LTC3834 Datasheet, PDF (11/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834
OPERATION (Refer to Functional Diagram)
When the RUN pin is pulled low to disable the LTC3834, or
when V_IN drops below its undervoltage lockout threshold
of 3.7V, the TRACK/SS pin is pulled low by an internal
MOSFET. When in undervoltage lockout, the controller is
disabled and the external MOSFETs are held off.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Continuous Conduction)
(PLLIN/MODE Pin)
The LTC3834 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 a DC voltage below 0.8V (e.g., SGND). 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 0.8V and
less than INTV_CC – 0.5V.
When the LTC3834 is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-tenth of the maximum sense voltage even though the
voltage on the I_TH pin indicates a lower value. If the aver-
age inductor current is lower than the load current, the
error amplifier EA will decrease the voltage on the I_TH pin.
When the I_TH voltage drops below 0.4V, 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 0.425V.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3834 draws to
only 30μA. 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 the LTC3834 is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
current comparator (RI_CMP) turns off the bottom external
MOSFET just before the inductor current reaches zero,
preventing it from reversing and going negative, thus
operating in discontinuous operation.
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_TH 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
advantages of lower output 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 skip-
ping mode or clocked by an external clock source to
use the phase-locked loop (see Frequency Selection and
Phase-Locked Loop section), the LTC3834 operates in
PWM pulse skipping mode at light loads. In this mode,
constant-frequency operation is maintained down to ap-
proximately 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.
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