 ∗ R C ) - (31 ∗ R H )
If the denominator of R2 becomes zero, or nearly so, R2
can be left out, effectively making it infinite; however,
this equation places a limit on the effective range of temperature for any given thermistor. In most cases, this
does not present a problem as it corresponds to about a
30°C range. The thermistor value at which charge is inhibited because of overtemperature can now be represented by this term:
RTHERM =
6
1.25 ∗ R 1 ∗ R 2
3.75 ∗ R 2 - 1.25 ∗ R 1
Using the bq2000/T to Control Fast Charge
VCC
2
VCC
VSS
VCC
7
2
30kΩ
bq2000
TS
VSS
VCC
7
R1
bq2000
5
TS
5
R2
20kΩ
Inhibit
2000TDCI.eps
2000TDC.eps
Figure 10. Temperature Defeating
Configuration with Inhibit
Figure 9. Temperature Defeating
Configuration
Some applications emphasize not starting charge above a
certain temperature more than exactly where charging
terminates for an overtemperature condition. If this is
the case, the following two equations can be substituted
for those above, with the understanding that RH now represents the high-temperature prequalification resistance
of the NTC thermistor, and the term to the extreme right
represents the thermistor value at cutoff:
the voltage excursion resulting from the thermistor’s
variation with temperature. Thus, in the case of the
bq2000T, the most critical specification is the charge
range, usually the high-temperature cutoff, while the second constraint is the rate of change of temperature with
time, which is the criterion of charge termination. Generally, the temperature at which the ∆T/∆t condition
should apply is approximately 5°C above the expected
ambient. This condition is ratiometric to the supply voltage but corresponds to a rate of 31mV/min with a 5V VCC
voltage (VCC/161). The following system of equations calculates two resistor values (R1, connected between VCC
and the thermistor and R2, connected in parallel with the
thermistor), which bias the thermistor connected inside
the battery pack to the negative terminal of the battery:
1.125 ∗ R 1 ∗ R 2
3.875 ∗ R 2 - 1.125 ∗ R 1
RT30 ∗ R2 ∗ 5
−
(RT3 ∗ R2) + R1 ∗ (RT30 + R2)
Constraining the
High-Temperature Start of
Charge
RT31 ∗ R2 ∗ 5
= 0.031
(RT31 ∗ R2) + R1 ∗ (RT31 + R2)
Where:
R1 =
2 ∗ RH ∗ RC
RC - RH
R2 =
2 ∗R H ∗ R C
RC - 3 ∗ RH
RTHI ∗ R2 ∗ 5
= 1.125
(RTHI ∗ R2) + R1 ∗ (RTHI + R2)
where RT30 and RT31 represent the thermistor value at
30°C and 31°C respectively, and RTHI represents the
high temperature cutoff resistance. Selecting a low beta
thermistor such as the Semitec 103ET-2 together with a
40°C cutoff affords a range of 0 to 40°C with a 1°C per
minute sensitivity at 30°C. The same sensitivity can be
attained with a range of 0 to 50°C by selecting a high
beta thermistor such as the Semitec 103GT-1. Note that,
Configuring the TS Input to the bq2000T for
∆T/∆t Termination
A Thevenin-equivalent circuit can be used to represent
the bias on the thermistor connected to the TS input. As
such, a circuit has only two characteristics capable of
modification, and only two constraints can be imposed on
7
Using the bq2000/T to Control Fast Charge
Selecting the Timing
Components
VCC
The RC pin of the bq2000/T provides for an infinitely
variable time-out range, so the user is not bound to binary multiples and submultiples of a 1C charge rate.
The time-out set by these pins expressed in minutes is
determined by the formula R ∗ C ∗ 35988, where R is in
ohms and C is in Farads. The value of R also determines
the rate of maintenance trickle following charge for
nickel-based chemistries. A C value of 0.13µF or greater
selects a top-off algorithm to follow fast charge for
pulse-trickle duty cycles of 8.33% or less. top-off is characterized by an interval equal to the time-out during
