Datasheet

LTC4020
11
4020fd
For more information www.linear.com/LTC4020
Using the same resistor values for battery voltage pro-
gramming, or R
FB1
= R
MIN1
and R
FB2
= R
MIN2
, yields an
instant-on voltage that is 85% of V
FLOAT
(CC/CV charging)
or V
ABSORB
(lead-acid charging):
V
OUT(MIN)
= 0.85 • V
FLOAT/ABSORB
BAT (Pin 20): Battery Voltage Monitor Pin. This pin serves as
the positive reference for the LTC4020 ideal diode function.
If a system load occurs that is large enough to collapse
the DC/DC converter output while charging is terminated
or disabled, and the battery is disconnected (PowerPath
FET is high impedance), the ideal diode function engages
the PowerPath. This function powers the system load from
the battery, and modulates the PowerPath FET gate such
that the system output voltage is maintained with 14mV
across the PowerPath FET, provided the voltage drop due
to R
DS(ON)
< 14mV. This allows large loads to be accom-
modated without excessive power dissipation in the body
diode of the PowerPath FET.
BGA
TE (Pin 21): PowerPath FET Gate Driver Output.
This pin is controls the multiple functions of the Power
-
Path FET.
This pin
is pulled low during a normal charging cycle,
minimizing the FET series impedance between the DC/
DC converter output and the battery.
The BGATE pin is also forced low when the DC/DC converter
is disabled, maintaining a low impedance connection to
power the system from the battery.
When BGATE is pulled low, CSP BGATE is limited inter
-
nally to 9.5V, so if CSP > 9.5
V,
BGATE is maintained by
an internal clamp at CSP – BGATE = 9.5V. The BGATE
pin must be near ground or at the clamp voltage for C/10
detection to occur.
If the battery voltage is lower than the instant-on threshold
(see V
FBMIN
), BGATE servos the PowerPath FET imped-
ance such that a voltage drop between the CSN pin and
the BAT pin is created while batter
y charging continues. If
the V
CSN
– V
BAT
voltage exceeds 0.4V, maximum charge
current is reduced to decrease power dissipation in the
PowerPath FET.
When the DC/DC converter is enabled, but the battery
charge cycle has terminated, BGATE is pulled high to
disconnect the battery from the converter output. The
battery is also disconnected in the same manner during
NTC faults. The ideal diode function is active during these
periods, however, so if a system load occurs that is larger
than what the DC/DC converter can accommodate, the
battery can supply the required current, and the BGATE
pin will be servo controlled to force a voltage drop of only
14mV across the PowerPath FET. The ideal diode function
is disabled during bad battery faults.
If a PowerPath FET is not being used, such as with a lead-
acid charging application, connect a 0.1nF capacitor from
BGATE to CSN.
CSN (Pin 22): Battery Charger Current Sense Negative
Input. Connect this pin to the negative terminal of the
battery charge current sense resistor (RCS) through a
100Ω resistor. Connect a filter capacitor between this
pin and the CSP pin for ripple reduction. See Applications
Information section.
The value of the sense resistor is related to the maximum
battery charge current (I
CSMAX
):
R
CS
= 0.05/I
CSMAX
This pin also serves as the negative reference for the
LTC4020 ideal diode function (see BAT).
CSP (Pin 23): Battery Charger Current Sense Positive
Input. Connect this pin to the positive terminal of the
battery charge current sense resistor (R
CS
) through a
100Ω resistor. Connect a filter capacitor between this
pin and the CSN pin for ripple reduction. See Applications
Information section.
The value of the sense resistor is related to the maximum
battery charge current (I
CSMAX
) such that:
R
CS
= 0.05/I
CSMAX
CSOUT (Pin 24): Current Sense Amplifier Output and
Charge Current Monitor. Connect 100pF capacitor to
ground.
pin FuncTions
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