High-Efficiency 3A Battery Chargers Use LM2576 Regulators AN-946
National Semiconductor
Application Note 946
Chester Simpson
May 1994
High-Efficiency 3A Battery
Chargers Use LM2576
This paper describes two LM2576-based designs which
provide up to 3A of current for battery charging.
3A Battery Charger Has Built-In
Overcharge Protection
This design is a 3A battery charger intended for use with
5-cell Ni-Cd or Ni-MH battery packs (but can be modified to
suit other numbers of cells). The circuit includes automatic
shutoff that occurs when the battery temperature rises 10§C
above ambient.
This battery charger shown in
Figure 1
was developed spe-
cifically for applications using either
Nickel-Metal Hydride (Ni-MH)
batteries that will
accept a 3A fast-charge rate, and provides automatic shut-
off of the high-current charge when the battery is full.
After shutoff, a continuous (low level) charge current is used
to ‘‘trickle charge’’ the battery which keeps it topped off and
prevents charge loss due to any internal leakage.
The trickle charge rate used must always be low enough
that the amount of gas developed within the cell is small
enough that it can recombine, thus preventing pressure
build-up and venting (opening of the cell’s internal vent to
release pressure). The maximum safe trickle charging rate
is determined by the size and type of battery (this is covered
later in the paper).
Notes (Unless Otherwise Specified):
All capacitance values are in mF.
All resistors are 5%, (/4W.
U1 requires small heatsink (RTH k15§C/W).
C1 and C4 must be low ESR aluminum electrolytic.
U5D is not used.
FIGURE 1. 3A Charger with Overcharge Protection
C1995 National Semiconductor Corporation RRD-B30M75/Printed in U. S. A.
The critical specification for a battery is its
Amp-hour (A-hr)
which is numerically equal to the maximum amount
of current the battery can supply to a load for one hour
before the cell reaches its
end-of-life voltage
(usually tak-
en as 1.0V/cell for Ni-Cd and Ni-MH batteries.
When a battery is charged or discharged at a current that is
equal to its A-hr rating,
this is known as the ‘‘c’’ rate.
Most Ni-Cd and Ni-MH batteries can be safely charged at a
1c rate, as long as they are not overcharged. However, the
battery temperature must be within a range of about 15§Cto
45§C (the reasons are detailed later in this paper).
The nemesis of all rechargeable batteries is overcharge . . .
although some battery types tolerate it better than others,
the results of overcharge range from minor damage to cata-
strophic failure.
In the case of Ni-Cd, which is the most popular rechargea-
ble battery type presently in use, sustained overcharge
causes increasing pressure within the battery that eventual-
ly causes the cell’s vent to open and release oxygen. This
has a detrimental effect on the battery, although it may still
retain some useful capacity.
If Ni-MH batteries are overcharged, they will also build up
pressure and release gas: however, the gas released will be
hydrogen, which is extremely explosive near spark or flame.
One battery manufacturer created an interesting euphe-
mism for some of the unfortunate accidents in cases where
Ni-MH batteries were overcharged:
Rapid Spontaneous
There are several ways to detect end-of-charge for Ni-Cd or
Ni-MH batteries, but one way that is both simple and reliable
is called a DT detector. It measures both the ambient tem-
perature and the battery temperature and cuts off the high
current charger when the battery rises a pre-set amount
above ambient. This design uses a 10§C rise as the cutoff
point (which is recommended by most battery makers), but
can be easily adjusted by changing resistor values.
Ni-Cd cells are perfectly suited for DT cutoff techniques,
because their charge process is
(they get
when a discharged battery is being re-
charged). Even at fast charge rates, the battery will not be-
gin to heat until it is nearly fully recharged. At that point, the
battery is no longer converting the electrical current into a
chemical reaction, so it must be dissipated as heat. The
resulting increase in temperature provides a very accurate
indicator that it is time to stop charging.
The Ni-MH battery is not quite as accommodating: the re-
charge cycle is
(the battery gets slightly
during recharge) but still shows a fairly well defined in-
crease in temperature when the battery is fully charged. Us-
CDT detection point will give good results in most
cases, and is recommended by the battery makers.
