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LT3651EUHE-4.1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LT3651-4.1/LT3651-4.2
13
365142ff
For more information www.linear.com/LT3651-4.1
duty cycle increases. Adding a soft-start capacitor to the
RNG/SS pin may be helpful as well at start-up
BAT Output Decoupling
It is recommended that the LT3651 charger output have
a decoupling capacitor. If the battery can be disconnected
from the charger output this capacitor is required. The
value of this capacitor (C
BAT
) is related to the minimum
operational V
IN
voltage such that:
C
BAT
≈ 20µF +
350µF
V
IN(MIN)
The voltage rating on C
BAT
must meet or exceed the bat-
tery float voltage.
R
SENSE
: Charge Current Programming
The LT3651 charger is configurable to charge at average
currents as high as 4A (see Figure 1). If RNG/SS maximum
voltage is not limited, the inductor sense resistor, R
SENSE
,
has 95mV across it at maximum charge current so:
R
SENSE
=
0.095V
I
MAX(AVG)
where I
MAX(AVG)
is the maximum average charge current.
R
SENSE
is 24mΩ for a 4A charger.
Inductor Selection
The primary criteria for inductor value selection in the
LT3651 charger is the ripple current created during switch
-
ing. Ripple current, ∆I
MAX
, is typically set within a range of
25% to 35% of the maximum charge current, I
MAX
. This
percentage typically gives a good compromise between
losses due to ripple and inductor size. An approximate
formula for inductance is:
L =
V
BAT
+ V
F
∆I
MAX
• f
OSC
MHz
( )
• 1–
V
BAT
+ V
F
V
IN
+ V
F
µH
( )
Worse-case ripple is at high V
IN
and high V
BAT
. V
F
is the
forward voltage of the synchronous switch (approximately
0.14V at 4A). Figure 2 shows inductance for the case of a
4A charger. The inductor must have a saturation current
equal to or exceeding the maximum peak current in the
inductor. Peak current is I
MAX
+ ∆I
MAX
/2.
Magnetics vendors typically specify inductors with maxi-
mum RMS and saturation current ratings. Select an induc-
tor that has a saturation current rating at or above peak
current, and an RMS rating above I
MAX
. Inductors 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 approximately:
V
BAT
f
OSC(MHz)
• 1–
V
BAT
V
IN(MAX)
V •µs
( )
APPLICATIONS INFORMATION
Figure 2. Inductance (L) vs Maximum V
IN
Figure 1. Programming Maximum Charge Current Using R
SENSE
V
IN(MAX)
(V)
5
L (µH)
2
3
25
365142 F02
1
0
10
15
20
30
4
I
MAX
= 4A
f
OSC
= 1MHz
25% TO 35% RIPPLE
SW
BOOST
SENSE
R
SENSE
LT3651
BAT
365142 F01
+
LT3651-4.1/LT3651-4.2
14
365142ff
For more information www.linear.com/LT3651-4.1
System Input Current Limit
The LT3651 contains a PowerPath
TM
control feature to
help manage supply load currents. The charger adjusts
charger output current in response to a system load so as
to maintain a constant input supply load. If overall input
supply current exceeds the programmed maximum value
the charge current is diminished in an attempt to keep
supply current constant. One application where this is
helpful is if you have a current limited input supply. Setting
the maximum input current limit below the supply limit
prevents supply collapse.
A resistor, R
CL
, is placed between the input supply and the
system and charger loads as shown in Figure 3.
The LT3651 sources 50µA from the I
LIM
pin, so a voltage
is developed by simply connecting a resistor to ground.
The voltage on the I
LIM
pin corresponds to 11.5 times the
maximum voltage across the input sense resistor (R
CL
).
Input current limit is defined by:
I
INPUT(MAX)
=
V
ILIM
11.5 • R
CL
=
50µA • R
ILIM
11.5 • R
CL
The programming range for I
LIM
is 0V to 1V. Voltages higher
than 1V have no effect on the maximum input current. The
default maximum sense voltage is 95mV and is obtained
if R
ILIM
is greater than 20k or if the pin is left open.
For example, say you want a maximum input current of
2A and the charger is designed for 4A maximum average
charge current, which is 1A V
IN
referred (4A time duty
cycle). Using the full I
LIM
range, the maximum voltage
across R
CL
is 95mV. So R
CL
is set at 95mV/2A = 48mΩ.
