LTC3869-2_15 Datasheet, PDF (12/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
Operation
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
The LTC3869 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode,
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to a DC
voltage below 0.6V (e.g., SGND). To select pulse-skipping
mode of operation, tie the MODE/PLLIN pin to INTV_CC. To
select Burst Mode operation, float the MODE/PLLIN pin.
When a controller is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-third of the maximum sense voltage even though the
voltage on the I_TH 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 I_TH pin. When
the I_TH voltage drops below 0.5V, the internal sleep signal
goes high (enabling sleep mode) and the top MOSFET is
turned off immediately, but the bottom MOSFET is turned
off when the inductor current reaches zero.
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
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_REV) turns off the bottom external MOSFET just before
the inductor current reaches zero, preventing it from
reversing and going negative. Thus, the controller oper-
ates 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. In this mode, the efficiency at light loads is lower than
in Burst Mode operation. However, continuous mode has
the advantages of lower output ripple and less interference
with audio circuitry.
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.
Single Output Multiphase Operation
The LTC3869 can be used for single output multiphase
converters by making these connections
• Tie all of the I_TH pins together.
• Tie all of the V_FB pins together.
• Tie all of the TK/SS pins together.
• Tie all of the RUN pins together.
LTC3869 has excellent current matching performance
between channels to ensure that there are equal thermal
stress for both channels.
Frequency Selection and Phase-Locked Loop
(FREQ and MODE/PLLIN Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency operation
increases efficiency by reducing MOSFET switching losses,
but requires larger inductance and/or capacitance to main-
tain low output ripple voltage. The switching frequency of
the LTC3869 controller can be selected using the FREQ pin.
If the MODE/PLLIN pin is not being driven by an external
clock source, the FREQ pin can be used to program the
controller’s operating frequency from 250kHz to 780kHz.
There is a precision 10µA current flowing out of the FREQ
pin, so the user can program the controller’s switch-
ing frequency with a single resistor to SGND. A curve
is provided later in the application section showing the
relationship between the voltage on the FREQ pin and
switching frequency.
When the MODE/PLLIN pin is connected to INTV_CC, the
LTC3869 operates in PWM pulse-skipping mode at light
loads. 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.,
A phase-locked loop (PLL) is integrated on the LTC3869
to synchronize the internal oscillator to an external clock
source that is connected to the MODE/PLLIN pin. The
controller is operating in forced continuous mode when
it is synchronized.
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