which the battery is pulse-charged at a 1/16th duty cycle.
R1
bq2000
THERM
TS
GND
100K
0.01µF
R2
BAT-
Fig11.eps
The main consideration in selecting the timing components is the rate at which charge is being restored to the
battery expressed as a fraction (or multiple) of its rated
capacity. As most battery manufacturers deliver batteries
that exceed their rated capacities, the recommended
time-out period is that which allows 33% more charge
than the rated capacity to be returned to the battery during the timed charge interval. Thus, for a battery charged
at a 1C rate, the timeout interval is 80 minutes. This interval allows for up to a 20% overcapacity that may occur
in new batteries, a small allowance for charge efficiency,
and a small overcharge to insure cell balance. Batteries
charged at rates below C/4 may need to add significant
additional time to accommodate charge inefficiency. Note
that the 14-hour time-out normally recommended for
NiCd batteries charged at C/10 is because of the poor
charge efficiency at this rate of charge. Li-Ion or Lead
Acid batteries require voltage regulation as part of the
charge algorithm and normally take longer to charge, and
the bq2000/T has incorporated a time-out doubler that is
activated if the battery achieves voltage regulation.
Figure 11. Configuring the TS Input
in solving these equations, R1 ≥ R2 implies that there will
be no cold temperature fault.
For ease of design, a table of values for R1 and R2 is presented below for various popular thermistors and temperature ranges.
Thermistor
Semitec 103AT-2
Semitec 103ET-2
Semitec 103GT-1
Philips 2322-640-63103
or
Fenwal197-103LA6-A01
Keystone
RL0703-5744-103-S1
Range (°C)
R1
R2
-1.6–40
0.7–45
4–50
8–55
0–40
1.8–45
4.6–50
8.2–55
-10.8–40
-6–45
-0.2–50
6.9–55
-7.9–40
-4–45
1.3–50
7.2–55
-9.6–40
-5.2–45
0.6–50
7.1–55
17.8K
14.7K
12.4K
10.5K
19.1K
15.8K
13.3K
11.5K
13.7K
11.3K
9.31K
7.87K
14.7K
12.1K
10K
8.45K
14K
11.5K
9.53K
8.06K
45.3K
33.2K
23.7K
23.2K
69.8K
45.3K
34.8K
29.4K
17.4K
14.7K
12.7K
11.5K
21K
17.4K
15K
13.3K
18.7K
15.8K
13.7K
12.4K
Here then is a design procedure :
8
1.
Express the charge rate as a fraction or multiple of
the capacity.
2.
Divide 80 minutes by this fraction or multiple to
determine the desired time-out expressed in minutes.
3.
Divide this result by 35,988 to determine the R·C
product.
4.
If top-off is desired, choose C ≥ 0.13µF. If not,
choose C ≤ 0.07µF.
5.
Calculate R from time-out = R ∗ C ∗ 35,988. An additional condition on top-off is that R < 300K.
(Even with the time-out capacitor and a resistor of
less than 300k, a time-out of 23 hours can be realized.)
Using the bq2000/T to Control Fast Charge
bq2000
2000sch.eps
Figure 12. Dual-Chemistry Buck Regulator with High-Side Current Sensing
6.
7.
11. R = 94.6KΩ
Verify that the trickle-pulse rate selected from the
graph multiplied by the charge rate determined in
step 1, is less than or equal to 1/32 for NiCd or 1/64
for NiMH, and that R < 500000.
12. Pulse-trickle = C/50 (All parameters within
specified limits.) For standard values, choose R =
95.3KΩ and C = 0.047µF.
Failing any condition imposed in steps 5 and 6, return to step 4 and choose a larger value for C
within the limits specified.
Charge Termination Considerations
Example: NiCd Battery
Fast charge terminates when any of the following conditions is fulfilled:
1.
C/2
1.
2.
160 minutes
The average voltage at the BAT input to the
bq2000/T declines by PVD threshold from its highest previous value (bq2000/T).
3.
R ∗ C = 4.4459·10-3
2.
4.
Choose C = 0.001µF (1000pF)
The signal at the TS input declines at a rate of
VTERM (bq2000T).
5.
R = 4.446MΩ
3.
Regulation voltage is attained at the BAT input
