LTC3899_15 Datasheet, PDF (16/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Operation (Refer to the Functional Diagrams)
the TRACK/SS voltage rises linearly from 0V to 0.8V/1.2V
(and beyond up to about 4V), the output voltage V_OUT rises
smoothly from zero (V_IN for the boost) to its final value.
Alternatively the TRACK/SS pins for buck channels 1 and 2
can be used to cause the start-up of V_OUT to track that of
another supply. Typically, this requires connecting to the
TRACK/SS pin an external resistor divider from the other
supply to ground (see Applications Information section).
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
The LTC3899 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 1.1V 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 (30%
for the boost) 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 ITH volt-
age 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 LTC3899 draws.
If one channel is in sleep mode and the other two are
shut down, the LTC3899 draws only 29μA of quiescent
current (with DRVSET = 0V). If two channels are in sleep
mode and the other shut down, it draws only 34μA of
quiescent current. If all three controllers are enabled in
sleep mode, the LTC3899 draws only 39μA 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 top external
MOSFET (the bottom external MOSFET for the boost) 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 (the top external MOSFET for the boost) just
before the inductor current reaches zero, preventing it
from reversing and going negative. Thus, the controller
operates discontinuously.
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 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 LTC3899 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 LTC3899 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 (bottom for the boost)
to stay off for the same number of cycles (i.e., skipping
pulses). The inductor current is not allowed to reverse (dis-
continuous 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.
3899f
16
For more information www.linear.com/LTC3899