Since the Ni-MH battery normally gives off heat during re-
charge, the 10§C ‘‘window’’ may have to be adjusted to suit
the characteristics of the specific cell: The window must be
wide enough to prevent premature cutoff from ‘‘normal’’
heating, but narrow enough to detect the temperature rise
which occurs at full charge (and execute appropriate charge
Any new design that uses Ni-MH batteries should be care-
fully evaluated to verify accurate end-of-charge termination
because of the potential for battery explosion if hydrogen is
With Ni-Cd or Ni-MH cells, the 1c (fast)
charge rate can only be safely used if the battery tem-
perature is in the range of about 15§Cto45
low temperatures,
gas recombination within NiCd and
NiMH batteries does not occur as easily, which limits the
amount of charging current that can be safely used before
venting will occur. If low-temperature (k15§C) recharging is
required, consult the battery maker for safe charging current
A battery that is recharged at
elevated temperature
retain substantially less energy than a battery recharged at
25§C. At high temperatures (l35§C) gas generation within
the cell occurs at a much lower state of charge, meaning
that the cell will not accept as much charge (compared to
25§C) for a given amount of cell temperature rise.
The poor charging efficiency seen at high battery tempera-
tures means that extremely long recharge times (at low
charging currents) are required to deliver full (25§C) capacity
of charge to a ‘‘hot’’ battery.
All batteries lose charge internally due to self-discharge,
usually occurring due to leakage paths through the battery
separators (insulators). The amount of leakage is depen-
dent primarily on battery age and usage, with leakage in-
creasing dramatically in batteries that are old or have com-
pleted many cycles of charge and discharge.
Trickle charging is a continuous low-level charging current
that tops off the total charge in the battery, and prevents
any energy loss that would occur due to leakage.
maximum safe trickle charging current for a typical
Ni-Cd cell is about 0.1c,
this being the maximum charge
rate at which all of the gas developed internally is able to
recombine (so there is no internal pressure buildup that
would cause venting).
For Ni-MH batteries, the maximum (safe) trickle charge
rate is lower
(one manufacturer specifies c/40). This is an
important difference between Ni-Cd and Ni-MH batteries,
and must not be exceeded for continuous charging.
In this design, the trickle charge current is provided by the
resistor labeled RTR (see
Figure 1
). This current flows any
time VIN is present, regardless of operation of the high-cur-
rent charger. When the high-current charger is operating,
the total charging current is the
of the trickle current
and the current provided by U1.
Once the input voltage VIN and the desired trickle charge
current ITR are known, the value for RTR is found using
Ohm’s Law:
RTR e(VIN b7b0.7)/ITR
The maximum power dissipation in RTR must also be calcu-
lated (when selecting a resistor, make sure the power rating
is greater than the value calculated below):
PMAX (RTR)e(VIN b4b0.7)2/RTR
Note that the power dissipation in the resistor is dependent
on the battery voltage. As the battery voltage increases, the
voltage drop across RTR decreases (causing the power dis-
sipation to decrease).
In the above equation, a battery voltage of 4V is assumed
as a worst-case minimum value for battery operating volt-
age for a five-cell battery pack (which would provide the
maximum power dissipation for RTR).
5-cell Ni-Cd or Ni-MH battery which is being trickle
charged (after being fully recharged) will read about 7V,
which will produce the
power dissipation in resistor
The 3A of charging current provided by the fast-charger is
obtained from an LM2576, which is a buck regulator that
switches at 52 kHz. Because it is a switcher, it allows the
user the option of using a wider input voltage range and still
retaining high power conversion efficiency (about 80% @3A
with VIN in the 10V 14V range).
The LM2576 IC (U1) is used to provide a charging current
that is independent of the battery voltage. Whenever the
ON/OFF pin is held low, U1 will source current into the
battery through D3. A current-control feedback loop is es-
tablished using U5B, R12, and associated components.
R12 is used as a current shunt, and it provides a voltage to
the input of U5B that is proportional to the charging current.
U5B functions as an amplifier with a gain of 8.5, which caus-
es the output of U5B to be 1.23V when the current through
R12 is about 2.9A. The 1.23V signal on the feedback pin of
U1 will ‘‘lock’’ the loop at this value of charging current.
