9
LTC3808
3808f
OPERATIO
U
Main Control Loop
The LTC3808 uses a constant frequency, current mode
architecture. During normal operation, the top external
P-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 amplifier (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 current
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 P-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 IRCMP, or the beginning of
the next cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN and TRACK/SS Pins)
The LTC3808 is shut down by pulling the RUN pin low. In
shutdown, all controller functions are disabled and the
chip draws only 9µA. The TG output is held high (off) and
the BG output 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
connections on the TRACK/SS pin. The start-up of V
OUT
is controlled by the LTC3808’s internal soft-start when
TRACK/SS is connected to V
IN
. During soft-start, the error
amplifier 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
final 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 amplifier 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 LTC3808 regulates the V
FB
voltage to the TRACK/SS pin instead of the 0.6V reference.
Therefore V
OUT
of the LTC3808 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 final value of V
X
should be larger
than that of V
OUT
, and the resistor divider on V
X
has 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 LTC3808 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. The 0.4V
threshold between forced continuous operation and pulse
skipping mode can be used in secondary winding regula-
tion as described in the Auxiliary Winding Control Using
SYNC/MODE Pin discussion in the Applications Informa-
tion section.
When the LTC3808 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 inductor
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 MOSFET is turned off.
(Refer to Functional Diagram)
