LTC4011EFE Evaluation Kit Quick Start Guide

LTC4011EFE Evaluation Kit Quick Start Guide
Quick Start Manual For Linear Technology Demo Circuit DC674A
LTC4011 Stand-Alone NiMH/NiCd 2A Battery Charger
Description
2.
Demonstration circuit DC674A is a complete PWM
Fast Battery Charger capable of charging 3, 4, 8 or
10 series connected NiMH or NiCd cells. Other cell
combinations can also be charged (up to 16 cells) by
installing an appropriate resistor on the board.
Charge termination includes - V, T/ t, peak voltage and a safety timer. Included on the board are
jumpers to select NTC thermistor functions, battery
chemistry, number of series cells, safety timer length
and four LEDs to indicate charger status. A power
path FET is also included that provides a low voltage drop path between the battery and an external
system load when the input power is removed.
T/ t method - measures the rate of temperature rise vs. time. During a fast charge, the battery temperature begins to rise. When it approaches full charge, the battery temperature
will begin to rise quickly. When the rate of
temperature rise reaches a fixed number of
°C/minute (depending on battery chemistry) the
charge cycle terminates. NiMH batteries add a
top-off charge that continues charging at a
lower current until the timer ends the charge
cycle.
For either of these charge termination methods to
work correctly, the charge current must be programmed to a sufficiently high current level (C/2 to
2C), but charging at 2C may result in excessive cell
temperature, especially small, high capacity NiMH
cells. The letter “C” is a measure of cell capacity in
mAHr. For example, to charge an 1800 mAHr battery at 1C would require a charge current of 1.8A.
The power portion of the circuit consists of a
550kHz synchronous step-down constant current
source which provides 2A of charge current. Other
charge currents can be selected by changing the
sense resistor value, and for higher charge current (2
to 4A), the PC board has a dual layout for the top
switch to allow a larger FET to be used.
Also included on this board is a power path
MOSFET that provides a low resistance path between the battery and a load (SYSTEM LOAD)
when the input power is removed. When the input
voltage is present, the load is powered from the input
voltage.
Charging NiMH or NiCd batteries.
Nickel based batteries are typically charged with a
constant current and allowing the battery voltage to
rise to the level it requires (within limits) to force
this current. When batteries are fast charged (1 to 2
hours) the charge cycle must be terminated when the
battery reaches full charge. Detecting when the battery is fully charged is usually done by one or more
methods.
The PC board used for this demo circuit is a 4 layer
board with generous amounts of copper surrounding
the heat producing components. It also includes vias
to internal copper layers to maximize the power dissipation capabilities. The programmed charge current of 2A with input voltages up to 25V are obtained with a moderate PC board temperature rise,
and 3A (with a sense resistor change) can be realized
by limiting the input voltage to approximately 15V.
With the dual layout board, higher charge current
and higher input voltages are possible by using a
larger (SO8) top MOSFET and a larger inductor.
1. – V method - measures the battery voltage as it
is being charged with a constant current. The
voltage will rise as the battery accepts charge.
When it approaches full charge, the battery
voltage will begin rising faster, reach a peak,
then begin to drop. When the battery voltage
drops a fixed number of mV (depending on battery chemistry), the battery is fully charged and
the charge cycle terminates.
Design files for this circuit board are available. Call
the LTC Factory.
1
Demo Features Include:
Jumper to select battery chemistry, NiMH or NiCd
Jumper to select number of cells
Jumper to select NTC Thermistor function
Jumper to select safety timer period
Additional PC pads for custom timer periods and custom # of cells
2A Constant Charge Current
LEDs to indicate charger status
o READY – OK to charge
o TOC – Top-Off charge, (NiMH with T/ t termination only)
o CHRG – Charging
o FAULT - Defective battery
Typical Demo Board Specifications
Minimum Input Voltage
(3 series cells) - 6.3V
(4 series cells) - 8.3V
(6 series cells) - 12.3V
10 series cells) - 20.3
(n series cells) - number of cells x 2V + 0.3V
Maximum Input Voltage - 25V (limited by capacitor voltage rating)
Constant Charge Current - 2A (±8%)
Safety Timer - 45 min, 1.5 Hrs, 3 Hrs, 4.3 Hrs (±8%)
- V Termination (NiCd) - 20mV (± 2mV)
- V Termination (NiMH) - 10mV (± 1.5mV)
T/ t Termination (NiCd) - 2°C/min (±0.16°C) Temperature Rise
T/ t Termination (NiMH) - 1°C/min (±0.14°C) Temperature Rise
Precharge Current - 400mA (±80mA)
Top Off Charge Current - 200mA (±50mA)
Test Equipment Required for Demo Board Evaluation
Lab Power supply for input power
0 to 25V @ 2A
Digital multimeter for measuring single cell voltage 3-1/2 digits
Digital multimeter for measuring battery voltage (VBAT)
3-1/2 digits
