AN26

Application Note 26
Fast charging batteries with Zetex high current
PNP transistors and benchmarq controller ICs
Neil Chadderton
Introduction
Fast charge controller ICs
The advances of digital technology and a
waiting market have created a huge demand
for portable products including cellular
telephones, PDAs, laptop computers, CD
players, and gaming systems. By their nature,
all of these products must derive their energy
from an integral rechargeable cell/battery pack
that must periodically be re-charged from a
mains outlet. The type of battery employed, or
its chemistry, depends on the overall pricing
available to the product - this will depend on
the perceived product lifetime, the required
energy/weight ratio, and which marketable
advantages there may be apparent from
reducing the number of re-charge cycles.
There are a number of fast charge controller
ICs available that perform all the necessary
monitoring and regulation control functions
on one monolithic IC. The Benchmarq
Microelectronics series of fast charge
controller ICs monitor the terminal voltage to
determine state of charge, and also the
temperature of the battery pack (via a
thermistor) to prevent deterioration of the
separator material. Fast charge termination is
effected by any of the following:
For the latest generations of products, users
are expecting higher performance, less
re-charge cycles, longer battery pack life and
faster charge times. All of these parameters are
very dependent on the manner in which the cell
or battery pack is treated during the charging
process. This includes consideration of the
preferred cell charging method, thermal
concerns, and pre-charge conditions.
Delta temperature/delta time (∆T/∆t)
Negative delta voltage (-∆V)
Maximum temperature
Maximum time
Maximum voltage
To allow cost effective IC designs, typical fast
charge controller ICs are manufactured on
existing CMOS processes, that do not readily
lend themselves to incorporating on-chip pass
elements within system cost constraints.
Therefore the IC controls an external discrete
transistor that is used within a linear regulator,
or a step-down frequency modulated DC-DC
converter topology to provide a suitable
current source for charging.
ISSUE 2 - SEPTEMBER 2003
AN26 - 1
SEMICONDUCTORS
Figure 1. Fast charge circuit (Benchmarq Microelectronics) using the ZTX788B
ISSUE 2 - SEPTEMBER 2003
SEMICONDUCTORS
AN26 - 2
In general, switched mode circuits are
preferred due to the higher efficiencies
possible, and the smaller packaged pass
devices required to perform the sourcing
function. For example, a comparison of the
power losses for a four `C’ cell battery pack
charged from a 12V DC source at 2A, leads to
values of greater than 12W for the linear
control system, but less than 2W for the
switched mode option.
The external pass transistor used within either
circuit mode can be either a PNP Bipolar or a
P-Channel MOSFET. For the linear regulator
mode, the device must be capable of being
heatsunk to remove the power dissipated,
which will probably dictate that the product
will be a large packaged, large silicon area
device which will define a costly component.
For the switched mode option, there is more
choice available. Due to the higher silicon
efficiency of the Bipolar technology compared
with MOSFETs, high current fast switching
PNP transistors are available that use a fraction
of the silicon area required for comparably
specified MOSFETs. This allows the bipolar
chip option to be encapsulated within a smaller
package, and therefore presents a lower cost to
the designer.
The PNP transistor chosen for this particular
application was the ZTX788B - one member of
the ZTX788B to ZTX796A series. These devices
are manufactured using a high gain emitter
process to produce a Super-β hFE range. The
ZTX788B is a 15V, 3A continuous TO92
compatible part that exhibits very low VCE(sat),
and can therefore replace much larger
packaged components (such as TO220, TO126
and D-Pak) in switching applications. Figure 2
demonstrates the on-state voltage of the `788B
as a function of load current with base drive
current as parameter. Tables 1, 2 and 3
(Appendix A) summarize the pertinent
parameters for the ZTX788B and other devices
of interest for this application.
Figure 2. VCE(sat) v collector current chart for
the ZTX788B
The circuit shown in Figure 1 is one example of
a circuit topology developed by Benchmarq
using Zetex E-Line PNP transistors. The circuit
allows the fast charging of Ni-Cd or Ni-MH
battery packs for laptop computers and similar
powered portable systems at a charge current
of 2.3A, (defined by the current sense resistor
R9). The potential divider formed by RB1 and
RB2 is used to feedback a cell corrected level to
the IC voltage sense input, that is then used to
define the battery pack voltage. Other
ICs/circuits are available from Benchmarq for
higher current charging and for other cell
chemistries.
ISSUE 2 - SEPTEMBER 2003
AN26 - 3
SEMICONDUCTORS
The circuit employs a circuit modification to
provide an active turn-off for the transistor.
[This doesn’t however impact on the cost
advantage over the MOSFET alternative].
Figure 3 shows the area around the pass
transistor with the relevant level shift and
turn-off components. The small signal
switching transistor Q2, provides level
translation from the “MOD” output of the
controller IC, and its emitter resistor defines
the base current for Q3, the ZTX788B. The
inductor L2, Schottky diode (1N5820), and the
10µF capacitor are the Buck converter
(step-down) components. The 1k base emitter
resistor for Q3 would, in the absence of Q1, L1
and the signal diode (1N4148), provide passive
turn-off for the pass transistor.
Figures 4 and 5 help to demonstrate the
difference in turn-off performance for the two
methods. These waveforms were recorded
from a charger circuit using the bq2004 and
ZTX789A for a 5 cell battery (output voltage to
charge to 6V), and an input voltage of 12V. Both
figures show the output from the IC controller
(from the “MOD” pin), and the voltage from the
collector of the ZTX789A with respect to
ground.
