LTC1734L View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1734L' PDF : 12 Pages View PDF
LTC1734L
APPLICATIONS INFORMATION
Table 1. PNP Pass Transistor Selection Guide
Maximum PD (W)
Mounted on Board
at TA = 25°C
0.2
Package Style
SC-70
ZETEX Part Number
0.2
SC-70
0.5
SOT-23
FMMT549
0.625
SOT-23
FMMT720
1
SOT-89
FCX589 or BCX69
1.1
SOT-23-6
ZXT10P12DE6
1 to 2
SOT-89
FCX717
2
SOT-223
FZT589
2
SOT-223
BCP69 or FZT549
0.75
FTR
1
ATV
2
SOT-89
ROHM Part Number
UMT4403
UMT2907A
2SB822
2SB1443
2SA1797
Comments
Smallest Size
Smallest Size
Low VCESAT
Very Low VCESAT, High Beta
Very Low VCESAT, High Beta, Small
Very Low VCESAT, High Beta
Low VCESAT
Low VCESAT
Low VCESAT
Low VCESAT
Once the maximum power dissipation and VCE(MIN) are
known, Table 1 can be used as a guide in selecting a
suitable PNP transistor. In the table, very low VCESAT is
less than 0.25V, low VCESAT is 0.25V to 0.5V and the others
are 0.5V to 0.8V all depending on the current required. 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 PRF 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 PRF18BA471QB1RB
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 180mA charger. Should the
thermistor reach 125°C, the charge current will drop to
60mA and inhibit any further increase in temperature.
Stability
The LTC1734L contains two control loops: constant volt-
age and constant current. To maintain good AC stability in
10
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:
CMAX(pF)
=
400k
RPROG
1734lf
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