A fast-charge current value other than 2.9A can be set by
adjusting the values of R7, R9, or R12. These values (which
set the overall gain of the stage) should be adjusted so that
the output of U5B is 1.23V at the desired amount of fast-
charge current.
The crucial part of fast charging a battery (especially if it is
Ni-MH) is
knowing when to stop.
This design uses a DT
detector that measures
the battery temperature and
the ambient temperature, and shuts down the fast-charge
current source when the battery is a10§C
above ambient.
This method is superior to techniques which sense only bat-
tery temperature. Single-ended temperature sensing may
not accurately measure charge: a ‘‘cold’’ battery will have to
heat up too much before the detection point is reached
(overcharging it), while a ‘‘hot’’ battery will terminate charge
long before full charge has been delivered to the battery
(because its temperature starts out too near the detection
Two LM35 temperature sensors (U3 and U4) provide output
voltages of 10 mV/§C (proportional to their temperature). U3
is used to measure the ambient, while U4 measures the
battery temperature.
Note: U4 must be in contact with the metal case of the
battery to accurately measure its temperature. The plastic
sleeve around the battery may have to be opened up to
allow flush contact. Best results are obtained if the sensor is
between two batteries
(touching both).
Monitoring more than one battery virtually eliminates the
possibility that the sensor happens to be reading a bad
(shorted) cell which will not heat up and provide charge ter-
mination. In some laptops, multiple sensors are used so that
all battery cells are monitored, with charge termination oc-
curring when
cell temperature reaches the trip level.
The 78L05 regulator (U2) is used to provide a 5V source to
power the LM35 sensors and also acts as a reference point
for resistive divider R2 and R3. Resistors R1 and R11 are
used to sink current (since the LM35 can not).
U5C acts as a comparator which controls the on/off pin of
the high-current charging source (U1). When the output of
U5C is low, the 3A current source is turned on. When the
output of U5C is high, U1 is turned off and LED1 is lit which
indicates that the charger has completed the high-current
charge phase and is now trickle charging.
Hysteresis is built into U5C (see R13), which effectively
‘‘latches’’ the output of U5C high after it completes the fast-
charge portion of the cycle (it stays latched until the input
power is cycled on and off). Without hysteresis, the charger
would again turn on the 3A charger after the fully-charged
battery had cooled during trickle charging.
The signals that are sent to U5C are derived from the tem-
perature sensors. They cannot be compared directly, since
detection must occur when the signal coming from U4 (the
battery sensor) is 100 mV above the signal coming from U3
(the ambient sensor).
In this design, the signal from U3 is DC level shifted up
about 0.1V by U5A and its associated components. R2 and
R3 set a 0.1V reference point for U5A, whose output volt-
age is the voltage at the output of U3 added to the 0.1V
With the signal from U3 level shifted by an amount that is
equal to 10§C, U5C can be used to compare the level-shift-
ed signal from U3 to the signal from U4. When these two
are equal, the temperature sensed by U4 (the battery) will
be 10§C above the temperature sensed by U3 (the ambient).
This is the point where shutdown of the 3A charger occurs,
and trickle charging continues.
3A Battery Charger Has Logic-
Level Current Controls
This design is a 3A battery charger with logic-level controls,
allowing a logic controller to adjust the battery charging cur-
rent to any one of four rates. The circuit was designed to
implement mP-based charging control in a system that oper-
ates from Ni-Cd or Ni-MH batteries.
The circuit shown in
Figure 2
is a 3A (maximum) battery
charger that uses a 52 kHz switching converter to step
down the input DC voltage and regulate the charging cur-
rent flowing into the battery. The switching regulator main-
tains good efficiency over a wide input voltage range, which
allows the use of a cheap, poorly regulated ‘‘DC wall adap-
tor’’ for the input source.
The key feature of this circuit is that it allows the mP control-
ler inside the PC to select from one of four different charg-
ing currents by changing the logic levels at two bits. The
various charge levels are necessary to accommodate both
Ni-Cd and Ni-MH type batteries, as they require slightly dif-
ferent charge methods.
Both Ni-Cd and Ni-MH batteries can be charged at the high-
current ‘‘c’’ rate up until the end-of-charge limit is reached,
but the two batteries must be trickle-charged differently
(trickle charging is a continuous, low-current charging rate
that keeps the battery ‘‘topped off’’ after the high-current
charge cycle has delivered about 95% of the battery’s total
charge capacity).
