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LTC3586EUFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36
LTC3586/LTC3586-1
19
3586fb
Thermal Regulation
To optimize charging time, an internal thermal feedback
loop may automatically decrease the programmed charge
current. This will occur if the die temperature rises to
approximately 110°C. Thermal regulation protects the
LTC3586/LTC3586-1 from excessive temperature due to
high power operation or high ambient thermal conditions
and allows the user to push the limits of the power handling
capability with a given circuit board design without risk of
damaging the LTC3586/LTC3586-1 or external components.
The benefit of the LTC3586/LTC3586-1 thermal regulation
loop is that charge current can be set according to actual
conditions rather than worst-case conditions with the as-
surance that the battery charger will automatically reduce
the current in worst-case conditions.
A flow chart of battery charger operation can be seen in
Figure 4.
Low Supply Operation
The LTC3586/LTC3586-1 incorporate an undervoltage
lockout circuit on V
OUT
which shuts down all four general
purpose switching regulators when V
OUT
drops below
V
OUTUVLO
. This UVLO prevents unstable operation.
FAULT Pin
FAULT is a bi-directional pin with an open-drain output used
to indicate a fault condition on any of the general purpose
regulators. If any of the four regulators are enabled, and
their corresponding FB pin voltage does not rise to within
8% of the internal reference voltage (0.8V) within 14ms, a
fault condition will be reported by FAULT going low. This,
in turn, will disable all of the regulators. Alternatively, the
regulators can be all disabled simultaneously by driving
FAULT low externally. This fault condition can be cleared
only if all of the ENABLE inputs are pulled low for at least
3.6µs. Since FAULT is an open-drain output, it requires
a pull-up resistor to the input voltage of the monitoring
microprocessor or another appropriate power source
such as LD03V3.
If any of the ENABLE pins is tied high during start-up,
the FAULT pin can erroneously report a fault condition.
To avoid such an event, the ENABLE pins should be tied
high
through a lowpass filter (comprised of a 1k resistor
and a 0.1µF capacitor) to the same power source to which
FAULT pin is pulled up.
General Purpose Buck Switching Regulators
The LTC3586/LTC3586-1 contain two 2.25MHz constant-
frequency current mode buck switching regulators. Each
buck regulator can provide up to 400mA of output current.
Both buck 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, disk drive or
other logic circuitry. Both buck converters support 100%
duty cycle operation (low dropout mode) when their input
voltage drops very close to their output voltage. To suit a
variety of applications, selectable mode functions can be
used to trade-off noise for efficiency. Two modes are avail-
able to control the operation of the LTC3586/LTC3586-1’s
buck regulators. At moderate to heavy loads, the pulse-
skip mode provides the least noise switching solution. At
lighter loads, Burst Mode operation may be selected. The
buck regulators include soft-start to limit inrush current
when powering on, short-circuit current protection and
switch node slew limiting circuitry to reduce radiated
EMI. No external compensation components are required.
The operating mode of the buck regulators can be set by
the MODE pin. The buck converters can be individually
enabled by the EN1 and EN2 pins. Both buck regulators
have
a fixed feedback servo voltage of 800mV. The buck
regulator input supplies V
IN1
and V
IN2
will generally be
connected to the system load pin V
OUT
.
Buck Regulator Output Voltage Programming
Both buck regulators can be programmed for output volt-
ages greater than 0.8V. The output voltage for each buck
regulator is programmed using a resistor divider from the
buck regulator output connected to the feedback pins (FB1
and FB2) such that:
V V
R
R
OUTX FBX
= +
1
2
1
where V
FB
is fixed at 0.8V and X = 1, 2. See Figure 4.
