8
LTC3737
3737fa
OPERATIO
U
(Refer to Functional Diagram)
Main Control Loop
The LTC3737 uses a constant frequency, current mode
architecture with the two controller channels operating
180 degrees out of phase. During normal operation, each
external P-channel power MOSFET is turned on when the
clock for that channel sets the RS latch, and turned off
when the current comparator (I
CMP
) resets the latch. The
peak inductor current at which I
CMP
resets the RS latch is
determined by the voltage on the I
TH
pin, which is the
output of each error amplifier (EAMP). The V
FB
pin re-
ceives 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.
Shutdown, Soft-Start and Tracking Start-Up
(RUN/SS and TRACK Pins)
The LTC3737 is shut down by pulling the RUN/SS pin low.
In shutdown, all controller functions are disabled and the
chip draws only 9µA. The PGATE outputs are held high
(off) in shutdown. Releasing RUN/SS allows an internal
0.7µA current source to charge up the RUN/SS pin. When
the RUN/SS pin reaches 0.65V, the LTC3737’s two con-
trollers are enabled.
The start-up of V
OUT1
is controlled by the LTC3737’s
internal soft-start. During soft-start, the error amplifier
EAMP compares the feedback signal V
FB1
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 increased by connecting
the optional external soft-start capacitor, C
SS
, between the
RUN/SS and SGND pins. As the RUN/SS pin continues to
rise linearly from approximately 0.65V to 1.3V (being
charged by the internal 0.7µA current source), the EAMP
regulates V
FB1
linearly from 0V to 0.6V.
The start-up of V
OUT2
is controlled by the voltage on the
TRACK pin. When the voltage on the TRACK pin is less
than the 0.6V internal reference, the LTC3737 regulates
the V
FB2
voltage to the TRACK pin instead of the 0.6V
reference. Typically, a resistor divider on V
OUT1
is con-
nected to the TRACK pin to allow the start-up of V
OUT2
to
“track” that of V
OUT1
. For one-to-one tracking during start-
up, the resistor divider would have the same values as the
divider on V
OUT2
that is connected to V
FB2
.
If no tracking function is desired, then the TRACK pin can
be tied to V
IN
. Note, however, that in this situation, there
would be no (internal or external) soft-start on V
OUT2
.
Light Load Operation (Burst Mode Operation or Pulse
Skipping Mode) (SYNC/MODE Pin)
The LTC3737 can be enabled to enter high efficiency Burst
Mode operation at low load currents. To select Burst Mode
operation, tie the SYNC/MODE pin to a DC voltage above
0.6V (e.g., V
IN
). To disable Burst Mode operation and
enable PWM pulse skipping mode, connect SYNC/MODE
to a DC voltage below 0.6V (e.g., SGND). In this mode, the
efficiency is lower at light loads. However, pulse skipping
mode has the advantages of lower output ripple and less
interference to audio circuitry.
When a controller is in Burst Mode operation, the peak
current in the inductor is set to approximate one-fourth of
the maximum sense voltage even when the voltage on the
I
TH
pin indicates a lower value. If the average inductor
current is greater 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 LTC3737 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 opera-
tion by turning on the external P-channel MOSFET on the
next cycle of the internal oscillator.
When the SYNC/MODE pin is clocked by an external clock
source to use the phase-locked loop (see Frequency
Selection and Phase-Locked Loop), the LTC3737 operates
in PWM pulse skipping mode at light loads.
