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Synchronous operation maximizes efficiency at full load, where
resistive drops in the switching MOSFET and the synchronous rectifier
dominate the power losses. As the load drops and switching losses become a
larger factor, the LTC1702/LTC1703 automatically shifts into
discontinuous mode, where the synchronous rectifier MOSFET turns off before the
end of a switching cycle to prevent reverse current flow in the inductor. As
the load current continues to decrease, the LTC1702/LTC1703
switches modes again and enters Burst Mode, where it will only switch as
required to keep the output in regulation, skipping cycles whenever possible
to reduce switching losses to a bare minimum. With no output load
in Burst Mode, the supply current for the entire system drops to the
3mA quiescent current drawn by each side of the LTC1702/LTC1703. Each
side can be shutdown independently; with both sides shut down, the
LTC1702/LTC1703 enters a sleep mode where it draws less than 50µA.
Inside the LTC1702/LTC1703
The LTC1702/LTC1703 features peerless regulation and
transient response, due to both to its high switching frequency and a
carefully designed internal architecture (Figure 2). Much of the transient
response improvement comes from a new feedback amplifier design.
Unlike conventional switching regulator designs, the LTC1702/LTC1703
use a true 25MHz gain-bandwidth op amp as the feedback amplifier (FB in
Figure 2). This allows the use of an optimized compensation scheme
that can tailor the loop response more precisely that the traditional RC
from COMP to ground. A "type 3" feedback circuit (Figure 3) typically allows
the loop to be crossed over beyond 50kHz while maintaining good stability,
significantly enhancing load transient response. Two additional high
speed comparators (MIN and MAX in Figure 2) run in parallel with the main
feedback amplifier, providing virtually instantaneous correction to
sudden changes in output voltage. In a typical application, the
LTC1702/LTC1703 will correct the duty cycle and have the output voltage
headed back in the right direction the very next switching cycle after a
transient load is applied.
Figure 2. LTC1702/LTC1703 block diagram
The positive input of the feedback op amp is connected to an
onboard reference trimmed to 800mV ±3mV. DC output error due to the
reference and the feedback amplifier are inside 0.5% and DC load and line
regulation are typically better than 0.1%, providing excellent DC accuracy.
The 800mV reference level allows the LTC1702/LTC1703 to provide
regulated output voltages as low as 0.8V without additional external
components. This reference performance, combined with the high speed
internal feedback amplifier and properly chosen external components,
allows the LTC1702 to provide output regulation tight enough for virtually
any microprocessor, today or in the future. For those Intel processors that
use VID control,
the LTC1703 with its onboard 5-bit VID output voltage control is the
best solution.
Another architecture trick inside the LTC1702/LTC1703 reduces
the required input capacitance with virtually no performance penalty.
The LTC1702/LTC1703 includes a single master clock, which drives the
two sides such that side 1 is 180° out of phase from side 2. This
technique, known as two-phase switching, has the effect of doubling the frequency
of the switching pulses seen by the input capacitor and significantly
reducing their RMS value. With two-phase switching, the input capacitor is
sized as required to support a single side at maximum load. As the load
increases at the other side, it tends to cancel, rather than add to, the RMS
current seen by the input capacitor; hence no additional capacitance needs to
be added.
External Components
The other half of the performance equation is made up by the
external components used with the LTC1702/LTC1703. The 550kHz clock
frequency and the low 5V input voltage allow the use of external inductors in the
1µH range or lower (LEXT in Figure 1)
while still keeping inductor ripple current under control. This low
inductance value helps in two ways: it reduces the energy stored in the inductor
during each switching cycle, reducing the physical core size required; and
it raises the attainable di/dt at the output of the circuit, decreasing the
time that it takes for the circuit to correct for sudden changes in load
current. This, in turn, reduces the amount of output capacitance
(COUT in Figure 1) required to support the output
voltage during a load transient. Together with the reduced capacitance at
the input due to the LTC1702/LTC1703's two-phase internal switching,
this significantly reduces the amount of total capacitance needed,
compared to a conventional design running at 300kHz or less.
Each side of an LTC1702/LTC1703 circuit requires a pair of
N-channel power MOSFETs to complete the power switching path. These are
chosen for low RDS(ON) and minimum gate charge, to minimize conductive
losses with heavy loads and switching losses at lighter loads. MOSFET types
that work well with the LTC1702/LTC1703 include the IRF7805 and IRF7811 from
International Rectifier, the Si9802 and Si9804 from Siliconix and the FDS6670A
from Fairchild.
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