LTC3788 Datasheet, PDF (13/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
OPERATION
Light Load Current Operation—Burst Mode Operation,
Pulse-Skipping or Continuous Conduction
(PLLIN/MODE Pin)
The LTC3788 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 ground (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 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 approxi-
mately 30% of the maximum sense voltage even though
the voltage on the ITH 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 ITH
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.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3788 draws.
If one channel is shut down and the other channel is in
sleep mode, the LTC3788 draws only 125μA of quiescent
current. If both channels are in sleep mode, the LTC3788
draws only 200μA 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 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 bottom 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 (IR) turns off the top external MOSFET
just before the inductor current reaches zero, preventing
it from reversing and going negative. Thus, the controller
operates in discontinuous current operation.
In forced continuous operation or when clocked by an
external clock source to use the phase-locked loop (see
the 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 advantages of lower output
voltage ripple and less interference to audio circuitry, as
it maintains constant-frequency operation independent
of load current.
When the PLLIN/MODE pin is connected for pulse-skip-
ping mode, the LTC3788 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 bottom 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.
3788f
13