Datasheet

LTC4156
45
4156f
Multilayer ceramic chip capacitors typically have excep-
tional ESR performance. MLCCs combined with a tight
board layout and an unbroken ground plane will yield very
good performance and low EMI emissions.
The actual in-circuit capacitance of a ceramic capacitor
should be measured with a small AC signal and DC bias,
as is expected in-circuit. Many vendors specify the capaci-
tance versus voltage with a 1V
RMS
AC test signal and, as
a result, overstate the capacitance that the capacitor will
present in the application. Using similar operating condi-
tions as the application, the user must measure, or request
from the vendor, the actual capacitance to determine if the
selected capacitor meets the minimum capacitance that
the application requires.
Programming the Input and Battery Charge
Current Limits
The LTC4156 features independent resistor programma-
bility of the input current limit and battery charge current
limit to facilitate optimal charging from a wide variety of
input power sources. The battery charge current should
be programmed based on the size of the battery and its
associated safe charging rate. With the full-scale (default)
charge current programmed with a resistor between PROG
and GND, all other I
2
C selectable charge current settings
are
lower and may be appropriate for custom charge
algorithms at extreme temperature or battery voltage. If
the battery charge current limit requires more power than
is available from the selected input current limit, the input
current limit will be enforced and the battery will charge
with less than the programmed current. Thus, the battery
charger should be programmed optimally for the battery
without concern for the input
source.
Resistive Inputs and Test Equipment
Care must be exercised in the laboratory while evaluat-
ing the LTC4156 with inline ammeters. The combined
resistance of the internal current sense resistor and fuse
of many meters can be 0.5Ω or more. At currents of 3A
to 4A, it is possible to drop several volts across the meter,
possibly resulting in unusual voltage readings or artificially
high switch duty cycles.
A resistive connection to the source of input power can
be particularly troublesome. With the undervoltage cur-
rent limit feature enabled, the switching regulator output
power will be automatically reduced to prevent V
BUS
from
falling below 4.3V. This feature greatly improves tolerance
of resistive input power sources (from either undersized
wiring and connectors or test equipment) and facilitates
stable behavior, but if engaged, it will result in
much less
power delivery to the system load and battery, depending
on the magnitude of input resistance.
If the undervoltage current limit feature is disabled and the
input power source is resistive, the voltage will continue
to fall through the falling undervoltage lockout threshold,
eventually shutting down that input channel and resetting
its input current limit back to the default setting. When
the input voltage recovers, the
channel will restart in the
default current limit setting.
APPLICATIONS INFORMATION