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LT3650EDD-8.2#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LT3650-8.2/LT3650-8.4
10
36508284fd
operaTion
The I
TH
error voltage corresponds linearly to average cur-
rent sensed across the inductor current sense resistor,
allowing maximum charge current control by limiting the
effective voltage range of I
TH
. A clamp limits this voltage
to 1V which, in turn, limits the current sense voltage to
100mV. This sets the maximum charge current, or the
current delivered while the charger is operating in constant-
current (CC) mode, which corresponds to 100mV across
R
SENSE
. This maximum charge current level can also be
manipulated through the RNG/SS pin (see the RNG/SS:
Dynamic Charge Current Adjust and RNG/SS: Soft-Start
sections).
If the voltage on the BAT pin is below V
BAT(PRE)
, the LT3650
engages precondition mode. During the precondition inter-
val, the charger continues to operate in constant-current
mode, but the maximum charge current is reduced to 15%
of the maximum programmed value as set by R
SENSE
.
When the charger output voltage on the BAT pin approaches
the float voltage (V
BAT(F LT)
), the charger transitions into
constant-voltage (CV) mode, and charge current is reduced
from the maximum value. As this occurs, the I
TH
voltage
falls from the limit clamp and servos to lower voltages.
The
IC monitors the I
TH
voltage as it is reduced, and
detection of the C/10 charge current is achieved when
I
TH
= 0.1V. If the charger is configured for C/10 termina-
tion, this threshold is used to terminate the charge cycle.
Once the charge cycle is terminated, the CHRG status
pin becomes high impedance and the charger enters low
current standby mode.
The LT3650 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 (EOC) 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 into the battery until the programmed EOC
time has elapsed, at which time the charge cycle will
terminate. At EOC, when the charging cycle terminates, if
the battery did not achieve at least 97.5% of the full float
voltage, charging is
deemed unsuccessful. The LT3650
will then re-initiate, and charging will continue for another
full timer cycle.
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 LT3650 terminates a charging cycle, whether
through C/10 detection or by reaching timer EOC, 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 er-
ror 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 85µA. 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.
LT3650-8.2/LT3650-8.4
11
36508284fd
V
IN
Input Supply
The LT3650 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 recommended to minimize voltage glitches on V
IN
. The
V
IN
decoupling capacitor (C
VIN
) absorbs all input switching
ripple current in the charger, so it must have an adequate
ripple current rating. RMS ripple current (I
CVIN(RMS)
) is:
I
CVIN(RMS)
I
CHG(MAX)
•
V
BAT
V
IN
•
V
IN
V
BAT
−1
1
2
which has a maximum at V
IN
= 2 • V
BAT
, where:
I
CVIN(RMS)
= I
CHG(MAX)
/2
The simple worst-case of 1/2 • I
CHG(MAX)
is commonly
used for design.
Bulk capacitance is a function of desired input ripple volt-
age (∆V
IN
), and follows the relation:
C
IN(BULK)
= I
MAX
•
V
BAT
/ V
IN
∆V
IN
µF
( )
10µF is typically adequate for most charger applications.
BOOST Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of switch transistor. Oper-
ating range of the BOOST pin is 0V to 8.5V, as referenced
to the SW pin. Connect a 1µF or greater capacitor from
the BOOST pin to the SW pin.
The voltage on the decoupling capacitor is refreshed
through a diode, with the anode connected to/from either
the battery output voltage or an external source, and the
cathode connected to the BOOST pin. Rate the diode av-
erage current greater than 0.1A, and its reverse voltages
greater than V
IN(MAX)
.
V
IN
/ BOOST Start-Up Requirement
The LT3650 operates with a V
IN
range of 9V to 32V,
however, a start-up voltage requirement exists due to
the nature of the nonsynchronous step-down switcher
topology used for the charger. If there is no BOOST supply
available, the internal switch requires (V
IN
– V
SW
) > 3V
to operate. This requirement does not exist if the BOOST
supply is available and (V
BOOST
– V
SW
) > 2V.
When an LT3650 charger is not switching, the SW pin is at
the same
potential as the battery, which can be as high as
V
BAT(F LT)
. For reliable start-up, the V
IN
supply must be at
least 3V above the SW pin. The minimum start-up speci-
fication of V
IN
at or above 11.5V provides ample margin
to satisfy this requirement. Once switching begins, the
BOOST supply capacitor gets charged such that (V
BOOST
– V
SW
) > 2V, and the V
IN
requirement no longer applies.