The recommended trickle-charge rate for a Ni-Cd is about
c/10, but for Ni-MH most manufacturers recommend that
the charge rate not exceed c/40. If a continuous charge
rate greater than c/40 is applied to a Ni-MH battery, the
internal pressure can build up to the point where the battery
will vent hydrogen gas. This is detrimental to the life of the
Ni-MH battery and potentially dangerous for the user (hydro-
gen gas is easily ignited).
The circuit shown in
Figure 2
was designed to charge a 3A-
hr Ni-Cd or Ni-MH battery with high efficiency, using logic-
level signals to control the charging current. The four select-
able charge rates are 3A, 0.75A, 0.3A, and 0.075A which
correspond to charge rates of c, c/4, c/10, and c/40 for the
3A-hr battery used in this application.
The unregulated DC input voltage is stepped down using an
LM2576 3A buck regulator, providing up to 3A of current to
charge the battery.
In order to regulate the amount of charging current flowing
into the battery, a current control loop is implemented using
op-amp U2. The voltage drop across the sense resistor R8
provides a voltage to U2 that is proportional to the charging
Note: The 0.05Xvalue for R8 was specified by the custom-
er in this application to minimize the power dissipated in this
resistor. If a higher Ohmic value is used (more resistance), a
larger sense voltage is developed and a less precise
(cheaper) op-amp can be used at U2, since the input offset
voltage would not be as critical (of course, increasing the
value of R8 also increases its power dissipation).
When the current-control loop is operating, the voltage at
the feedback pin of U1 is held at 1.23V. The battery charg-
ing current that corresponds to this voltage is dependent on
the overall gain of U2 and the attenuators made up of Q1,
Q2 and the resistors R10, R11, R2 and R3.
Turning Q1 on (by putting a ‘‘1’’ on logic input ‘‘A’’) provides
an increase of 4:1 in load current. The load current is higher
with Q1 on because R2 and R3 divide down the output of
U2 by 4:1, requiring U2 to output a higher voltage to get the
1.23V on the feedback line of U1. Higher voltage at the
output of U2 means that more charging current is flowing
through R8 (also the battery).
The operation of Q2 is similar to Q1: when Q2 is turned on
by putting a logic ‘‘1’’ on input ‘‘B’’, the load current is in-
creased by a factor of 10:1. This is because when Q2 is on,
the sense voltage coming from R8 is divided down by R10
and R11, requiring ten times as much signal voltage across
R8 to get the same voltage at the non-inverting input of U2.
Although both attenuating dividers could have been placed
on the input side of U2, putting the 4:1 divider at the output
improves the accuracy and noise immunity of the amplifier
U2 (because the voltage applied to the input of U2 is larger,
this reduces the input-offset voltage error and switching
noise degradation).
R5, R6, and D2 are included to provide a voltage-control
loop in the case where the battery is disconnected. These
components prevent the voltage at the cathode side of D3
from rising above about 8V when there is no path for the
charging current to return (and the current control loop
would not be operational).
Capacitor C2 is included to filter some of the 52 kHz noise
present on the control line coming from U2. Adding this
component improved the accuracy of the measured charg-
ing current on the breadboard (compared to the predicted
design values).
Notes (Unless Otherwise Specified):
Note 1: All resistors are in X, 5% tolerance, (/4W
Note 2: All capacitors are in mF
Note 3: Q1 and Q2 are made by SUPERTEX
Note 4: For 3A current, U1 requires small heatsink (RTH s15§C/W)
Logic Logic Nominal Battery Measured Battery Power Conversion
Input ‘‘A’’ Input ‘‘B’’ Charging Charging Current (A) Efficiency (%)
Current (A) with VIN e10V with VIN e10V
1 1 3.0 (C RATE) 3.06 77
0 1 0.75 (C/4 RATE) 0.78 79
1 0 0.30 (C/10 RATE) 0.30
0 0 0.075 (C/40 RATE) 0.077
FIGURE 2. 3A Battery Charger With Logic-Level Current Controls
AN-946 High-Efficiency 3A Battery Chargers Use LM2576 Regulators
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