Datasheet

10
LTC1734
Once the maximum power dissipation and V
CE(MIN)
are
known, Table 1 can be used as a guide in selecting some
PNPs to consider. In the table, very low V
CESAT
is less than
0.25V, low V
CESAT
is 0.25V to 0.5V and the others are 0.5V
to 0.8V all depending on the current. See the manufacturer’s
data sheet for details. All of the PNP transistors are rated
to carry at least 1A continuously as long as the power
dissipation is within limits. The Stability section addresses
caution in the use of high beta PNPs.
Should overheating of the PNP transistor be a concern,
protection can be achieved with a positive temperature
coefficient (PTC) thermistor, wired in series with the
current programming resistor and thermally coupled to
the transistor. The PTH9C chip series from Murata has a
steep resistance increase at temperature thresholds from
85°C to 145°C making it behave somewhat like a thermo-
stat switch. For example, the model PTH9C16TBA471Q
thermistor is 470Ω at 25°C, but abruptly increase its
resistance to 4.7k at 125°C. Below 125°C, the device
exhibits a small negative TC. The 470Ω thermistor can be
added in series with a 1.6k resistor to form the current
programming resistor for a 700mA charger. Should the
thermistor reach 125°C, the charge current will drop to
238mA and inhibit any further increase in temperature.
Stability
The LTC1734 contains two control loops: constant voltage
and constant current. To maintain good AC stability in the
constant voltage mode, a capacitor of at least 4.7µF is
usually required from BAT to ground. The battery and
interconnecting wires appear inductive at high frequen-
cies, and since these are in the feedback loop, this capaci-
tance may be necessary to compensate for the inductance.
This capacitor need not exceed 100µF and its ESR can
range from near zero to several ohms depending on the
inductance to be compensated. In general, compensation
is optimal with a capacitance of 4.7µF to 22µF and an ESR
of 0.5Ω to 1.5Ω.
Using high beta PNP transistors (>300) and very low ESR
output capacitors (especially ceramic) reduces the phase
margin, possibly resulting in oscillation. Also, using high
value capacitors with very low ESRs will reduce the phase
margin. Adding a resistor of 0.5Ω to 1.5Ω in series with
the capacitor will restore the phase margin.
In the constant current mode, the PROG pin is in the
feedback loop, not the battery. Because of this, capaci-
tance on this pin must be limited. Locating the program
resistor near the PROG pin and isolating the charge
current monitoring circuitry (if used) from the PROG pin
with a 1k to 10k resistor may be necessary if the capaci-
tance is greater than that given by the following equation:
C
k
R
MAX pF
PROG
()
=
400
APPLICATIONS INFORMATION
WUU
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Table 1. PNP Pass Transistor Selection Guide
Maximum P
D
(W)
Mounted on Board
at T
A
= 25°C Package Style ZETEX Part Number ROHM Part Number Comments
0.5 SOT-23 FMMT549 Low V
CESAT
0.625 SOT-23 FMMT720 Very Low V
CESAT,
High Beta
1 SOT-89 FCX589 or BCX69
1.1 SOT-23-6 ZXT10P12DE6 Very Low V
CESAT,
High Beta, Small
1 to 2 SOT-89 FCX717 Very Low V
CESAT,
High Beta
2 SOT-223 FZT589 Low V
CESAT
2 SOT-223 BCP69 or FZT549
0.75 FTR 2SB822 Low V
CESAT
1 ATV 2SB1443 Low V
CESAT
2 SOT-89 2SA1797 Low V
CESAT
10 (T
C
= 25°C) TO-252 2SB1182 Low V
CESAT,
High Beta