ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 10

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3827-1
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
(PLLIN/MODE Pin)
The LTC3827-1 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 a DC voltage below 0.8V (e.g., SGND). To
select forced continuous operation, tie the PLLIN/MODE
pin to INTV
PLLIN/MODE pin to a DC voltage greater than 0.8V and
less than INTV
When a controller 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
average inductor current is lower than the load current,
the error amplifier EA will decrease the voltage on the
I
internal sleep signal goes high (enabling “sleep” mode)
and both external MOSFETs are turned off. The I
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 LTC3827-1
draws. If one channel is shut down and the other
channel is in sleep mode, the LTC3827-1 draws only
80μA of quiescent current. If both channels are in sleep
mode, the LTC3827-1 draws only 115μA 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
of the EA, the sleep signal goes low, and the controller
10
TH
pin. When the I
CC
. To select pulse-skipping mode, tie the
CC
U
– 0.5V.
TH
TH
TH
(Refer to Functional Diagram)
pin indicates a lower value. If the
pin is reconnected to the output
voltage drops below 0.4V, the
TH
pin
resumes normal operation by turning on the top exter-
nal 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 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, 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
mode, the efficiency at light loads is lower than in Burst
Mode operation. However, continuous 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 LTC3827-1 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
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 (discontinu-
ous operation). This mode, like forced continuous opera-
tion, 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.
TH
pin, just as in normal operation. In this
CMP
may remain tripped for
38271fd