LTC3857-1_15 Datasheet, PDF (12/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857-1
OPERATION (Refer to the Functional Diagram)
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
The LTC3857-1 can be enabled to enter high efficiency
Burst Mode operation, constant frequency pulse-skipping
mode, or forced continuous conduction mode at low load
currents. To select Burst Mode operation, tie the PLLIN/
MODE pin to GND. To select forced continuous operation,
tie the PLLIN/MODE pin to INTV_CC. To select pulse-skipping
mode, tie the PLLIN/MODE pin to a DC voltage greater
than 1.2V and less than INTV_CC – 1.3V.
When a controller is enabled for Burst Mode operation,
the minimum peak current in the inductor is set to ap-
proximately 15% 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 error amplifier, EA, will decrease the voltage
on the I_TH pin. When the I_TH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off. The I_TH pin is
then disconnected from the output of the EA and parked
at 0.450V.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3857-1 draws.
If one channel is shut down and the other channel is in
sleep mode, the LTC3857-1 draws only 50μA of quiescent
current. If both channels are in sleep mode, the LTC3857-1
draws only 65μA of quiescent current. In sleep mode,
the load current is supplied by the output capacitor. As
the output voltage decreases, the EA’s output begins to
rise. When the output voltage drops enough, the I_TH pin
is reconnected to the output of the EA, the sleep signal
goes low, and the controller resumes normal operation
by turning on the top external MOSFET on the next cycle
of the internal oscillator.
When a controller is enabled for Burst Mode operation, the
inductor current is not allowed to reverse. The reverse cur-
rent comparator, IR, turns off the bottom external MOSFET
just before the inductor current reaches zero, preventing
it from reversing and going negative. Thus, the controller
operates in discontinuous operation.
In forced continuous operation or clocked by an external
clock source to use the phase-locked loop (see Frequency
Selection and Phase-Locked Loop section), the induc-
tor 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_TH pin, just as in normal
operation. In this mode, the efficiency at light loads is lower
than in Burst Mode operation. However, continuous opera-
tion has the advantage of lower output voltage ripple and
less interference to audio circuitry. In forced continuous
mode, the output ripple is independent of load current.
When the PLLIN/MODE pin is connected for pulse-skipping
mode, the LTC3857-1 operates in PWM pulse-skipping
mode at light loads. In this mode, constant frequency
operation is maintained down to approximately 1% of
designed maximum output current. At very light loads, the
current comparator, ICMP, may remain tripped for several
cycles and force the external top MOSFET to stay off for
the same number of cycles (i.e., skipping pulses). The
inductor current is not allowed to reverse (discontinuous
operation). This mode, like forced continuous operation,
exhibits low output ripple as well as low audio noise and
reduced RF interference as compared to Burst Mode
operation. It provides higher low current efficiency than
forced continuous mode, but not nearly as high as Burst
Mode operation.
Frequency Selection and Phase-Locked Loop
(FREQ and PLLIN/MODE Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3857-1’s controllers
can be selected using the FREQ pin.
If the PLLIN/MODE pin is not being driven by an external
clock source, the FREQ pin can be tied to SGND, tied to
INTV_CC or programmed through an external resistor. Tying
FREQ to SGND selects 350kHz while tying FREQ to INTV_CC
selects 535kHz. Placing a resistor between FREQ and
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