DN54 - A 4-Cell Ni-Cad Regulator/Charger for Notebook Computers

A 4-Cell Ni-Cad Regulator/Charger for
Notebook Computers - Design Note 54
Tim Skovmand
pack can be used to power a 5V notebook computer
system. Inexpensive N-channel power MOSFETs have
very low on resistance and can be used to switch power
with low voltage drop between the battery pack and the
5V logic circuits.
The new LTC ®1155 dual power MOSFET driver delivers
12V of gate drive to two N-channel power MOSFETs
when powered from a 5V supply with no external
components required. This ability, coupled with its
micropower current demands and protection features,
makes it an excellent choice for high side switching
applications which previously required more expensive
P-channel MOSFETs.
Figure 1 shows how a battery charger and an extremely
low voltage drop 5V regulator can be built using the
new LTC1155 and three inexpensive power MOSFETs.
A notebook computer power supply system is a good
example of an application which benefits directly from
this high side driving scheme. A 4-cell, Ni-Cad battery
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Figure 1. The LTC1155 Dual MOSFET Driver Provides Gate Drive
and Protection for a 4-Cell Ni-Cad Charger and Regulator
11/91/54_conv
Quick Charge Battery Charger
One-half of the LTC1155 dual MOSFET driver controls
the charging of the battery pack. The 9V, 2A current
limited wall unit is switched directly into the battery pack
through an extremely low resistance MOSFET switch,
Q2. The gate drive output, Pin 2, generates about 13V
of gate drive to fully enhance Q1 and Q2. The voltage
drop across Q2 is only 0.17V at 2A and, therefore, can
be surface mounted to save board space.
An inexpensive thermistor, RT1, measures the battery
temperature and latches the LTC1155 off when the
temperature rises to 40°C by pulling low on Pin 1, the
drain sense input. The window comparator also ensures
that battery packs which are very cold (<10°C) are not
quick charged.
Q1 drives an indicator lamp during quick charge to let
the computer operator know that the battery pack is
being charged properly. When the battery temperature
rises to 40°C, the LTC1155 latches off and the battery
charge current flowing through R9 drops to 150mA.
Extremely Low Voltage Drop Regulator
A 4-cell Ni-Cad battery pack produces about 6V when
fully charged. This voltage will drop to about 4.5V
when the batteries are nearly discharged. The second
half of the LTC1155 provides gate voltage drive, Pin 7,
for an extremely low voltage drop MOSFET regulator.
The LT1431 controls the gate of Q4 and provides a
regulated 5V output when the battery is above 5V.
When the battery voltage drops below 5V, Q4 acts as
a low resistance switch between the battery and the
regulator output.
A second power MOSFET, Q3, connected between the
9V supply and the regulator output “bypasses” the main
regulator when the 9V supply is connected. This means
that the computer power is taken directly from the AC
line while the charger wall unit is connected. The LT1431
provides regulation for both Q3 and Q4 and maintains
a constant 5V at the regulator output. The diode string
made up of diodes D2-D4 ensure that Q3 conducts all
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the regulator current when the wall unit is plugged in
by separating the two gate voltages by about 2V.
R14 acts as a current sense for the regulator. The regulator latches off at 3A when the voltage drop between
the second drain sense input, Pin 8, and the supply,
Pin 6, rises above 100mV. R10 and C3 provide a short
delay. The μP can restart the regulator by turning the
second input, Pin 5, off and then back on.
The regulator is switched off by the μP when the battery voltage drops below 4.6V. The standby current for
the 5V, 2A regulator is less than 10μA. The regulator
is switched on again when the battery voltage rises
during charging.
Very Low Power Dissipation
The power dissipation in the notebook computer is very
low. The current limited wall unit dissipates the bulk of
the power created by quick charging the battery pack.
Q2 dissipates less than 0.5W. R9 dissipates about 0.7W.
Q4 dissipates about 2W for a very short period of time
when the batteries are fully charged and dissipates less
than 0.5W as soon as the battery voltage drops to 5V.
The three integrated circuits shown are micropower
and dissipate virtually no power.
Cost Effective and Efficient Power System
The circuit shown in Figure 1 consumes very little board
space. The LTC1155 is available in a 8-pin SO package
and the three power MOSFETs can also be housed in
SO packaging. Q4 must be heat sinked properly for the
short period of time that the battery voltage is above
5.5V. (Consult the MOSFET manufacturer data sheet
for SO heat sink recommendations).
The LTC1155 allows the use of inexpensive N-channel
MOSFET switches to directly connect power from
a 4-cell Ni-Cad battery pack to the charger and the
load. This technique is very cost effective and is also
very efficient. Nearly all the battery power is delivered
directly to the load to ensure maximum operating time
from the batteries.
For applications help,
call (408) 432-1900
dn54f_conv BA/GP 1191 180K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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© LINEAR TECHNOLOGY CORPORATION 1991
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