LTC3822-1
9
38221f
OPERATION
Main Control Loop
The LTC3822-1 uses a constant frequency, current mode
architecture. During normal operation, the top external
N-channel power MOSFET is turned on when the clock
sets the RS latch, and is turned off when the current
comparator (ICMP) resets the latch. The peak inductor
current at which ICMP resets the RS latch is determined
by the voltage on the I
TH
pin, which is driven by the output
of the error amplifi er (EAMP). The V
FB
pin receives the
output voltage feedback signal from an external resistor
divider. This feedback signal is compared to the internal
0.6V reference voltage by the EAMP. When the load cur-
rent increases, it causes a slight decrease in V
FB
relative
to the 0.6V reference, which in turn causes the I
TH
voltage
to increase until the average inductor current matches the
new load current. While the top N-channel MOSFET is off,
the bottom N-channel MOSFET is turned on until either
the inductor current starts to reverse, as indicated by the
current reversal comparator RICMP, or the beginning of
the next cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN and TRACK/SS Pins)
The LTC3822-1 is shut down by pulling the RUN pin low.
In shutdown, all controller functions are disabled and the
chip draws only 7.5µA. The TG and BG outputs are held
low (off) in shutdown. Releasing the RUN pin allows an
internal 0.7µA current source to pull up the RUN pin to
V
IN
. The controller is enabled when the RUN pin reaches
1.1V.
The start-up of V
OUT
is based on the three different con-
nections on the TRACK/SS pin. The start-up of V
OUT
is
controlled by the LTC3822-1’s internal soft-start when
TRACK/SS is connected to V
IN
. During soft-start, the error
amplifi er EAMP compares the feedback signal V
FB
to the
internal soft-start ramp (instead of the 0.6V reference),
which rises linearly from 0V to 0.6V in about 1ms. This
allows the output voltage to rise smoothly from 0V to
its fi nal value while maintaining control of the inductor
current.
The 1ms soft-start time can be changed by connecting
the optional external soft-start capacitor C
SS
between the
TRACK/SS and GND pins. When the controller is enabled
by releasing the RUN pin, the TRACK/SS pin is charged up
by an internal 1µA current source and rises linearly from
0V to above 0.6V. The error amplifi er EAMP compares the
feedback signal V
FB
to this ramp instead, and regulates
V
FB
linearly from 0V to 0.6V.
When the voltage on the TRACK/SS pin is less than the
0.6V internal reference, the LTC3822-1 regulates the V
FB
voltage to the TRACK/SS pin instead of the 0.6V reference.
Therefore V
OUT
of the LTC3822-1 can track an external
voltage V
X
during start-up. Typically, a resistor divider on
V
X
is connected to the TRACK/SS pin to allow the start-up
of V
OUT
to “track” that of V
X
. For coincident tracking during
start-up, the regulated fi nal value of V
X
should be larger
than that of V
OUT
, and the resistor divider on V
X
should
have the same ratio as the divider on V
OUT
that is connected
to V
FB
. See detailed discussions in the Run and Soft-Start/
Tracking Functions in the Applications Information
section.
Light Load Operation (Burst Mode Operation,
Continuous Conduction or Pulse Skipping Mode)
(SYNC/MODE Pin)
The LTC3822-1 can be programmed for either high effi -
ciency Burst Mode operation, forced continuous conduc-
tion mode or pulse skipping mode at low load currents. To
select Burst Mode operation, tie the SYNC/MODE pin to
V
IN
. To select forced continuous operation, tie the SYNC/
MODE pin to a DC voltage below 0.4V (e.g., GND). Tying
the SYNC/MODE to a DC voltage above 0.4V and below
1.2V (e.g., V
FB
) enables pulse skipping mode.
When the LTC3822-1 is in Burst Mode operation, the peak
current in the inductor is set to approximate one-fourth of
the maximum sense voltage even though the voltage on
the I
TH
pin indicates a lower value. If the average induc-
tor current is higher than the load current, the EAMP will
decrease the voltage on the I
TH
pin. When the I
TH
voltage
drops below 0.85V, the internal SLEEP signal goes high
and the external MOSFETs are turned off.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3822-1 draws.
The load current is supplied by the output capacitor. As
the output voltage decreases, the EAMP increases the I
TH
voltage. When the I
TH
voltage reaches 0.925V, the SLEEP
(Refer to Functional Diagram)
