LTC4020_15 Datasheet, PDF (10/42 Pages) Linear Technology – 55V Buck-Boost Multi-Chemistry Battery Charger

LTC4020
PIN FUNCTIONS
The RNG/SS pin is pulled low during periods when charg-
ing is disabled, including NTC faults, bad battery faults,
and normal charge cycle termination. This allows for a
graceful start after faults and when initiating new charge
cycles, should soft-start functionality be implemented.
Both a soft-start capacitor and a programming resistor
can be implemented in parallel.
RNG/SS voltage can also be manipulated using an active
device, such as employing a pull-down transistor to dis-
able charge current or to dynamically servo maximum
charging current. Because this pin is internally pulled to
ground during fault conditions, active devices with low-
impedance pull up capability cannot be used.
See Applications Information section.
NTC (Pin 16): Battery Temperature Monitor Pin. Connect
a 10kΩ, β = 3380 NTC thermistor from this pin to ground.
The NTC pin is the input to the negative temperature coef-
ficient temperature monitoring circuit. This pin sources
50µA, and monitors the voltage created across the 10kΩ
thermistor. When the voltage on this pin is above 1.35V
(0°C) or below 0.3V (40°C), charging is disabled and an
NTC fault is signaled at the STAT1 and STAT2 status pins.
If the internal timer is being used, the timer is paused,
suspending the charging cycle until the NTC fault condi-
tion is relieved. There is approximately 5°C of temperature
hysteresis associated with each of the temperature thresh-
olds. The NTC function remains enabled while thermistor
resistance to ground is less than 250kΩ. If this function
is not desired, leave the NTC pin unconnected or connect
a 10k resistor from the NTC pin to ground. The NTC pin
contains an internal clamp that prevents excursions above
2V, so the pin must not be pulled high with a low imped-
ance source. A low impedance element can be used to
pull the pin to ground.
VFB (Pin 17): Battery Voltage Feedback Pin. Battery volt-
ages are programmed through a feedback resistor divider
placed from the BAT pin to FBG.
During CC/CV charging, the battery voltage references are:
Float Voltage (V_FLOAT) = 2.5V
Trickle Charge Voltage (V_TRK) = 1.75V
Auto-Restart Voltage (V_RESTART) = 2.4375
During lead-acid charging, the battery voltage references
are:
Absorption Voltage (V_ABSOR) = 2.5V
Float Charge Voltage (V_FLT) = 2.3125V
Trickle Charge Voltage (V_TRK) = 1.75V
Bulk Recharge Voltage (V_BULK) = 2.1875V
With R_FB1 connected from BAT to VFB and R_FB2 connected
from VFB to FBG, the ratio of (R_FB1/R_FB2) for the desired
programmed battery float voltage (CC/CV charging) or
absorption voltage (lead-acid charging) follows the relation:
R_FB1/R_FB2 = (V_FLOAT/ABSORB)/2.5 – 1
(V)
FBG (Pin 18): Voltage Feedback Divider Return. This pin
contains a low impedance path to signal ground, used
as the ground reference for voltage monitoring feedback
resistor dividers. When V_IN is not present or the LTC4020
is in shutdown, this pin becomes high impedance, elimi-
nating current drain from the battery associated with the
feedback resistor dividers.
V_FBMIN (Pin 19): Minimum voltage feedback pin for instant-
on operation. Minimum DC/DC converter output voltage
(V_OUTMIN) is programmed using this pin for instant-on
functionality. V_OUTMIN is programmed through a feedback
resistor divider placed from the CSP pin to FBG.
If the battery voltage is below the voltage level programmed
using this pin, the LTC4020 controls the external Pow-
erPath FET as a linear pass element, allowing the DC/DC
converter output to achieve the minimum programmed
voltage. Maximum battery charge current is reduced as
the voltage across the PowerPath FET increases to control
power dissipation.
The internal V_FBMIN voltage reference is 2.125V. With a
resistor (R_MIN1) connected from CSP to V_FBMIN, and a
resistor (R_MIN2) connected from V_FBMIN to FBG, the ratio
of these resistors for the desired minimum converter
10
For more information www.linear.com/LTC4020
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