LTC3892_15 Datasheet, PDF (14/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Operation (Refer to the Functional Diagrams)
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
The LTC3892/LTC3892-1 can be enabled to enter high
efficiency Burst Mode operation, 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 1.1V and
less than INTV_CC – 1.3V. This can be done by connecting
a 100kΩ resistor between PLLIN/MODE and INTV_CC.
When a controller is enabled for Burst Mode operation,
the minimum peak current in the inductor is set to ap-
proximately 25% of the maximum sense voltage even
when the voltage on the ITH pin indicates a lower value.
If the average inductor current is higher than the load cur-
rent, the error amplifier, EA, will decrease the voltage on
the ITH pin. When the ITH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off. The ITH 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 LTC3892/LTC3892-
1 draws. If one channel is in sleep mode and the other
channel is shut down, the LTC3892/LTC3892-1 draws only
29μA of quiescent current (with DRVSET = 0V). If both
channels are in sleep mode, it draws only 34μA of quies-
cent 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 ITH 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
current comparator (I_R) turns off the bottom external
MOSFET just before the inductor current reaches zero,
preventing it from reversing and going negative. Thus,
the controller operates discontinuously.
In forced continuous operation or when clocked by an
external clock source to use the phase-locked loop (see
Frequency Selection and Phase-Locked Loop section),
the inductor current is allowed to reverse at light loads or
under large transient conditions. The peak inductor cur-
rent is determined by the voltage on the ITH pin, just as
in normal operation. In this mode, the efficiency at light
loads is lower than in Burst Mode operation. However,
continuous operation 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. Clocking the LTC3892/LTC3892-1 from
an external source enables forced continuous mode (see
the Frequency Selection and Phase-Locked Loop section).
When the PLLIN/MODE pin is connected for pulse-skipping
mode, the LTC3892/LTC3892-1 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, I_CMP, 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 (discon-
tinuous 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 LTC3892/LTC3892-1’s
controllers can be selected using the FREQ pin.
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