MC33340PG BATTERY MANAGEMENT

MC33340, MC33342
Battery Fast Charge
Controllers MC33340, MC33342
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INTRODUCTION
Nickel Cadmium and Nickel Metal Hydride batteries
require precise charge termination control to maximize cell
capacity and operating time while preventing overcharging.
Overcharging can result in a reduction of battery life as well
as physical harm to the end user. Since most portable
applications require the batteries to be charged rapidly, a
primary and usually a secondary or redundant charge sensing
technique is employed into the charging system. It is also
desirable to disable rapid charging if the battery voltage or
temperature is either too high or too low. In order to address
these issues, an economical and flexible fast charge controller
was developed.
The MC33340/342 contains many of the building blocks
and protection features that are employed in modern high
performance battery charger controllers that are specifically
designed for Nickel Cadmium and Nickel Metal Hydride
batteries. The device is designed to interface with either
primary or secondary side regulators for easy implementation
of a complete charging system. A representative block diagram
in a typical charging application is shown in Figure 8.
The battery voltage is monitored by the Vsen input that
internally connects to a voltage to frequency converter and
counter for detection of a negative slope in battery voltage. A
timer with three programming inputs is available to provide
backup charge termination. Alternatively, these inputs can be
used to monitor the battery pack temperature and to set the
over and undertemperature limits also for backup charge
termination.
Two active low open collector outputs are provided to
interface this controller with the external charging circuit.
The first output furnishes a gating pulse that momentarily
interrupts the charge current. This allows an accurate method
of sampling the battery voltage by eliminating voltage drops
that are associated with high charge currents and wiring
resistances. Also, any noise voltages generated by the
charging circuitry are eliminated. The second output is
designed to switch the charging source between fast and
trickle modes based upon the results of voltage, time, or
temperature. These outputs normally connect directly to a
linear or switching regulator control circuit in non-isolated
primary or secondary side applications. Both outputs can be
used to drive optoisolators in primary side applications that
require galvanic isolation. Figure 9 shows the typical charge
characteristics for NiCd and NiMh batteries. OPERATING DESCRIPTION
The MC33340/342 starts up in the fast charge mode when
power is applied to VCC. A change to the trickle mode can
occur as a result of three possible conditions. The first is if
the Vsen input voltage is above 2.0 V or below 1.0 V. Above
2.0 V indicates that the battery pack is open or disconnected,
while below 1.0 V indicates the possibility of a shorted or
defective cell. The second condition is when the
MC33340/342 detects a fully charged battery by measuring
a negative slope in battery voltage. The MC33340/342
recognize a negative voltage slope after the preset holdoff
time (thold) has elapsed during a fast charge cycle. This
indicates that the battery pack is fully charged. The third
condition is either due to the battery pack being out of a
programmed temperature range, or that the preset timer
period has been exceeded.
There are three conditions that will cause the controller to
return from trickle to fast charge mode. The first is if the Vsen
input voltage moved to within the 1.0 to 2.0 V range from
initially being either too high or too low. The second is if the
battery pack temperature moved to within the programmed
temperature range, but only from initially being too cold.
Third is by cycling VCC off and then back on causing the
internal logic to reset. A concise description of the major
circuit blocks is given below.
Negative Slope Voltage Detection
A representative block diagram of the negative slope
voltage detector is shown in Figure 10. It includes a
Synchronous Voltage to Frequency Converter, a Sample
Timer, and a Ratchet Counter. The Vsen pin is the input for
the Voltage to Frequency Converter (VFC), and it connects
to the rechargeable battery pack terminals through a
resistive voltage divider. The input has an impedance of
approximately 6.0 M and a maximum voltage range of
-1.0 V to VCC + 0.6 V or 0 V to 10 V, whichever is lower.
The 10 V upper limit is set by an internal zener clamp that
provides protection in the event of an electrostatic discharge.
The VFC is a charge-balanced synchronous type which
generates output pulses at a rate of FV = Vsen (24 kHz).
