LTC3809-1
9
38091fc
Main Control Loop
The LTC3809-1 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 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 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 LTC3809-1 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 con-
nections on the TRACK/SS pin. The start-up of V
OUT
is
controlled by the LTC3809-1’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 al-
lows 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 LTC3809-1 regulates the V
FB
voltage to the TRACK/SS pin instead of the 0.6V reference.
Therefore V
OUT
of the LTC3809-1 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
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)
(MODE Pin)
The LTC3809-1 can be programmed for either high effi ciency
Burst Mode operation, forced continuous conduction mode
or pulse-skipping mode at low load currents. To select
Burst Mode operation, tie the MODE pin to V
IN
. To select
forced continuous operation, tie the MODE pin to a DC
voltage below 0.4V (e.g., GND). Tying the MODE pin 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 regulation as described in the
Auxiliary Winding Control Using the MODE Pin discussion
in the Applications Information section.
When the LTC3809-1 is in Burst Mode operation, the peak
current in the inductor is set to approximately 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 MOSFET is turned off.
OPERATION
(Refer to Functional Diagram)
