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LT3651EUHE-4.1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LT3651-4.1/LT3651-4.2
10
365142ff
For more information www.linear.com/LT3651-4.1
OPERATION
Overview
The LT3651 is a complete Li-Ion battery charger, addressing
wide input voltage (up to 32V) and high currents (up to
4A). High charging efficiency is produced with a constant
frequency, average current mode synchronous step-down
switcher architecture.
The charger includes the necessary circuitry to allow for
programming and control of constant current, constant
voltage (CC/CV) charging with both current only and timer
termination. High charging efficiency is achieved by the
switcher by using a bootstrapped supply for low switch
drop for the high side driver and a MOSFET for the low
side (synchronous) switch.
Maximum charge current is set with an external sense re
-
sistor in series with the inductor and is adjustable through
the RNG/SS pin. T
otal system input current is monitored
with an input sense resistor and is used to maintain con
-
stant
input current by regulating battery charge current.
It is adjustable through the I
LIM
pin.
If the battery voltage is low, charge current is automatically
reduced to 15% of the programmed current to provide
safe battery preconditioning. Once the battery voltage
climbs above the battery precondition threshold, the IC
automatically increases the maximum charge current to
the full programmed value.
Charge termination can occur when charge current de
-
creases to one-tenth the programmed maximum charge
current
(C/10
termination). Alternately, termination can
be time based through the use of an internal program
-
mable charge cycle control timer. When using the timer
termination, charging continues beyond the C/10 level to
“top-off” a battery. Charging typically terminates three
hours after initiation. When the timer-based scheme is
used, bad battery detection is also supported. A system
fault is triggered if a battery stays in precondition mode
for more than one-eighth of the total charge cycle time.
Once charging is terminated and the LT3651 is not actively
charging, the IC automatically enters a low current standby
mode in which supply bias currents are reduced to 100µA.
If the battery voltage drops 2.5% from the full charge float
voltage, the LT3651 engages an automatic charge cycle
restart. The IC also automatically restarts a new charge
cycle after a bad battery fault once the failed battery is
removed and replaced with another battery.
After charging is completed the input bias currents on the
pins connecting to the battery are reduced to minimize
battery discharge.
The LT3651 contains provisions for a battery temperature
monitoring circuit. Battery temperature is monitored by
using a NTC thermistor located with the battery. If the
battery temperature moves outside a safe charging range
of 0°C to 40°C the charging cycle suspends and signals
a fault condition.
The LT3651 contains two digital open-collector outputs,
which provide charger status and signal fault conditions.
These binary coded pins signal battery charging, standby
or shutdown modes, battery temperature faults and bad
battery faults.
A precision undervoltage lockout is possible by using a
resistor divider on the shutdown pin (SHDN). The input
supply current is 17µA when the IC is in shutdown.
General Operation (See Block Diagram)
The LT3651 uses an average current mode control loop
architecture to control average charge current. The LT3651
senses charger output voltage via the BAT pin. The dif
-
ference between this voltage and the internal float volt-
age reference is integrated by the voltage error amplifier
(V
-EA). The amplifier output voltage (I
TH
) corresponds
to the desired average voltage across the inductor sense
resistor, R
SENSE
, connected between the SENSE and BAT
pins. The I
TH
voltage is divided down by a factor of 10,
and provides a voltage offset on the input of the current
error amplifier (C-EA). The difference between this im
-
posed voltage and the current sense resistor voltage is
integrated by C-EA. The resulting voltage (V
C
) provides a
voltage that is compared against an internally generated
ramp and generates the switch duty cycle that controls
the charger’s switches.
The I
TH
error voltage corresponds linearly to average
current sensed across the inductor current sense resistor.
