LTC3736EUF-2#PBF Linear Technology, LTC3736EUF-2#PBF Datasheet - Page 10

• Manufacturer Part Number: LTC3736EUF-2#PBF
• Description: IC CTRLR SW SYNC DUAL 2PH 24QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3736EUF-2#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz ~ 750kHz
• Voltage - Input: 2.75 ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

OPERATION
LTC3736-2
Main Control Loop
The LTC3736-2 uses a constant-frequency, current mode
architecture with the two controllers operating 180 degrees
out-of-phase. During normal operation, the top 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
inductor current at which I
determined by the voltage on the I
by the output of the error amplifi er (EAMP). The V
receives the output voltage feedback signal from an ex-
ternal 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
I
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, I
beginning of the next cycle.
Shutdown, Soft-Start and Tracking Start-Up (RUN/SS
and TRACK Pins)
The LTC3736-2 is shut down by pulling the RUN/SS pin
low. In shutdown, all controller functions are disabled and
the chip draws only 9μA. The TG outputs are held high
(off) and the BG outputs low (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 LTC3736-2’s two controllers are enabled.
The start-up of V
internal soft-start. During soft-start, the error amplifi er
EAMP compares the feedback signal V
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 increased by connecting the
optional external soft-start capacitor C
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
the V
10
TH
FB
voltage to increase until the average inductor current
relative to the 0.6V reference, which in turn causes the
FB1
proportionally linearly from 0V to 0.6V.
OUT1
(Refer to Functional Diagram)
is controlled by the LTC3736-2’s
CMP
CMP
) resets the latch. The peak
resets the RS latch is
TH
SS
pin, which is driven
FB1
between the RUN/
to the internal
RCMP
, or the
FB
pin
The start-up of V
TRACK pin. When the voltage on the TRACK pin is less
than the 0.6V internal reference, the LTC3736-2 regulates
the V
erence. Typically, a resistor divider on V
to the TRACK pin to allow the start-up of V
that of V
resistor divider would have the same values as the divider
on V
Light Load Operation (Pulse-Skipping or Continuous
Conduction) (SYNC/FCB Pin)
The LTC3736-2 can be enabled to enter high effi ciency
pulse-skipping operation or forced continuous conduc-
tion mode at low load currents. To select pulse-skipping
operation, tie the SYNC/FCB pin to a DC voltage above
0.6V (e.g., V
the SYNC/FCB to a DC voltage below 0.6V (e.g., SGND).
This 0.6V threshold between pulse-skipping operation
and forced continuous mode can be used in secondary
winding regulation as described in the Auxiliary Winding
Control Using SYNC/FCB Pin discussion in the Applications
Information section.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by the
voltage on the I
on every cycle (constant frequency) regardless of the I
pin voltage. In this mode, the effi ciency at light loads is
lower than in pulse-skipping operation. However, continu-
ous mode has the advantages of lower output ripple and
less interference with audio circuitry.
When the SYNC/FCB pin is tied to a DC voltage above
0.6V or when it is clocked by an external clock source to
use the phase-locked loop (see Frequency Selection and
Phase-Locked Loop), the LTC3736-2 operates in PWM
pulse-skipping mode at light loads. In this mode, the
current comparator I
cycles and force the external P-channel MOSFET to stay
off for the same number of cycles. The inductor current is
not allowed to reverse, though (discontinuous operation).
This mode, like forced continuous operation, exhibits low
output ripple as well as low audio noise and reduced RF
interference. However, it provides low current effi ciency
OUT2
FB2
OUT1
voltage to the TRACK pin instead of the 0.6V ref-
that is connected to V
IN
. For one-to-one tracking during start-up, the
). To select forced continuous operation, tie
TH
OUT2
pin. The P-channel MOSFET is turned
CMP
is controlled by the voltage on the
may remain tripped for several
FB2
.
OUT1
OUT2
is connected
to track
37362fb
TH