LTC4080EMSE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4080EMSE Datasheet PDF : 20 Pages
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U
OPERATIO
Dropout Operation
When the BAT pin voltage approaches VOUT, the duty cycle
of the switching regulator approaches 100%. When VBAT
is approximately equal to VOUT, the regulator is said to be
in dropout. In dropout, the main switch (MP2) stays on
continuously with the output voltage being equal to the
battery voltage minus the voltage drops across the main
switch and the inductor.
LTC4080
Global Thermal Shutdown
The LTC4080 includes a global thermal shutdown which
shuts off the entire part (both battery charger and switch-
ing regulator) if the die temperature exceeds 160°C. The
LTC4080 resumes normal operation once the temperature
drops approximately 14°C.
APPLICATIO S I FOR ATIO
BATTERY CHARGER
Programming Charge Current
The battery charge current is programmed using a single
resistor from the PROG pin to ground. The charge current
is 400 times the current out of the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
RPROG
=
400
•
1V
IBAT
, IBAT
=
400
•
1V
RPROG
The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage and using
the following equation:
IBAT
=
VPROG
RPROG
• 400
Stability Considerations
The LTC4080 battery charger contains two control loops:
constant-voltage and constant-current. 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 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
In constant-current mode, the PROG pin voltage is in
the feedback loop, not the battery voltage. Because of
the additional pole created by 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 is loaded with a capacitance,
CPROG, the following equation should be used to calculate
the maximum resistance value for RPROG:
RPROG
≤
2π
1
• 105 •
CPROG
Average, rather than instantaneous, battery current may be
of interest to the user. For example, when the switching
regulator operating in low-current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
filter can be used on the PROG pin to measure the average
battery current as shown in Figure 2. A 10k resistor has
been added between the PROG pin and the filter capacitor
to ensure stability.
LTC4080
PROG
GND
10k
RPROG
CFILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
4080 F02
Figure 2. Isolating Capacitive Load
on PROG Pin and Filtering
4080fb
15
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