LTC3802_15 Datasheet, PDF (19/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
As the output load current decreases in continuous mode,
the average current in the inductor will reach a point where
it drops below half the ripple current. At this point, the
current in the inductor will reverse during a portion of the
switching cycle, or begin to flow from the output back to
the input. This does not adversely affect regulation, but
does cause additional losses as a portion of the inductor
current flows back and forth through the resistive power
switches, giving away a little more power each time and
lowering the efficiency. There are some benefits to allow-
ing this reverse current flow: the circuit will maintain
regulation even if the load current drops to zero and the
output ripple voltage and frequency remain constant at all
loads, easing filtering requirements. However, continuous
mode at low output current does cause losses in effi-
ciency. A portion of the inductor current flows back and
forth through the resistive power switches, causing I²R
losses. The drivers continue to switch QT and QB on and
off once a cycle. Each time an external MOSFET is turned
on, the internal driver must charge its gate to a potential
above the MOSFET’s source voltage; when the MOSFET is
turned off, that charge is lost to ground or SW. At the high
switching frequencies, the lost gate charges can add up to
tens of millicoulombs. As the load current continues to
drop, these charges quickly become the dominant power
loss term, reducing efficiency once again.
I_RIPPLE
I_AVERAGE
TIME
Figure 5a. Continuous Mode
3802 F05a
I_RIPPLE
TIME
I_AVERAGE
3802 F05b
Figure 5b. Burst Mode Operation
To minimize the switching loss and reverse current flow at
light loads, the LTC3802 switches to a second mode of
operation: Burst Mode operation (Figure 5b). In Burst
Mode operation, at the end of the QB cycle, if the inductor
current approaches zero or goes negative, the LTC3802
turns off both drivers. The actual cutoff threshold is
proportional to the I_MAX setting and is equal to:
– ^VIMAX – 3mV
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The –3mV built-in offset overcomes the random mis-
match in the burst compararator trip point and allows
Burst Mode operation at no load.
Once both MOSFETs shut off, the voltage at the SW pin will
float around V_OUT, and the inductor current and the voltage
across the inductor will be close to zero. This prevents
current from flowing backwards in QB, eliminating that
power loss term.
The moment the LTC3802 enters Burst Mode operation,
both drivers skip a number of switching cycles until the
internal 36µs timeout forces the switcher to return to
continuous operation. This timeout eliminates the audible
noise from certain types of inductors when they are lightly
loaded. After the 36µs timeout, the LTC3802 forces one
continuous mode cycle and checks the inductor current at
the end of the period. If it is still too small, it enters Burst
Mode operation again. This pattern repeats until the out-
put is loaded. The LTC3802 returns to continuous mode
operation if it detects that CMPIN potential is 12mV below
or 15mV above its nominal bandgap voltage. Immediately
after returning to continuous mode operation, the regula-
tor output might continue to droop slightly until the feed-
back loop responds and requests an increase in duty cycle.
During sudden transient steps, the regulator output ripple
is limited by the feedback loop transient response and is
independent of the mode of operation.
The small 15mV and –12mV offset at the POS and NEG
RESET comparators ensure that after a transient load step,
the LTC3802 returns to continuous mode quickly. This
minimizes the output ripple under Burst Mode operation.
For proper Burst Mode operation, the LTC3802 requires
very precise CMPIN and FB sensing. To realize this,
CMPIN and FB must use the same resistive divider values
3802f
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