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LT3652HVEMSE#PBF

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LT3652HV
10
3652hvfb
For more information www.linear.com/LT3652HV
APPLICATIONS INFORMATION
Overview
LT3652HV is a complete monolithic, mid-power, multi-
chemistry buck battery charger, addressing high input
voltage applications with solutions that require a minimum
of external components. The IC uses a 1MHz constant fre-
quency, average-current mode step-down architecture.
The LT3652HV incorporates a 2A switch that is driven
by a bootstrapped supply to maximize efficiency during
charging cycles. Wide input range allows operation to full
charge from voltages as high as 34V. A precision threshold
shutdown pin allows incorporation of UVLO functionality
using a simple resistor divider. The IC can also be put into
a low-current shutdown mode, in which the input supply
bias is reduced to only 15µA.
The LT3652HV employs an input voltage regulation loop,
which reduces charge current if a monitored input voltage
falls below a programmed level. When the LT3652HV is
powered by a solar panel, the input regulation loop is used
to maintain the panel at peak output power.
The LT3652HV automatically enters a battery precondition
mode if the sensed battery voltage is very low. In this mode,
the charge current is reduced to 15% of the programmed
maximum, as set by the inductor sense resistor, R
SENSE
.
Once the battery voltage reaches 70% of the fully charged
float voltage, the IC automatically increases maximum
charge current to the full programmed value.
The LT3652HV can use a charge-current based C/10
termination scheme, which ends a charge cycle when
the battery charge current falls to one tenth of the pro-
grammed maximum charge current. The LT3652HV also
contains an internal charge cycle control timer, for timer-
based termination. When using the internal timer, the
IC combines C/10 detection with a programmable time
constraint, during which the charging cycle can continue
beyond the C/10 level to top-off a battery. The charge
cycle terminates when a specific time elapses, typically 3
hours. When the timer-based scheme is used, the IC also
supports bad battery detection, which triggers a system
fault if a battery stays in precondition mode for more than
one eighth of the total charge cycle time.
Once charging is terminated, the LT3652HV automati-
cally enters a low-current standby mode where supply
bias currents are reduced to 85µA. The IC continues to
monitor the battery voltage while in standby, and if that
voltage falls 2.5% from the full-charge float voltage, the
LT3652HV 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.
The LT3652HV contains provisions for a battery tem
-
perature monitoring circuit. This feature monitors battery
temperature using a thermistor during the charging cycle.
If the battery temperature moves outside a safe charg-
ing range of 0°C to 40°C, the IC suspends charging and
signals a fault condition until the temperature returns to
the safe charging range.
The LT3652HV 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.
General Operation (See Block Diagram)
The LT3652HV uses average current mode control loop
architecture, such that the IC servos directly to average
charge current. The LT3652HV senses charger output
voltage through a resistor divider via the V
FB
pin. The
difference between the voltage on this pin and an internal
3.3V voltage reference is integrated by the voltage error
amplifier (V-EA). This amplifier generates an error volt-
age on its output (I
TH
), which corresponds to the average
current sensed across the inductor current sense resistor,
R
SENSE
, which is connected between the SENSE and BAT
pins. The I
TH
voltage is then divided down by a factor of
10, and imposed on the input of the current error amplifier
(C-EA). The difference between this imposed voltage and
the current sense resistor voltage is integrated, with the
resulting voltage (V
C
) used as a threshold that is compared
against an internally generated ramp. The output of this
comparison controls the charger’s switch.
The I
TH
error voltage corresponds linearly to average
current 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 con-
LT3652HV
11
3652hvfb
For more information www.linear.com/LT3652HV
APPLICATIONS INFORMATION
stant-current (CC) mode, which corresponds to 100mV
across R
SENSE
. The I
TH
voltage is pulled down to reduce
this maximum charge current should the voltage on the
V
IN_REG
pin falls below 2.7V (V
IN_REG(TH)
) or the die tem-
perature approaches 125°C.
