LTC4121/LTC4121-4.2
16
4121fc
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protection has been engaged) a DC current, I
LOWBAT
, is
applied to the BAT pin from the INTV
CC
supply. When
the battery voltage rises above V
LOWBAT
, the switching
regulator is enabled and charges the battery at a trickle
charge level of 10% of the full scale charge current (in
addition to the DC I
LOWBAT
current). Trickle charging of
the battery continues until the sensed battery voltage rises
above the trickle charge threshold, V
TRKL
, or V
TRKL_42
.
When the battery rises above the trickle charge threshold
the full scale charge current is applied and the DC trickle
charge current is turned off. If the battery remains below
the trickle charge threshold for more than 30 minutes,
charging terminates and the fault status pin is asserted
to indicate a bad battery. After a bad battery fault, the
LTC4121 automatically restarts a new charge cycle once
the failed battery is removed and replaced with another
battery. The LTC4121-4.2 monitors the BATSNS pin volt-
age to sense LOWBAT and TRKL conditions.
Precision Run/Shutdown Control
The LTC4121 remains in a low power disabled mode until
the RUN pin is driven above V
EN
(typically 2.45V). While
the LTC4121 is in disabled mode, current drain from the
battery is reduced to extend battery lifetime, the status pins
are both de-asserted, and the FBG pin is high impedance.
Charging can be stopped at any time by pulling the RUN
pin below 2.25V. The LTC4121 also offers an extremely
low operating current shutdown mode when the RUN pin
is pulled below V
SD
(typically about 0.7V). In this condition
less than 20µA is pulled from the supply at IN. Tie the RUN
pin to a resistive divider from the IN supply to program
the voltage where the LTC4121
turns on. Examples are
shown in Figures 9 and 10.
Differential Under Voltage Lockout
The LTC4121 monitors the difference between the battery
voltage, V
BAT
, and the input supply voltage, V
IN
. If the
difference (V
IN
– V
BAT
) falls to ∆V
DUVLO
, all functions are
disabled and the part is forced into shutdown mode until
(V
IN
– V
BAT
) rises above the ∆V
DUVLO
rising threshold. The
LTC4121-4.2 monitors the V
BATSNS
and V
IN
pin voltages
to sense DUVLO condition.
OPERATION
User Selectable Switching Regulator Operating
Frequency
The LTC4121 uses a constant-frequency synchronous
step-down switching regulator architecture to pro-
duce high operating efficiency. The nominal operating
frequency, f
OSC
, is programmed by pulling the FREQ pin to
either INTV
CC
or to GND to obtain a switching frequency
of 1.5MHz or 750kHz, respectively. The high operating
frequency allows the use of smaller external components.
Selection of the operating frequency is a trade-off between
efficiency, component size, and margin from the minimum
on-time of the switcher. Operation at lower frequency
improves efficiency by reducing internal gate charge and
switching losses, but requires larger inductance values to
maintain low output ripple. Operation at higher frequency
allows the use of smaller components, but may require
sufficient margin from the minimum on-time at the lowest
duty cycle if fixed-frequency switching is required.
PWM Dropout Detector
If the input voltage approaches the battery voltage, the
LTC4121 may require duty cycles approaching 100%. This
mode of operation is known as dropout. In dropout, the
operating frequency may fall well below the programmed
f
OSC
value. If the top switch remains on for eight clock
cycles, the dropout detector activates and forces the
bottom switch on for the remainder of that clock cycle
or until the inductor current decays to zero. This avoids
a potential source of audible noise when using ceramic
input or output capacitors and prevents the boost sup-
ply capacitor for the top gate drive from discharging. In
dropout operation, the actual charge current may not be
able to reach the full-scale programmed value. In such a
scenario the analog charge current monitor function does
not represent actual charge current being delivered.
Burst Mode
®
Operation
At low charge currents, for example during constant-
voltage mode, the LTC4121 automatically enters Burst
Mode operation. In Burst Mode operation the switcher is
periodically forced into standby mode in order to improve