Digital multimeter for measuring charge current
4 cell, 2000 mAHr NiCd or NiMH battery pack with suitable 10k NTC thermistor
10
20W power resistor for System Load (optional)
2
Jumper Descriptions
Jumper Name
Jumper Description
NTC
Selects thermistor function for sensing
battery temperature
OUT - Disables thermistor function
BAT - Selects thermistor in battery pack
BRD - Selects fixed 10k resistor on board
CHEM
Selects battery chemistry
NiMH - Nickel Metal Hydride charge termination
NiCd - Nickel Cadmium charge termination
Cells
Selects number of series cells to be
charged
3 cells
4 cells
6 cells
10 cells
X number of cells, select appropriate R14 value
Time
Programs maximum charge time
4.3Hr
3Hr - Use for charging at C/2
1.5Hr - Use for charging at 1C
0.75Hr - Use for charging at 2C
Figure 1. Demonstration Circuit Evaluation Setup
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Demo Circuit Evaluation
in Figure 2. When the single cell voltage reaches a
peak, then begins to drop slightly, or when the battery temperature rise exceeds a fixed number (depending on battery chemistry) of °C/minute, the
charge cycle ends, or in the case of a NiMH battery,
a TOPOFF charge begins. The TOPOFF charge
only takes place for a NiMH battery and only if the
termination occurs because of the rate of temperature rise ( T/ t). There is no TOPOFF charge for
NiCd batteries.
Begin evaluation by placing the jumpers for NTC,
CHEM, # of CELLS and TIME in the correct location. If the battery has a thermistor to measure the
battery temperature, move the NTC jumper (JP1) to
the BAT position. If a thermister is not used, locate
the jumper to the OUT position. For charging a
2000mAHr battery, select the 1.5 hour position on
J2. Locate the CHEM (chemistry) jumper to the appropriate position for the type of battery being
charged.
After a complete charge cycle has ended, the
charger continues to monitor the battery voltage. If
the single cell voltage drops below 1.325V because
of either self discharge or the system load, a recharge cycle will begin.
Connect the Input Power Supply, Battery (with
thermistor) and Optional System Load (10 Ohm
20W resistor) to the demonstration circuit as shown
in Figure 1. Connect a digital voltmeter to the single
cell voltage (Vcell) terminal located near the bottom
of the board. If desired, a meter can be placed between the BAT terminal and the positive side of the
battery to measure charge current.
Terminals are provided on the board to externally
pause the charge cycle as well as terminals to monitor the single cell voltage (Vcell) and the thermistor
voltage (NTC).
Switch on the input power supply and adjust the
voltage to approximately 12V. The battery voltage
will determine what will happen next. The single
cell voltage (Vcell) is the average voltage of only
one of the series connected cells which the charger
uses to determine the condition of the battery pack.
If the single cell voltage is less than 350mV, no
charging takes place. If the single cell voltage is
between 350mV and 900mV, the READY LED and
the CHRG LED come on and the battery begins a
trickle charge at 400mA or 20% of the programmed
current for a period of time equal to 1/12 of the total
time. If the cell voltage does not exceed 900mV after this time period, the LTC4011 indicates a
FAULT, and the trickle charge stops. In the event
that a battery is defective resulting in the single cell
voltage exceeding 1.9V, the trickle charge will stop
and the FAULT LED will come on. If the FAULT
LED comes on, the charger must be reset by either
removing and reapplying the input voltage or replacing the battery.
Additional pads are located on the PC board for optional components. R14 can be added to change the
number of cells being charged. R23 can be added to
program different timer periods, and C6 (small
10µF @ 35V aluminum or tantalum surface mount
capacitor) and C7 (0.22µF, 25V, X5R ceramic capacitor) to assure stable operation when long leads
exist between the input power source and the
charger.
Once the single cell voltage exceeds 900mV, the
charger begins charging with the programmed constant current. During charging, the battery voltage
and battery temperature (provided an NTC thermistor is used) are constantly monitored. See curve
shown
The - V that occurs when charging a NiMH battery
is less than a NiCd. This makes - V termination
somewhat more difficult to detect for a NiMH battery, therefore it is recommended to include a thermistor in the NiMH battery packs.
Please refer to the LTC4011 data sheet for additional information.
Differences between NiCd and NiMH batteries
NiCd batteries exhibit very little temperature rise
during most of the charge cycle. The temperature
rise mainly occurs near the end of the charge cycle.
NiMH batteries are different in this respect, where
the battery temperature rise continues during the
charge cycle and accelerates near the end of the
charge cycle.
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Figure 2. NiMH Charge Profile (1C Charge Rate)
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