The Sample Timer circuit provides a 95 kHz system clock
signal (SCK) to the VFC. This signal synchronizes the FV
output to the other Sample Timer outputs used within the
detector. At 1.38 second intervals the Vsen Gate output goes
low for a 33 ms period. This output is used to momentarily
interrupt the external charging power source so that a precise
voltage measurement can be taken. As the Vsen Gate goes
low, the internal Preset control line is driven high for 11 ms.
During this time, the battery voltage at the Vsen input is
allowed to stabilize and the previous FV count is preloaded.
At the Preset high-to-low transition, the Convert line goes
high for 22 ms. This gates the FV pulses into the ratchet
counter for a comparison to the preloaded count. Since the
Convert time is derived from the same clock that controls the
VFC, the number of FV pulses is independent of the clock
frequency. If the new sample has more counts than were
preloaded, it becomes the new peak count and the cycle is
repeated 1.38 seconds later. If the new sample has two fewer
counts, a less than peak voltage event has occurred, and a
register is initialized. If two successive less than peak
voltage events occur, the -V ‘AND’ gate output goes high
and the Fast/Trickle output is latched in a low state,
signifying that the battery pack has reached full charge
status. MC33340, MC33342
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Negative slope voltage detection starts after 60 ms have
elapsed in the fast charge mode. This does not affect the
Fast/Trickle output until the holdoff time (thold) has elapsed
during the fast charge mode. Two scenarios then exist.
Trickle mode holdoff is implemented to ignore any initial
drop in voltage that may occur when charging batteries that
have been stored for an extended time period. If the negative
slope voltage detector senses that initial drop during the
holdoff time, and the input voltage rises as the battery
charges, the Fast/Trickle output will remain open. However,
if the negative slope voltage detector senses a negative drop
in voltage during the holdoff time and the input voltage
never rises above that last detected level, the Fast/Trickle
output will latch into a low state. The negative slope voltage
detector has a maximum resolution of 2.0 V divided by
1023 mV, or 1.955 mV per count with an uncertainty of
±1.0 count. This yields a detection range of 1.955 mV to
5.865 mV. In order to obtain maximum sensing accuracy,
the R2/R1 voltage divider must be adjusted so that the Vsen
input voltage is slightly less than 2.0 V when the battery pack
is fully charged. Voltage variations due to temperature and
cell manufacturing must be considered.Fast Charge Timer
A programmable backup charge timer is available for fast
charge termination. The timer is activated by the Time/Temp
Select comparator, and is programmed from the t1/Tref
High, t2/Tsen, and t3/Tref Low inputs. If one or more of these
inputs is allowed to go above VCC - 0.7 V or is left open, the
comparator output will switch high, indicating that the timer
feature is desired. The three inputs allow one of seven
possible fast charge time limits to be selected. The
programmable time limits, rounded to the nearest whole
minute, are shown in Table 1.
Over/Under Temperature Detection
A backup over/under temperature detector is available
and can be used in place of the timer for fast charge
termination. The timer is disabled by the Time/Temp Select
comparator when each of the three programming inputs are
held below VCC - 0.7 V.
Temperature sensing is accomplished by placing a
negative temperature coefficient (NTC) thermistor in
thermal contact with the battery pack. The thermistor
connects to the t2/Tsen input which has a 30 A current
source pull-up for developing a temperature dependent
voltage. The temperature limits are set by a resistor that
connects from the t1/Tref High and the t3/Tref Low inputs to
ground. Since all three inputs contain matched 30 A
current source pull-ups, the required programming resistor
values are identical to that of the thermistor at the desired
over and under trip temperature. The temperature window
detector is composed of two comparators with a common
input that connects to the t2/Tsen input.
