LTC3557_15 Datasheet, PDF (17/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
OPERATION
NTC Thermistor
The battery temperature is measured by placing a negative
temperature coefficient (NTC) thermistor close to the bat-
tery pack. The NTC circuitry is shown in Figure 8. To use
this feature connect the NTC thermistor, R_NTC, between
the NTC pin and ground and a bias resistor, R_NOM, from
V_NTC to NTC. R_NOM should be a 1% resistor with a value
equal to the value of the chosen NTC thermistor at 25°C
(R25). A 100k thermistor is recommended since thermistor
current is not measured by the LTC3557/LTC3557-1 and
will have to be considered for USB compliance.
The LTC3557/LTC3557-1 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 (float) 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 LTC3557/
LTC3557-1 is also designed to pause charging when the
value of the NTC thermistor increases to 3.25 times the value
of R25. For a Vishay “Curve 1” thermistor this resistance,
325k, corresponds to approximately 0°C. The hot and cold
comparators each have approximately 3°C of hysteresis
to prevent oscillation about the trip point. Grounding the
NTC pin disables all NTC functionality.
General Purpose Step-Down Switching Regulators
The LTC3557/LTC3557-1 includes three 2.25MHz constant
frequency current mode step-down switching regulators
providing 400mA, 400mA and 600mA each. All step-down
switching regulators can be programmed for a minimum
output voltage of 0.8V and can be used to power a micro-
controller core, microcontroller I/O, memory or other logic
circuitry. All step-down switching regulators support 100%
duty cycle operation (low dropout mode) when the input
voltage drops very close to the output voltage and are also
capable of Burst Mode operation for highest efficiencies at
light loads (Burst Mode operation is pin selectable). The
step-down switching regulators also include soft-start to
limit inrush current when powering on, short-circuit current
protection, and switch node slew limiting circuitry to reduce
EMI radiation. No external compensation components are
required for the switching regulators.
A single MODE pin sets all step-down switching regulators
in Burst Mode or pulse-skip mode operation, while each
regulator is enabled individually through their respective
enable pins (EN1, EN2 and EN3). It is recommended that
the step-down switching regulator input supplies (V_IN1 and
V_IN2) be connected to the system supply pin (V_OUT). This
allows the undervoltage lock out circuit on the V_OUT pin
(V_OUT UVLO)to disable the step-down switching regulators
when the V_OUT voltage drops below V_OUT UVLO threshold.
If driving the step-down switching regulator input supplies
from a voltage other than V_OUT the regulators should
not be operated outside the specified operating range as
operation is not guaranteed beyond this range.
Step-Down Switching Regulator Output Voltage
Programming
Figure 2 shows the step-down switching regulator
application circuit. The full-scale output voltage for each
step-down switching regulator is programmed using a
resistor divider from the step-down switching regulator
output connected to the feedback pins (FB1, FB2 and
FB3) such that:
V_OUTx
=
0.8V
•
⎛ R1
⎝⎜ R2
+
1^{⎞⎠⎟}
Typical values for R1 are in the range of 40k to 1M. The
capacitor C_FB cancels the pole created by feedback resistors
and the input capacitance of the FB pin and also helps
to improve transient response for output voltages much
greater than 0.8V. A variety of capacitor sizes can be used
for C_FB but a value of 10pF is recommended for most
applications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
EN
PWM
MODE ^CONTROL
V_IN
MP _SWx
L
MN
C_FB
FBx
0.8V
GND
V_OUTx
C_OUT
R1
R2
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Figure 2. Buck Converter Application Circuit
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