In low V
IN
applications, the BOOST supply can be powered
by an external source for start-up, eliminating the V
IN
start-up requirement.
V
BAT
Output Decoupling
An LT3650 charger output requires bypass capacitance
connected from the BAT pin to ground (C
BAT
). A 10µF ce-
ramic capacitor is required for all applications. In systems
where the battery can be disconnected from the charger
output, additional bypass capacitance may be desired for
visual indication of a no-battery condition (see the Status
Pins section).
If it is desired to operate a system load from the LT3650
charger output when the battery is disconnected, additional
bypass capacitance is required. In this type of application
with the charger being used as a DC/DC converter, exces-
sive ripple and/or
low amplitude oscillations can occur
without
additional output bulk capacitance. For these ap-
plications, place a 100µF low ESR nonceramic capacitor
(chip tantalum or organic semiconductor capacitors such
as Sanyo OS-CONs or POSCAPs) from BAT to ground,
in parallel with the 10µF ceramic bypass capacitor. This
additional bypass capacitance may also be required in
systems where the battery is connected to the charger
through long wires. The voltage rating on C
BAT
must meet
or exceed the battery float voltage.
applicaTions inForMaTion
LT3650-8.2/LT3650-8.4
12
36508284fd
R
SENSE
: Charge Current Programming
The LT3650 charger is configurable to charge at average
currents as high as 2A. Maximum charge current is set by
choosing an inductor sense resistor such that the desired
maximum average current through that sense resistor
creates a 100mV drop, or:
R
SENSE
=
0.1
I
MAX(AVG)
where I
MAX(AVG)
is the maximum average charge current.
A 2A charger, for example, would use a 0.05Ω sense
resistor.
to 35% of I
MAX
, so an inductor value can be determined
by setting 0.25 < ∆I
MAX
< 0.35.
Magnetics vendors typically specify inductors with
maximum RMS and saturation current ratings. Select an
inductor that has a saturation current rating at or above
(1+∆I
MAX
/2) • I
MAX
, and an RMS rating above I
MAX
. In-
ductors must also meet a maximum volt-second product
requirement. If this specification is not in the data sheet of
an inductor, consult the vendor to make sure the maximum
volt-second product is not being exceeded by your design.
The minimum required volt-second product is:
V
BAT
• 1 −
V
BAT
V
IN(MAX)
(V • µs)
applicaTions inForMaTion
Figure 1. Programming Maximum Charge Current Using R
SENSE
365082 F01
SW
BOOST
SENSE
BAT
R
SENSE
LT3650
Figure 2. 2A Charger Switched Inductor Value
(R
SENSE
= 0.05Ω) 25% to 35% I
MAX
Ripple Current
Figure 3. 1.3A Charger Switched Inductor Value
(R
SENSE
= 0.075Ω) 25% to 35% I
MAX
Ripple Current
Inductor Selection
The primary criteria for inductor value selection in an
LT3650 charger is the ripple current created in that inductor.
Once the inductance value is determined, an inductor must
also have a saturation current equal to or exceeding the
maximum peak current in the inductor. An inductor value
(L), given the desired amount of ripple current (∆I
MAX
)
can be approximated using the relation:
L =
10
∆I
MAX
•R
SENSE
•(V
BAT
+ V
F
)
• 1−
V
BAT
+ V
F
V
IN(MAX)
+ V
F
(µH)
In the previous relation, ∆I
MAX
is the normalized ripple
current, V
IN(MAX)
as the maximum operational voltage,
and V
F
is the forward voltage of the rectifying Schottky
diode. Ripple current is typically set within a range of 25%
MAXIMUM OPERATIONAL V
IN
VOLTAGE (V)
9
0
SWITCHED INDUCTOR VALUE (µH)
2
6
8
10
14
15
21
24
4
12
12
18
27
3230
365082 F02
9 15
21
24
12
18
27
3230
MAXIMUM OPERATIONAL V
IN
VOLTAGE (V)
0
SWITCHED INDUCTOR VALUE (µH)
2
6
8
10
20
14
4
16
18
12
365082 F03
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LT3650EDD-8.2#PBF

Mfr. #:
Buy LT3650EDD-8.2#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management High Voltage 2 Amp Monolithic Li-Ion Battery Charger
Lifecycle:
New from this manufacturer.
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