LTC3867_15 Datasheet, PDF (12/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
OPERATION
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Continuous Conduction)
The LTC3867 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 pin to 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 the 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 both external MOSFETs are turned off.
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 the 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 re-
versing 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_TH pin, just as in normal operation. 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.
When the MODE/PLLIN pin is connected to INTV_CC, the
LTC3867 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.,
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.
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 opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
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 200kHz to 1.2MHz.
There is a precision 20µA current flowing out of the FREQ
pin so that the user can program the controller’s switch-
ing frequency with a single resistor to SGND. A curve
is provided later in the Applications Information section
showing the relationship between the voltage on the FREQ
pin and switching frequency.
A phase-locked loop (PLL) is available on the LTC3867
to synchronize the internal oscillator to an external clock
source that is connected to the MODE/PLLIN pin. The PLL
loop filter network is integrated inside the LTC3867. The
phase‑locked loop is capable of locking any frequency
within the range of 250kHz to 1.1MHz. The frequency setting
resistor should always be present to set the controller’s
initial switching frequency before locking to the external
clock. The controller operates in forced continuous mode
when it is synchronized.
Sensing the Output Voltage with a
Differential Amplifier
The LTC3867 includes a low offset, high input impedance,
unity-gain, high bandwidth differential amplifier for ap-
plications that require true remote sensing. Sensing the
3867f
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