Datasheet

LTC4155
45
4155fc
Table 38. Recommended P-Channel Battery Charger MOSFETs
MANUFACTURER
PART
NUMBER
R
DS(ON)
(mΩ) V
T
(V) BV
DSS
(V)
Fairchild FDMC510P 7.6 –0.5 –20
Vishay Si7123DN 11.2 –1 –20
Vishay Si5481DU 24 –1 –20
V
BUS
and V
OUT
Bypass Capacitors
The style and value of the capacitors used with the LTC4155
determine several important parameters such as regulator
control loop stability and input voltage ripple. Because
the LTC4155 uses a step-down switching power supply
from V
BUS
to V
OUT
, its input current waveform contains
high frequency components. It is strongly recommended
that a low equivalent series resistance (ESR) multilayer
ceramic capacitor be used to bypass V
BUS
. Tantalum and
aluminum capacitors are not recommended because of
their high ESR. The value of the capacitor on V
BUS
directly
controls the amount of input ripple for a given load current.
Increasing the size of this capacitor will reduce the input
ripple. The USB specification allows a maximum of 10F
to be connected directly across the USB power bus. If the
overvoltage protection circuit is used to protect V
BUS
, then
its soft-starting nature can be exploited and a larger V
BUS
capacitor can be used if desired. If one or both of the input
channels are never used for USB, additional capacitance
placed upstream of the overvoltage protection NMOS de-
vices can absorb significant high frequency current ripple.
To prevent large V
OUT
voltage steps during transient load
conditions, it is also recommended that a ceramic capacitor
be used to bypass V
OUT
. The output capacitor is used in
the compensation of the switching regulator. At least 22F
with low ESR are required on V
OUT
. Additional capacitance
will improve load transient performance and stability.
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 LTC4155 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. Typically this rate is close
to “1C”, or equal to the current which would discharge
the battery in one hour. For example, a 2000mAH battery
would be charged with no more than 2A. 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 cus-
tom 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 LTC4155 with inline ammeters. The combined
resistance of the internal current sense resistor and fuse
of many meters can be 0.5 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
APPLICATIONS INFORMATION