LTC3809
8
3809fc
OPERATION
(Refer to Functional Diagram)
Main Control Loop
The LTC3809 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 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 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 and Soft-Start (RUN Pin)
The LTC3809 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 controlled by the LTC3809’s inter-
nal soft-start. During soft-start, the error amplifi er EAMP
compares the feedback signal V
FB
to the in t er na l s of t -s t ar t
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.
Light Load Operation (Burst Mode Operation,
Continuous Conduction or Pulse-Skipping Mode)
(SYNC/MODE Pin)
The LTC3809 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 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
regulation as described in the Auxiliary Winding Control
Using SYNC/MODE Pin discussion in the Applications
Information section.
When the LTC3809 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 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.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3809 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 signal goes low and the controller resumes normal
operation by turning on the external P-channel MOSFET
on the next cycle of the internal oscillator.
When the controller is enabled for Burst Mode or pulse-
skipping operation, the inductor current is not allowed to
reverse. Hence, the controller operates discontinuously.
