LTC4110EUHF LINER [Linear Technology], LTC4110EUHF Datasheet - Page 34

• Manufacturer Part Number: LTC4110EUHF
• Description: Battery Backup System Manager
• Manufacturer: LINER [Linear Technology]

LTC4110
APPLICATIONS INFORMATION
SOFT-START
The LTC4110 is soft-started with the 0.1μF capacitor on
the I
to 0.1V, then ramp up at a rate set by the internal 24μA
pull-up current and the external capacitor. Battery charging
current starts ramping up when I
and full current is achieved with I
0.1μF capacitor, time to reach full charge current is about
8ms and it is assumed that input voltage to the charger
will reach full value in less than 8ms. The capacitor can
be increased up to 1μF if longer input start-up times are
needed.
In any switching regulator, conventional timer-based soft-
starting can be defeated if the input voltage rises much
slower than the timeout period. This happens because
the switching regulators in the battery charger and the
computer power supply are typically supplying a fi xed
amount of power to the load. If input voltage comes up
slowly compared to the soft-start time, the regulators will
try to deliver full power to the load when the input voltage
is still well below its fi nal value. If the adapter is current
limited, it cannot deliver full power at reduced output
voltages and the possibility exists for a quasi “latch” state
where the adapter output stays in a current limited state at
reduced output voltage. For instance, if maximum charger
plus computer load power is 30W, a 15V adapter might
be current limited at 2.5A. If adapter voltage is less than
(30W/2.5A = 12V) when full power is drawn, the adapter
voltage will be pulled down by the constant 30W load
until it reaches a lower stable state where the switching
regulators can no longer supply full load. This situation
can be prevented by utilizing the DCDIV resistor divider,
set higher than the minimum adapter voltage where full
power can be achieved.
34
TH
pin. On start-up, the I
LTC4110
TH
BACK
DRIVE
pin voltage will rise quickly
TH
TH
voltage reaches 0.7V
at about 2V. With a
CL1
–
+
Figure 11. Back-Drive Protection
10mV
4110 F11
CLP
CLN
CALIBRATION MODE BACK-DRIVE CURRENT
PROTECTION
A resistor between CLP and CLN programs the minimum
supply forward current, this feature prevent the LTC4110
from back-driving the supply in calibration mode and
pulling the voltage higher when the system load is low.
The resistor value is given by
where
I
I
An RC fi lter may be required to fi lter out system load noise
as shown in Figure 11.
BATTERY AND CHARGER CURRENT SENSE
The LTC4110 uses two sense resistors to monitor and
control all charge and calibration currents: R
R
R
R
the battery for charge, and the current going out of the
battery for calibration. Before any current programming
can be done, the value of R
fi rst. Highest accuracy is achieved when full-scale current,
I
Although values greater than 100mV can be used to improve
accuracy, this requires larger sense resistors to handle
the extra heat and lower effi ciency. I
V
BDT
FR(MIN)
MAX
IN
100nF
SNS(FET)
SNS(BAT)
SNS(BAT)
R
CL
= back-drive current limit threshold, 10mV typical
10mV
is set to develop a 100mV drop across the resistor.
=
= minimum forward current in calibration mode
.
I
is used to monitor the DC current going into
FR MIN
I
BDT
(
5k
+
)
C
IN
TO
SYSTEM
LOAD
SNS(BAT)
must be determined
MAX
must be set to
SNS(BAT)
4110fa
and