Datasheet

LTC4020
12
4020fd
For more information www.linear.com/LTC4020
Pin output impedance is 100kΩ, so any loading for moni-
tors must be high impedance. The sense output voltage
follows the relation:
V
CSOUT
= 0.25 + 20 • (V
CSP
– V
CSN
)
CSOUT is only active while battery charger functions are
operating. CSOUT pin voltage is pulled to 0V after charge
cycle termination or during fault conditions.
I
LIMIT
(Pin 25): Switched Inductor Maximum Current
Programming Pin. This pin allows dynamic adjustment
of DC/DC converter maximum average inductor current,
and can be used to employ a soft-start function.
Setting the voltage on the I
LIMIT
pin reduces maximum
average inductor current from the value programmed.
The inductor current limit is reduced to the fraction of
programmed current (as per the sense resistors) corre-
sponding to the voltage set on the pin (in volts). This pin
has an effective range from 0 to 1V.
For example, with 0.5V on the pin, the maximum inductor
current will be reduced to 50% of the programmed value
set by the sense resistor values.
50µA is sourced from this pin, so maximum inductor cur-
rent can be programmed by connecting a single resistor
(R
ILIMIT
) from I
LIMIT
to ground, such that the voltage
dropped across the resistor is equivalent to the desired
pin voltage:
V
ILIMIT
= 50µA • R
ILIMIT
Soft-start functionality can be implemented by connecting
a capacitor (C
ILIMIT
) from I
LIMIT
to ground, such that the
time required to charge the capacitor is the desired soft-
start interval (T
SS2
). The voltage that corresponds to full
inductor current on the I
LIMIT
pin is 1V, so the relation for
this capacitor reduces to:
C
ILIMIT
= 50µA • T
SS2
I
LIMIT
voltage can also be manipulated using an active
device, such as employing a pull-down transistor to dis-
able DC/DC converter current or to dynamically servo
maximum current. Because this pin is internally pulled to
ground during portions of the converter power-up cycle,
active devices with low impedance pull-up capability can-
not be used.
V
FBMAX
(Pin 26): DC/DC Converter Output Feedback Pin.
Maximum DC/DC converter output voltage (V
OUTMAX
) is
programmed using this pin. When a battery charge cycle
is terminated or disabled, and the battery is disconnected
(PowerPath FET is high impedance), the converter output
will servo to this maximum voltage.
The internal V
FBMAX
voltage reference is 2.75V. With a
resistor (R
MAX1
) connected from CSP to V
FBMAX
and a
resistor (R
MAX2
) connected from V
FBMAX
to FBG, the ratio
of R
MAX1
/R
MAX2
for the desired maximum DC/DC converter
output voltage follows the relation:
R
MAX1
/R
MAX2
= (V
OUTMAX
/2.75) – 1
Using the same resistor values for battery voltage pro-
gramming, or R
FB1
= R
MAX1
and R
FB2
= R
MAX2
, yields an
instant-on voltage that is 110% of V
FLOAT
(CC/CV charging)
or V
ABSORB
(lead-acid charging):
V
OUTMAX
= 1.1 • V
FLOAT/ABSORB
ITH (Pin 27): DC/DC Converter Voltage Loop Compensation
Pin. See Applications Information section for compensa-
tion component selection details.
VC (Pin 28):
DC
/DC Converter Current Loop Compensation
Pin. See Applications Information section for compensa
-
tion component selection details.
BST2
(Pin 30): Boosted supply rail for V
OUT
side (step-
up) switch FETs. Connect a 1µF capacitor from this pin
to SW2. Connect a 1A Schottky diode cathode to this pin,
anode to INTV
CC
pin.
TG2 (Pin 31): V
OUT
side (step-up) synchronous switch
FET gate driver output.
SW2
(Pin 32): Switched node for step-up switches
.
Connect the switched inductor to this pin. Connect the
primary switch FET drain and synchronous switch FET
source to this pin.
pin FuncTions
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