Maximum charge current is controlled by clamping the
maximum voltage of I
TH
to 1V. This limits the maximum
current sense voltage (voltage across R
SENSE
) to 95mV
LT3651-4.1/LT3651-4.2
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365142ff
For more information www.linear.com/LT3651-4.1
OPERATION
setting the maximum charge current. Manipulation of
maximum charge current is possible through the RNG/SS
and I
LIM
pins (see the RNG/SS: Dynamic Charge Current
Adjust, RNG/SS: Soft-Start and I
LIM
Control sections).
If the voltage on the BAT pin (V
BAT
) is below V
BAT(PRE)
,
A7 initiates the precondition mode. During the pre-
condition interval, the charger continues to operate in
constant current mode, but the I
TH
clamp is reduced to
0.15V reducing charge current to 15% of the maximum
programmed value.
As V
BAT
approaches the float voltage (V
FLOAT
) the voltage
error amp V-EA takes control of I
TH
and the charger transi-
tions into constant voltage (CV) mode. As this occurs, the
I
TH
voltage falls from the limit clamp and charge current is
reduced from the maximum value. When the I
TH
voltage
falls below 0.1V, A8 signals C/10. If the charger is config-
ured for C/10 termination the charge cycle is terminated.
Once the charge cycle is terminated, the
CHRG
status
pin becomes high impedance and the charger enters low
current standby mode.
The LT3651 contains an internal charge cycle timer that
terminates a successful charge cycle after a programmed
amount of time. This timer is typically programmed to
achieve end-of-cycle in three hours, but can be configured
for any amount of time by setting an appropriate timing
capacitor value (C
TIMER
). When timer termination is used,
the charge cycle does not terminate after C/10 is achieved.
Because the CHRG status pin responds to the C/10 current
level, the IC will indicate a fully charged battery status,
but the charger will continue to source low currents. At
the programmed end of the cycle time the charge cycle
stops and the part enters standby mode. If the battery
did not achieve at least 97.5% of the full float voltage at
the end-of-cycle, charging is deemed unsuccessful and
another full-timer cycle is initiated.
Use of the timer function also enables bad battery detec
-
tion. This fault condition is achieved if the battery does
not respond to preconditioning and the charger remains
in (or enters) precondition mode after one-eighth of the
programmed
charge
cycle time. A bad battery fault halts
the charging cycle, the CHRG status pin goes high imped
-
ance and the FAULT pin is pulled low.
When the LT3651 terminates a charging cycle, whether
through C/10 detection or by reaching timer end-of-cycle,
the average current mode analog loop remains active but
the internal float voltage reference is reduced by 2.5%.
Because
the voltage on a successfully charged battery is
at the full float voltage, the voltage error amp detects an
overvoltage condition and rails low. When the voltage error
amp output drops below 0.3V, the IC enters standby mode,
where most of the internal circuitry is disabled and the
V
IN
bias current is reduced to <100µA. When the voltage
on the BAT pin drops below the reduced float reference
level, the output of the voltage error amp will climb, at
which point the IC comes out of standby mode and a new
charging cycle is initiated.
The system current limit allows charge current to be
reduced in order to maintain a constant input current.
Input current is measured via a resistor (R
CL
) that is
placed between the CLP and CLN pins. Power is applied
through this resistor and is used to supply both V
IN
of the
chip and other system loads. An offset produced on the
inputs of A12 sets the threshold. When that threshold is
achieved, I
TH
is reduced, lowering the charge current thus
maintaining the maximum input current.
50µA of current is sourced from I
LIM
to a resistor (R
ILIM
)
that is placed from that pin to ground. The voltage on I
LIM
determines the regulating voltage across R
CL
. 1V on I
LIM
corresponds to 95mV across R
CL
. The I
LIM
pin clamps
internally to 1V maximum.
If the junction temperature of the die becomes excessive,
A10 activates decreasing I
TH
and reduces charge current.
This reduces on-chip power dissipation to safe levels but
continues charging.