Typical values for R1 are in the range of 40k to 1M. The
capacitor C
FB
cancels the pole created by feedback resistors
operaTion
LTC3586/LTC3586-1
20
3586fb
operaTion
CLEAR EVENT TIMER
NTC OUT OF RANGE
CHRG CURRENTLY
HIGH-Z
INDICATE
NTC FAULT
AT CHRG
BATTERY STATE
CHARGE AT
1022V/R
PROG
RATE
PAUSE EVENT TIMER
INHIBIT CHARGER
CHARGE WITH
FIXED VOLTAGE
(4.200V)
RUN EVENT TIMER
CHARGE AT
100V/R
PROG
(C/10 RATE)
RUN EVENT TIMER
ASSERT CHRG LOW
POWER ON
TIMER > 30 MINUTES TIMER > 4 HOURS
BAT > 2.85V BAT < 4.1V
I
BAT
< C/10
NO
NO
YES
YES
YES
YES
YES
YES
NO
NO
BAT > 4.15VBAT < 2.85V
2.85V < BAT < 4.15V
NO
NO
NONO
INHIBIT CHARGING STOP CHARGING
INDICATE BATTERY
FAULT AT CHRG
BAT RISING
THROUGH 4.1V
BAT FALLING
THROUGH 4.1V
RELEASE CHRG
HIGH-Z
RELEASE CHRG
HIGH-Z
3586 F04
NO
YES
YES
YES
Figure 4. Flow Chart for Battery Charger Operation (LTC3586)
LTC3586/LTC3586-1
21
3586fb
and the input capacitance of the FBx 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 ap-
plications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
Buck Regulator Operating Modes
The LTC3586/LTC3586-1’s buck regulators include two
possible operating modes to meet the noise/ power needs
of a variety of applications.
In pulse-skip mode, an internal latch is set at the start of
every cycle which turns on the main P-channel MOSFET
switch. During each cycle, a current comparator compares
the peak inductor current to the output of an error amplifier.
The output of the current comparator resets the internal
latch which causes the main P-channel MOSFET switch to
turn off and the N-channel MOSFET synchronous rectifier
to turn on. The N-channel MOSFET synchronous rectifier
turns off at the end of the 2.25MHz cycle or if the current
through the N-channel MOSFET synchronous rectifier
drops to zero. Using this method of operation, the error
amplifier adjusts the peak inductor current to deliver the
required output power. All necessary compensation is
internal to the switching regulator requiring only a single
ceramic output capacitor for stability. At light loads, the
inductor current may reach zero on each pulse which will
turn off the N-channel MOSFET synchronous rectifier.
In this case, the switch node (SW1, SW2) goes high
impedance and the switch node voltage will “ring”. This
is discontinuous mode operation, and is normal behavior
for a switching regulator. At very light loads, the buck
regulators will automatically skip pulses as needed to
maintain output regulation.
operaTion
At high duty cycles (V
OUTx
> V
INx
/2) it is possible for the
inductor current to reverse, causing the buck regulator
to operate continuously at light loads. This is normal and
regulation is maintained, but the supply current will increase
to several milliamperes due to continuous switching.
In Burst Mode operation, the buck regulator automati-
cally
switches between fixed frequency P
WM operation
and hysteretic control as a function of the load current.
At light loads, the buck regulators operate in hysteretic
mode in which the output capacitor is charged to a volt-
age slightly higher than the regulation point. The buck
converter then goes into sleep mode, during which the
output capacitor provides the load current. In sleep mode,
most of the regulator’s circuitry is powered down, helping
conserve battery power. When the output voltage drops
below a predetermined value, the buck regulator circuitry
is powered on and the normal PWM operation resumes.
The duration for which the buck regulator operates in
sleep mode depends on the load current. The sleep time
decreases as the load current increases. Beyond a certain
load current point (about 1/4 rated output load current) the
step-down switching regulators will switch to a low noise
constant frequency PWM mode of operation, much the
same as pulse-skip operation at high loads. For applica-
tions that can tolerate some output ripple at low output
currents,
Burst Mode operation provides better
efficiency
than pulse skip at light loads while still providing the full
specified output current of the buck regulator.
The buck regulators
allow mode transition on the fly,
providing seamless transition between modes even under
load. This allows the user to switch back and forth between
modes to reduce output ripple or increase low current
efficiency as needed.
Buck Regulator in Shutdown
The buck regulators are in shutdown when not enabled for
operation. In shutdown, all circuitry in the buck regulator
is disconnected from the buck regulator input supply
leaving only a few nanoamps of leakage current. The
buck regulator outputs are individually pulled to ground
through a 10k resistor on the switch pins (SW1 and SW2)
when in shutdown.
V
INx
LTC3586/
LTC3586-1
L
SWx
R1 C
OUT
X = 1, 2
C
FB
V
OUTx
R2
3586 F05
FBx
GND
Figure 5. Buck Converter Application Circuit
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LTC3586EUFE#TRPBF

Mfr. #:
Buy LTC3586EUFE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management High Efficiency USB Power Manager + Dual Buck + Boost + Buck/Boost DC/DC
Lifecycle:
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