and the current tapers to IMIN threshold.
6.
R > 500KΩ (Selected capacitor is too small.)
4.
7.
Choose C = 0.01µF
The maximum temperature threshold at the TS input is exceeded.
8.
R = 444KΩ
5.
The timer expires.
Pulse-trickle = 1/10.7(>1/32) (Pulse-trickle exceeds recommended value for NiCd.)
Measurement accuracy made in 3 above may depend on
the degree to which the battery voltage is filtered. A
larger value of capacitance connected across the battery
leads to a more accurate termination measurement.
9.
10. Choose C = 0.047µF
9
Using the bq2000/T to Control Fast Charge
1N4148
R7
464K
D4
R8
69.8K
R11
69.8K
2N3904
Q4
Q3
2N3906
R10
487K
R14
100K
Q5
ZTX1149
+11-16VDC
470UF
25V
LOW ESR
C3
2N3904
Q2
R1
510 OHMS
68 OHMS
D2
R5
1N4148
C4
VCC
C2
10 UF
D1
1N751
R19
2K
C1
0.1
D5
STATUS
1
2
3
4
R15
C8
0.047UF
MOD
VCC
RC
TS
BAT+
0.02 OHM
1/2 W
D3
1N5822
R9
487K
C5
470UF
25VDC
R13
464K
1000PF
BATQ1
2N3904
R18
30K
U1
SNS
VSS
LED
BAT
R6
L1
75UH
R2
1K
8
7
6
5
R3
68 OHMS
100K
C7
0.1
bq2000
R17
20K
R16
53K
R12
61.9K
R4
1K
C6
2200PF
Figure 13. Simple Buck-Boost Design
Configuring the LED Output
Application Example: Simple Dual-Chemistry
Buck Regulator
The LED output is an open-drain MOSFET capable of
sinking up to 10mA of DC current. Unlike the CMOS inputs to the bq2000/T, the LED output is protected from
overvoltage by a punch-through ESD structure. Thus the
LED output can tolerate voltages within the recommended operating range, independent of the bias on the
VCC pin.
To safely accommodate two distinct battery chemistries
in the same charger, voltage regulation must supercede
completeness of charge. This requirement implies that,
for simplicity of charger design, the user must sacrifice
similarity of discharge voltage, but for similarity of pack
voltage under discharge, the user must default under
conditions of contact failure to voltage regulation for
safety reasons. A fixed cell-ratio usually applies for applications requiring similar discharge voltage—for example,
3 NiCd/NiMH to 1 Li-Ion. The safest design approach for
these packs is to include an additional negative battery
contact on the nickel chemistry packs that will adjust the
battery divider in the charger to accommodate the higher
charge voltage requirement. See Figure 12.
Layout Considerations
The bq2000/T makes its voltage and temperature measurements with the switching regulator and timing oscillator turned off. This prevents layout considerations from
affecting termination decisions. Layout is very important,
however, for predicting the performance of the switching
regulation function. One rule applies for high or low-side
regulation:
If packs of dissimilar voltage can be allowed, the user
may select a pack of higher output voltage for the voltage
regulated chemistry while the nickel chemistry pack has
a higher amp-hour rating. These pack cell ratios should
be 2 Li-Ion to 5 or fewer nickel cells or 2 lead acid to 3 or
fewer nickel cells. Care must be taken here to limit the
charge current to the nickel cells so they do not achieve
voltage regulation. If they do, they will not terminate for
−∆V. The safest approach to cell counts that risk this situation is to use the bq2000T.
Minimize the size of all input pin nodes. Locate all bypass, feedback, and filtering components adjacent to their
connected input pins, or power pins in the case of bypass.
Ground connections associated with any power or input
pin to the bq2000/T must be kept separate from all other
grounds and brought directly to the correct side of the
sense resistor. The resistor associated with the SNS input
filter must connect separately and alone to the opposite
side of the sense resistor.
10
Using the bq2000/T to Control Fast Charge