LT3651-4.1/LT3651-4.2
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For more information www.linear.com/LT3651-4.1
APPLICATIONS INFORMATION
OSC Frequency
A precision resistor to ground sets the LT3651 switcher
oscillator frequency, f
OSC
, permitting user adjustability
of the frequency value. Typically this frequency is in the
200kHz to 1MHz range. Power consideration may neces
-
sitate lower frequency operation especially if the charger
is operated with very high voltages. Adjustability also
allows the user to position switching harmonics if their
system requires.
The timing resistor, R
T
, value is set by the following:
R
T
=
54.9
f
OSC
MHz
( )
kΩ
( )
Set R
T
to 54.9k for 1MHz operation.
V
IN
Input Supply
The LT3651 is biased directly from the charger input supply
through the V
IN
pin. This supply provides large switched
currents, so a high quality, low ESR decoupling capacitor
is required to minimize voltage glitches on V
IN
. The V
IN
decoupling capacitor (C
VIN
) absorbs all input switching
ripple current in the charger. Size is determined by input
ripple voltage with the following equation:
C
IN(BULK)
=
I
MAX
• V
BAT
f
OSC
MHz
( )
• ∆V
IN
• V
IN
µF
(
)
where ∆V
IN
is the input ripple, I
MAX
is the maximum
charge current and f is the oscillator frequency. A good
starting point for ∆V
IN
is 0.1V. Worst-case conditions are
with V
BAT
high and V
IN
at minimum. So for a 8V V
IN(MIN)
,
I
MAX
= 4A and a 1MHz oscillator frequency:
C
IN(BULK)
=
= 21µF
The capacitor must have an adequate ripple current rating.
RMS ripple current, I
CVIN(RMS)
is approximated by:
I
CVIN(RMS)
≈ I
CHARGE(MAX)
•
V
BAT
V
IN
•
V
IN
V
BAT
– 1
which has a maximum at V
IN
= 2 • V
BAT
, where I
CVIN(RMS)
= I
CHARGE(MAX)
/2. In the example above that requires a
capacitor RMS rating of 2A.
Boost Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the high side switch
transistor. The BOOST voltage is normally created by con
-
necting a 1µF capacitor from the BOOST pin to the SW
pin. Operating range of the BOOST pin is 2V to 4.5V
, as
referenced to the SW pin.
The boost capacitor is normally charged via a diode con
-
nected from the batter
y or an external source through the
low side
switch. Rate the diode average current greater
than 0.1A and its reverse voltages greater than V
IN(MAX)
.
If an external supply that is greater than the input is avail-
able (V
BOOST
– V
IN
> 2V), it may be used in place of the
bootstrap capacitor and diode.
V
IN
,V
BOOST
Start-Up Requirement and Blocking
The LT3651 operates with a V
IN
range up to 32V. The
charger begins a charging cycle when the detected battery
voltage is below the 4.0V/4.1V auto-restart float voltage
and the part is enabled.
When V
IN
is below 6.3V and the BOOST capacitor is un-
charged, the high side switch would normally not have
sufficient head room to start switching. During normal
operation the low side switch is deactivated
when charge
current is very low to prevent reverse current in the in
-
ductor. However in order to facilitate start-up, the LT3651
enables the switch if V
BOOST
voltage is low. This allows
initial charging of the BOOST capacitor which then permits
the high side switch to saturate and efficiently operate.
The boost capacitor charges to full potential after a few
cycles. Because of potential issues when operating at very
high duty cycles that often occur at start-up, it is highly
recommended that the part SHDN pin be used to enable
part start-up once V
IN
is above 6.3V.
To prevent battery discharge with a shorted input supply,
a blocking Schottky diode or FET is recommended in se
-
ries with the input as shown in the Typical Applications.
Of course the voltage drops associated with the blocking
diode or FET as well as the input current sense resistor (if
used) and IR drops in the power path need to be accounted
for. Input currents increase at lower voltages because the
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LT3651EUHE-4.1#PBF

Mfr. #:
Buy LT3651EUHE-4.1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management Monolithic 4A High Voltage Li-Ion Battery Charger
Lifecycle:
New from this manufacturer.
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