LTC3827 Datasheet, PDF (24/36 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3827
APPLICATIONS INFORMATION
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The minimum on-time for the LTC3827 is approximately
180ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
200ns. This is of particular concern in forced continuous
applications with low ripple current at light loads. If the
duty cycle drops below the minimum on-time limit in this
situation, a significant amount of cycle skipping can occur
with correspondingly larger current and voltage ripple.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3827 circuits: 1) IC V_IN current, 2) INTV_CC
regulator current, 3) I²R losses, 4) Topside MOSFET
transition losses.
1. The V_IN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the 3.3V
linear regulator output. V_IN current typically results in
a small (<0.1%) loss.
2. INTV_CC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results
from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ moves
from INTV_CC to ground. The resulting dQ/dt is a cur-
rent out of INTV_CC that is typically much larger than the
control circuit current. In continuous mode, I_GATECHG
= f(Q_T + Q_B), where Q_T and Q_B are the gate charges of
the topside and bottom side MOSFETs.
Supplying INTV_CC power through the EXTV_CC switch
input from an output-derived source will scale the V_IN
current required for the driver and control circuits by
a factor of (Duty Cycle)/(Efficiency). For example, in a
20V to 5V application, 10mA of INTV_CC current results
in approximately 2.5mA of V_IN current. This reduces the
mid-current loss from 10% or more (if the driver was
powered directly from V_IN) to only a few percent.
3. I²R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. In continuous
mode the average output current flows through L and
R_SENSE, but is “chopped” between the topside MOSFET
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R_DS(ON), then the resistance
of one MOSFET can simply be summed with the resis-
tances of L, R_SENSE and ESR to obtain I²R losses. For
example, if each R_DS(ON) = 30mΩ, R_L = 50mΩ, R_SENSE
= 10mΩ and R_ESR = 40mΩ (sum of both input and
output capacitance losses), then the total resistance
is 130mΩ. This results in losses ranging from 3% to
13% as the output current increases from 1A to 5A for
a 5V output, or a 4% to 20% loss for a 3.3V output.
Efficiency varies as the inverse square of V_OUT for the
same external components and output power level. The
combined effects of increasingly lower output voltages
and higher currents required by high performance digital
systems is not doubling but quadrupling the importance
of loss terms in the switching regulator system!
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