 V(25)BULK - 2
3
+  ∗ (R3) = 2

 R1 + RT(25) ∗ R4 R2 


RT(25) + R4
Buck-Boost Design
This section illustrates a commonly encountered application, one in which a user wishes to design an automotive
charger for a number of cells whose voltage at end of
charge would exceed the supply. A nickel cell count of 8
would fit the requirement of a buck-boost implementation
if charged from a cigarette lighter. This circuit maintains
a constant current to the battery by compensating the
current regulation in the inductor for the difference between the input and output voltages from the supply.
The voltage inversion allows the inductor to remain a single winding. See Figure 13.


 V(25)FLOAT - 2
3.1 
+

 ∗ (R3) = 1.9
R2 
 R1 + RT(25) ∗ R4


RT(25) + R4


 V(HI)BULK - 2
3
+  ∗ (R3) = 2

 R1 + RT(HI) ∗ R4 R2 


RT(HI) + R4
Charging Lead-Acid Batteries
The bq2000/T charges lead-acid batteries where a
pulsed-charge algorithm is acceptable. The charger is designed so that (1) the bulk voltage (nominally 2.45 V/cell)
on the battery causes the regulation voltage to appear on
the BAT input and (2) the float voltage (nominally
2.275V/cell) causes the battery replaced voltage of 1.9V to
appear on the BAT input. The SNS input can be configured normally for current limit. For temperature compensation, an NTC thermistor is part of the resistive divider.
The design procedure is to solve the following four equations simultaneously for values R1, R2, R3, and R4:
where bulk and float voltages and thermistor values are
HI for the high-temperature limit, LO for the
low-temperature limit, and 25 for the value at 25°C. R1 is
connected from the battery positive terminal to the
higher voltage side of the thermistor. R2 is connected between the lower-voltage side of the thermistor and Vcc.
R3 is connected between the lower-voltage side of the
thermistor and ground of the IC. R4 is in parallel with
the thermistor. The voltage at the juncture of the thermistor, R4, R3, and R2 is applied to the BAT input through a
filter. The 3V and 3.1V values on the left side of the above
equations assume 5V for VCC. In applications where an
alternative voltage for VCC is used, these values would be
(VCC − 2) and (VCC − 1.9) respectively. A thermistor value
of 100KΩ or greater at 25°C is recommended.


 V(LO)BULK - 2
3
+  ∗ (R3) = 2

 R1 + RT(LOW) ∗ R4 R2 


RT(LOW) + R4
Q3
ZTX789
D3
+18V
1N4001
D4
1N4148
R4
1K
D6
1N5817
D5
1N4148
R8
154K
R15
300 OHMS
C5
1000PF
R3
1K
L1
100UH
Q2
2N3904
C9
470UF
Q1
2N3904
C4
R2
2K
D2
10UF
1N5231B
C6
2200PF
R1
D1
RED
1
2
3
4
C2
0.1
U1
SNS
VSS
LED
BAT
C8
0.047UF
C7
4.7PF
R11
30K
R14
300 OHMS
R9
53.6K
R10
115K
BT1
6 CELLS
C3
10UF
100K
MOD 8
VCC 7
6
RC 5
TS
R12
20K
bq2000
R5
1.1K
RT1
135-104LFW-J01
FENWAL
C1
0.1
R7
21.5K
R13
215K
R6
0.04 OHM
Pn1061a3.eps
Figure 14. Design Example for Lead-Acid Batteries
11
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necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed, except those mandated by government requirements.
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Copyright © 2000, Texas Instruments Incorporated
12
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