Figure 3. Active turn-off circuit for Bipolar transistors, allowing high efficiency DC-DC
conversion at high frequency
ISSUE 2 - SEPTEMBER 2003
SEMICONDUCTORS
AN26 - 4
Figure 4. Oscillograph of charger circuit employing passive turn-off for ZTX789A, showing:
1. Collector-to-0V waveform, and
2. IC “MOD” drive waveform. Note bipolar transistor storage and fall times. Channel 1:
5V/div, Channel 2, 2V/div, timebase at 2µs/div
Figure 4 shows the turn-off produced with a
passive turn-off circuit - a resistor. The storage
time of the bipolar transistor, or the time from
the falling edge of trace 2 to the start of the
falling edge of trace 1 is shown to be 1.6µs, and
the fall time is 340ns. This fall time could
represent a major power loss contributer and
therefore would (under some circumstances)
serve to limit the maximum load current.
ISSUE 2 - SEPTEMBER 2003
AN26 - 5
SEMICONDUCTORS
Figure 5. Oscillograph of charger circuit employing active turn-off for ZTX789A, showing:
1. Collector-to-0v waveform, and
2. IC “MOD” drive waveform. Note bipolar transistor storage and fall times. Channel 1:
5V/div, channel 2, 2V/div, timebase at 2µs/div
Figure 5 shows the turn-off produced with an
active turn-off circuit as shown in figure 3. The
storage time has reduced to less than 200ns,
and the fall time to 90ns. These switching
values are comparable to, or better than the
large P-Channel MOSFETs that would
otherwise be specified, and allow the bipolar
device to be operated to much higher currents
than possible with passive turn-off circuitry.
ISSUE 2 - SEPTEMBER 2003
SEMICONDUCTORS
AN26 - 6
Appendix A
BVCEO
IC(DC)
ICM
hFE @IC/VCE
VCE(sat)(max.)
@IC/IB
FMMT717
Device
12V
2.5A
10A
275(typ) 2.5A/2V
140mV
1A/10mA
Package
SOT23
ZXT13P12DE6
12V
4A
15A
450(typ)
1A/2V
90mV
1A/10mA
SOT23-6
ZXT13P20
20V
4A
10A
450(typ)
1A/2V
130mV
1A/10mA
SOT23-6
Table 1. PNP transistors for fast chargers <1A.
BVCEO
IC(DC)
ICM
VCE(sat)(max.)
@IC/IB
Package
ZTX788B
Device
15
3
8
300 min
hFE @IC/VCE
2A/2V
450mV
2A/10mA
E-Line
FZT788B
15
3
8
300 min
2A/2V
450mV
2A/10mA
SOT23-6
ZTX789A
25
3
8
200 min
2A/2V
450mV
2A/20mA
E-Line
FZT789B
25
3
8
200 min
2A/2V
450mV
2A/20mA
SOT23-6
ZTX90A
40
2
6
200 min
1A/2V
450mV
1A/10mA
E-Line
FZT90A
40
2
6
200 min
1A/2V
450mV
1A/10mA
SOT23-6
ZTX949
30
4.5
20
140(typ)
5A/1V
320mV
5A/300mA
E-Line
FZT949
30
4.5
20
140(typ)
5A/1V
320mV
5A/300mA
SOT23-6
ZTX951
60
4
15
140(typ)
4A/1V
300mV
4A/400mA
E-Line
FZT951
60
4
15
140(typ)
4A/1V
300mV
4A/400mA
SOT23-6
Table 2. PNP transistors for fast chargers 1 to 4A.
BVCEO
IC(DC)
ICM
hFE @IC/VCE
VCE(sat)(max.)
@IC/IB
Package
FZT1147A
Device
12V
5A
20A
340(typ) 2A/12V
130mV
1A/6mA
SOT223
FZT1148A
25V
4A
10A
320(typ)
240mV
1A/7mA
SOT223
2A/2V
Table 3. PNP transistors for linear and switch mode high current (5A) fast chargers.
ISSUE 2 - SEPTEMBER 2003
AN26 - 7
SEMICONDUCTORS
Application Note 26
ISSUE 2 - SEPTEMBER 2003
© Zetex plc 2003
Americas
Asia Pacific
Zetex GmbH
Streitfeldstraße 19
D-81673 München
Zetex Inc
700 Veterans Memorial Hwy
Hauppauge, NY 11788
Germany
Telefon: (49) 89 45 49 49 0
Fax: (49) 89 45 49 49 49
[email protected]
USA
Telephone: (1) 631 360 2222
Fax: (1) 631 360 8222
[email protected]
Zetex (Asia) Ltd
3701-04 Metroplaza Tower 1
Hing Fong Road
Kwai Fong
Hong Kong
Telephone: (852) 26100 611
Fax: (852) 24250 494
[email protected]
Europe
Zetex plc
Fields New Road
Chadderton
Oldham, OL9 8NP
United Kingdom
Telephone (44) 161 622 4444
Fax: (44) 161 622 4446
[email protected]
These offices are supported by agents and distributors in major countries world-wide.
This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used,
applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products
or services concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply of
any product or service.
For the latest product information, log on to www.zetex.com
SEMICONDUCTORS
AN26 - 8
Similar pages