LTC4015_15 Datasheet, PDF (18/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Operation
Power Path Ideal Diode Controllers
Input Current Regulation (ICL)
The LTC4015 features input and output ideal diode control-
lers. These controllers make up a power path that allows
power to be delivered to the system (V_SYS) by either V_IN or
V_BAT, whichever is greater. The input ideal diode provides
a one way path from V_IN to V_SYS. The output ideal diode
provides a one way path from V_BAT to V_SYS.
The ideal diode controllers consist of a precision ampli-
fier that drives the gate of a MOS transistor whenever the
voltage at V_SYS is approximately 15mV (V_FWD) below the
voltage at V_IN or V_BAT. Within the amplifier’s linear range,
the small signal resistance of the ideal diode will be quite
low, keeping the forward drop near 15mV. At higher cur-
rent levels, the MOS transistors will be in full conduction.
The input ideal diode controller assumes control of an
external NMOS transistor by modulating the gate voltage
of the NMOS transistor to allow current to flow from V_IN
to V_SYS while blocking current in the opposite direction to
prevent back driving V_IN. Additionally a fastoff comparator
shuts off the NMOS if V_IN falls 25mV below V_SYS.
The output ideal diode provides a path for V_BAT to power
V_SYS when V_IN is unavailable, while blocking current in the
opposite direction to prevent overcharging of the battery.
The output ideal diode controller controls an external PMOS
transistor by modulating the gate voltage of the PMOS
transistor. In addition to a fast-off comparator the output
ideal diode also has a fast-on comparator that turns on
the external MOSFET when V_SYS drops 45mV below V_BAT.
When limited power is available to the switching charger
because either the programmed input current limit or input
undervoltage limit is active, charge current will automati-
cally be reduced to prioritize power delivery to the system
load. Note that the LTC4015 only limits charge current,
but does not limit current from the input to the system
load—if the system load alone requires more power than
is available from the input after charge current has been
reduced to zero, V_SYS must fall to the battery voltage in
order for the battery to provide supplemental power. Note
that a system load fault can dissipate very large amounts
of power, as the system load current will not be limited
by the ideal diode controllers.
The LTC4015 contains a control loop, ICL (input current
limit), that automatically reduces charge current when the
overall average input current reaches a maximum level.
The input current regulation function can only reduce
charge current to zero, it cannot limit the overall input
current which is a function of the load on V_SYS.
This level is set by the combination of the current
sense resistor R_SNSI from CLP to CLN and either the
default 32mV servo voltage or a value programmed into
IIN_LIMIT_SETTING via the serial port. The maximum
servo voltage that can be programmed is 32mV. The
voltage across the sense resistor divided by its value
determines the target maximum possible input current. A
2mΩ resistor, for example, would have an upper limit of
input current of 16A using a 32mV servo voltage.
Input Undervoltage Regulation (UVCL) and Solar
Panel Maximum Power Point Tracking (MPPT)
The LTC4015 contains a control loop, UVCL (under
voltage current limit) that allows it to tolerate a resistive
connection to the input power source by automatically
reducing charge current as V_IN (as observed at the UVCLFB
pin using a V_IN voltage divider ) drops to a programmable
level (VIN_UVCL_SETTING). This circuit helps prevent
UVLO oscillations by regulating the input voltage above
the LTC4015’s undervoltage lockout level. The UVCL
function can only reduce charge current to zero, it cannot
limit the overall input current which is also a function of
the load on V_SYS.
Optionally, the LTC4015 includes a maximum power
point tracking (MPPT) algorithm to find and track the
VIN_UVCL_SETTING that delivers the maximum charge
current to the battery. If enabled by the MPPT pin or
by the mppt_en_i2c bit via the serial port, the MPPT
algorithm performs a sweep of VIN_UVCL_SETTING
values, measuring battery charge current at each setting.
When the sweep is completed, the LTC4015 applies the
VIN_UVCL_SETTING value corresponding to the maximum
battery charge current (i.e. the maximum power point).
The LTC4015 then tracks small changes in the maximum
power point by slowly dithering the VIN_UVCL_SETTING.
The LTC4015 periodically performs a new sweep of
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