If the voltage on the V
FB
pin is below 2.3V (V
FB(PRE)
),
the LT3652HV engages precondition mode. During the
precondition interval, 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 approaches the float volt
-
age, or the voltage on the V
FB
pin approaches 3.3V (V
FB(FLT)
),
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
volt-
age as it is reduced, and detection of C/10 charge current
is achieved when I
TH
= 0.1V. If the charger is configured
for C/10 termination, 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 LT3652HV 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 3 hours, but can be con
-
figured 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 when 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 continues 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 LT3652HV re-initiates, and
charging continues 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, such that the charger
remains in (or enters) precondition mode after 1/8th of
the programmed charge cycle time. A bad battery fault
halts the charging cycle, the CHRG status pin goes high-
impedance, and the FAULT pin is pulled low.
When the LT3652HV 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
over-voltage condition and I
TH
is pulled low. When the
voltage error amp output drops below 0.3V, the IC enters
standby mode, where most of the internal circuitry is dis
-
abled, and the V
IN
bias current is reduced to 85µA. When
the voltage on the V
FB
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.
V
IN
Input Supply
The LT3652HV is biased through a reverse-current block
-
ing element from the charger input supply to 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
,
where I
CHG(MAX)
is the maximum average charge current
(100mV/R
SENSE
). The above relation has a maximum at
V
IN
= 2 • V
BAT
, where:
I
CVIN(RMS)
= I
CHG(MAX)
/2.
The simple worst-case of ½ • I
CHG(MAX)
is commonly
used for design.
LT3652HV
12
3652hvfb
For more information www.linear.com/LT3652HV
Bulk capacitance is a function of desired input ripple volt-
age (ΔV
IN
), and follows the relation:
C
IN(BULK)
= I
CHG(MAX)
• (V
BAT
/V
IN
)/ΔV
IN
(µF)
Input ripple voltages above 0.1V are not recommended.
10µF is typically adequate for most charger applica
-
tions.
Charge Current Programming
The LT3652HV charger is configurable to charge at aver
-
age currents as high as 2A. Maximum charge current is
set by choosing an inductor sense resistor (R
SENSE
) such
that the desired maximum average current through that
sense resistor creates a 100mV drop, or:
R
SENSE
= 0.1/I
CHG(MAX)
where I
CHG(MAX)
is the maximum average charge cur-
rent. A 2A charger, for example, would use a 0.05Ω sense
resistor.
BOOST Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the switch transistor.
Operating range of the BOOST pin is 0V to 8.5V, as refer
-
enced 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 either the
battery output voltage or an external source, and the
cathode connected to the BOOST pin. Rate the diode
average current greater than 0.1A, and reverse voltage
greater than V
IN(MAX)
.
To refresh the decoupling capacitor with a rectifying diode
from the battery with battery float voltages higher than
8.4V, a >100mA Zener diode can be put in series with
the rectifying diode to prevent exceeding the BOOST pin
operating voltage range.
APPLICATIONS INFORMATION
Figure 1. Programming Maximum Charge
Current Using R
SENSE
SW
BOOST
SENSE
BAT
R
SENSE
LT3652HV
3652 F01
Figure 2. Zener Diode Reduces Refresh
Voltage for BOOST Pin
SW
BOOST
SENSE
BAT
LT3652HV
3652 F02
V
IN
/ BOOST Start-Up Requirement
The LT3652HV operates with a V
IN
range of 4.95V to 34V,
however, a start-up voltage requirement exists due to
the nature of the non-synchronous step-down switcher
topology used for the charger. If there is no BOOST supply
available, the internal switch requires (V
IN
– V
SW
) ≥ 3.3V
to reliably operate. This requirement does not exist if the
BOOST supply is available and (V
BOOST
– V
SW
) > 2V.
When an LT3652HV charger is not switching, the SW pin
is at the same potential as the battery, which can be as
high as V
BAT(FLT)
. As such, for reliable start-up, the V
IN
supply must be at least 3.3V above V
BAT(FLT)
. Once switch-
ing begins and the BOOST supply capacitor gets charged
such that (V
BOOST
– V
SW
) > 2V, the V
IN
requirement no
longer applies.
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LT3652HVEMSE#PBF

Mfr. #:
Buy LT3652HVEMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management Pwr Track 2A Bat Chr
Lifecycle:
New from this manufacturer.
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