LTC3709 Datasheet, PDF (11/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

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OPERATIO (Refer to Functional Diagram)
DRV_CC
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
DRV_CC pin. The top MOSFET driver is powered from a
floating bootstrap capacitor C_B. This capacitor is normally
recharged from DRV_CC through an external Schottky di-
ode D_B when the top MOSFET is turned off.
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both V_OUT+ and V_OUT– benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses. This sensing also
isolates the physical power ground from the physical
signal ground, preventing the possibility of troublesome
“ground loops” on the PC layout and preventing voltage
errors caused by board-to-board interconnects.
LTC3709
Dual Phase Operation
An internal phase-lock loop (PLL1) ensures that channel
2 operates exactly at the same frequency as channel 1 and
is also phase shifted by 180°, enabling the LTC3709 to
operate optimally as a dual phase controller. The loop filter
connected to the INTLPF pin provides stability to the PLL.
For external clock synchronization, a second PLL (PLL2)
is incorporated into the LTC3709. PLL2 will adjust the on-
time of channel 1 until its frequency is the same as the
external clock. When locked, the PLL2 aligns the turn on
of the top MOSFET of channel 1 to the rising edge of the
external clock. Compensation for PLL2 is through the
EXTLPF pin.
Second Channel Shutdown During Light Loads
When FCB is tied to V_CC, discontinuous mode is selected.
In this mode, no reverse current is allowed. The second
channel is off when I_TH is less than 0.8V for better
efficiency. When FCB is tied to ground, forced continuous
mode is selected, both channels are on and reversed
current is allowed.
3709f
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