The lower comparator senses the presence of an under
temperature condition. When the lower temperature limit is
exceeded, the charger is switched to the trickle mode. The
comparator has 44 mV of hysteresis to prevent erratic switching between the fast and trickle modes as the lower
temperature limit is crossed. The amount of temperature rise
to overcome the hysteresis is determined by the thermistorЃfs
rate of resistance change or sensitivity at the under
temperature trip point. The required resistance change is:
R(TLow  THigh) 
VH(T)
Iin

44 mV
30 A  1.46 k
The resistance change approximates a thermal hysteresis
of 2Ѓ‹C with a 10 k thermistor operating at 0Ѓ‹C. The under
temperature fast charge inhibit feature can be disabled by
biasing the t3/Tref Low input to a voltage that is greater than
that present at t2/Tsen, and less than VCC Ѓ| 0.7 V. Under
extremely cold conditions, it is possible that the thermistor
resistance can become too high, allowing the t2/Tsen input
to go above VCC Ѓ| 0.7 V, and activate the timer. This
condition can be prevented by placing a resistor in parallel
with the thermistor. Note that the time/temperature
threshold of VCC Ѓ| 0.7 V is a typical value at room
temperature. Refer to the Electrical Characteristics table
and to Figure 4 for additional information.
The upper comparator senses the presence of an over
temperature condition. When the upper temperature limit is
exceeded, the comparator output sets the Overtemperature
Latch and the charger is switched to trickle mode. Once the
latch is set, the charger cannot be returned to fast charge,
even after the temperature falls below the limit. This feature
prevents the battery pack from being continuously
temperature cycled and overcharged. The latch can be reset
by removing and reconnecting the battery pack or by cycling
the power supply voltage.
If the charger does not require either the time or
temperature backup features, they can both be easily
disabled. This is accomplished by biasing the t3/Tref Low
input to a voltage greater than t2/Tsen, and by grounding the
t1/Tref High input. Under these conditions, the Time/Temp
Select comparator output is low, indicating that the
temperature mode is selected, and that the t2/Tsen input is
biased within the limits of an artificial temperature window.
Charging of battery packs that are used in portable power
tool applications typically use temperature as the only
means for fast charge termination. The MC33340/342 can
be configured in this manner by constantly resetting the Ѓ|V
detection logic. This is accomplished by biasing the Vsen
input to 1.5 V from a two resistor divider that is connected
between the positive battery pack terminal and ground. The
Vsen Gate output is also connected to the Vsen input. Now,
each time that the Sample Timer causes the Vsen output to go
low, the Vsen input will be pulled below the undervoltage
threshold of 1.0 V. This causes a reset of the Ѓ|V logic every
1.38 seconds, thus disabling detection.
Operating Logic
The order of events in the charging process is controlled
by the logic circuitry. Each event is dependent upon the input
conditions and the chosen method of charge termination. A
table summary containing all of the possible operating
The MC33340 and MC33342 are monolithic control IC’s that are
specifically designed as fast charge controllers for Nickel Cadmium
(NiCd) and Nickel Metal Hydride (NiMH) batteries. These devices
feature negative slope voltage detection as the primary means for fast
charge termination. Accurate detection is ensured by an output that
momentarily interrupts the charge current for precise voltage
sampling. An additional secondary backup termination method can
be selected that consists of either a programmable time or temperature
limit. Protective features include battery over and undervoltage
detection, latched over temperature detection, and power supply input
undervoltage lockout with hysteresis. Fast charge holdoff time is the
only difference between the MC33340 and the MC33342. The
MC33340 has a typical holdoff time of 177 seconds and the
MC33342 has a typical holdoff time of 708 seconds.
• Negative Slope Voltage Detection with 4.0 mV Sensitivity
• Accurate Zero Current Battery Voltage Sensing
• High Noise Immunity with Synchronous VFC/Logic
• Programmable 1 to 4 Hour Fast Charge Time Limit
• Programmable Over/Undertemperature Detection
• Battery Over and Undervoltage Fast Charge Protection
• Power Supply Input Undervoltage Lockout with Hysteresis
• Operating Voltage Range of 3.25 V to 18 V
• 177 seconds Fast Change Holdoff Time (MC33340)
• 708 seconds Fast Change Holdoff Time (MC33342)
• Pb-Free Packages are Available