When the system load exceeds 1A (= 2A – 1A) charge
current is reduced such that the total input current stays
at 2A. When the system load is 2A the charge current is 0.
This feature only controls charge current so if the system
load exceeds the maximum limit and no other limitation
is designed, the input current exceeds the maximum
desired, though the charge current reduces to 0A. When
the input limiter reduces charge current it does not impact
the internal system timer if used. See Figure 4.
If reduced voltage overhead or better efficiency is required
then reduce the maximum voltage across R
CL
. So for
instance, a 10k R
ILIM
sets the maximum R
CL
voltage to
43mV. This reduction comes at the expense of slightly
increased limit variation.
APPLICATIONS INFORMATION
Figure 3. Input Current Limit Configuration
SYSTEM LOAD CURRENT (A)
INPUT CURRENT
CHARGE
CURRENT
(V
IN
REFERRED)
2
2
365142 F04
1
10
0
3
CURRENT (A)
Figure 4. Input Current Limit for 4A Maximum Charger
and 6A System Current Limit
CLP
CLN
SYSTEM LOAD
INPUT
SUPPLY
V
IN
R
ILIM
R
CL
LT3651
I
LIM
365142 F03
LT3651-4.1/LT3651-4.2
15
365142ff
For more information www.linear.com/LT3651-4.1
Note the LT3651 internally integrates the input limit
signals. This should normally provide sufficient filtering
and reduce the sensitivity to current spikes. For the best
accuracy take care in provide good Kelvin connections
from R
CL
to CLP, CLN.
Further flexibility is possible by dynamically altering the
I
LIM
pin. Different resistor values could be switched in
to create unique input limit conditions. The I
LIM
pin can
also be tied to a servo amplifier for other options. See the
information in the following section concerning I
RNG/SS
programming for examples.
RNG/SS: Dynamic Current Adjust
The RNG/SS pin gives the user the capability to adjust
maximum charge current dynamically. The part sources
50µA from the pin, so connecting a resistor to ground
develops a voltage. The voltage on the RNG/SS pin cor
-
responds to approximately ten times the maximum volt-
age across the charge current sense resistor, R
SENSE
. The
defining equations for charge current are:
I
MAX(RNG/SS)
=
V
RNG/SS
10.8 • R
SENSE
=
50µA • R
RNG/SS
10.8 • R
SENSE
I
MAX(RNG/SS)
is the maximum charge current.
The programming range for RNG/SS is 0V to 1V. Voltages
higher than 1V have no effect on the maximum charge
current. The default maximum sense voltage is 95mV
and is obtained if R
RNG/SS
is greater than 20k or if the
pin is left open.
For example, say you want to reduce the maximum charge
current to 50% of the maximum value. Set RNG/SS to 0.5V
(50% of 1V), imposing a 48mV maximum sense voltage.
Per the above equation, 0.5V on RNG/SS requires a 10k
resistor. If the charge current needs to be dynamically
adjustable then Figure 5 shows one method.
Active servos can also be used to impose voltages on the
RNG/SS pin, provided they can only sink current. Active
circuits that source current cannot be used to drive the
RNG/SS pin. An example is shown in Figure 6.
RNG/SS: Soft-Start
Soft-start functionality is also supported by the RNG/SS
pin. The 50µA sourced from the RNG/SS pin can linearly
charge a capacitor, C
RNG/SS
, connected from the RNG/SS
pin to ground (see Figure 7). The maximum charge current
follows this voltage. Thus, the charge current increases
from zero to the fully programmed value as the capacitor
charges from 0V to 1V. The value of C
RNG/SS
is calculated
based on the desired time to full current (t
SS
) following
the relation:
C
RNG/SS
= 50µA • t
SS
The RNG/SS pin is pulled to ground internally when charg-
ing is terminated so each new charging cycle begins with
a soft-start
cycle. RNG/SS is also pulled to ground during
bad battery and NTC fault conditions, so a graceful recovery
from these faults is possible.
APPLICATIONS INFORMATION
LT3651
RNG/SS
10k
365142 F05
LOGIC HIGH = HALF CURRENT
Figure 5. Using the RNG/SS Pin for
Digital Control of Maximum Charge Current
LT3651
RNG/SS
SERVO
REFERENCE
365142 F06
+
–
Figure 6. Driving the RNG/SS Pin
with a Current-Sink Active Servo Amplifier
LT3651
RNG/SS
C
RNG/SS
365142 F07
Figure 7. Using the RNG/SS Pin for Soft-Start
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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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