LTC3554-3_15 Datasheet, PDF (17/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
OPeration
Battery Charger Stability Considerations
The LTC3554’s battery charger contains both a constant-
voltage and a constant-current control loop. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1μF from BAT to
GND. Furthermore, a 100μF 1210 ceramic capacitor in
series with a 0.3Ω resistor from BAT to GND is required
to keep ripple voltage low if operation with the battery
disconnected is allowed.
High value, low ESR multilayer ceramic chip capacitors
reduce the constant-voltage loop phase margin, possibly
resulting in instability. Ceramic capacitors up to 22μF
may be used in parallel with a battery, but larger ceramics
should be decoupled with 0.2Ω to 1Ω of series resistance.
In constant-current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin has a parasitic capacitance,
C_PROG, the following equation should be used to calculate
the maximum resistance value for R_PROG:
R _PROG
≤
2π
•
1
100kHz
•
C_PROG
NTC Thermistor
The battery temperature is measured by placing a nega-
tive temperature coefficient (NTC) thermistor close to the
battery pack. To use this feature connect the NTC therm-
istor, R_NTC, between the NTC pin and ground and a bias
resistor, R_NOM, from V_BUS to NTC, as shown in Figure 1.
R_NOM should be a 1% resistor with a value equal to the
value of the chosen NTC thermistor at 25°C (R25). The
LTC3554 will pause charging when the resistance of the
NTC thermistor drops to 0.54 times the value of R25
or approximately 54k (for a Vishay curve 1 thermistor,
this corresponds to approximately 40°C). If the battery
charger is in constant-voltage mode, the safety timer also
pauses until the thermistor indicates a return to a valid
temperature. As the temperature drops, the resistance of
the NTC thermistor rises. The LTC3554 is also designed
to pause charging when the value of the NTC thermistor
increases to 3.17 times the value of R25. For a Vishay
curve 1 thermistor this resistance, 317k, corresponds to
approximately 0°C. The hot and cold comparators each
have approximately 3°C of hysteresis to prevent oscillation
about the trip point.
Alternate NTC Thermistors and Biasing
The LTC3554 provides temperature qualified charging if
a grounded thermistor and a bias resistor are connected
to NTC. By using a bias resistor whose value is equal to
the room temperature resistance of the thermistor (R25)
the upper and lower temperatures are preprogrammed
to approximately 40°C and 0°C, respectively (assuming
a Vishay curve 1 thermistor).
The upper and lower temperature thresholds can be
adjusted by either a modification of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modified but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tem-
perature trip points can be independently programmed
with the constraint that the difference between the upper
and lower temperature thresholds cannot decrease.
Examples of each technique are given below.
3554123ff
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