LTC3890-2 Datasheet, PDF (26/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-2
APPLICATIONS INFORMATION
The minimum on-time for the LTC3890-2 is approximately
95ns. However, as the peak sense voltage decreases the
minimum on-time gradually increases up to about 130ns.
This is of particular concern in forced continuous applica-
tions 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 cor-
respondingly 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 LTC3890-2 circuits: 1) IC V_IN current, 2) IN-
TV_CC regulator current, 3) I²R losses, 4) topside MOSFET
transition losses.
1. The V_IN current is the DC supply current given in the
Electrical Characteristics table, which excludes MOSFET
driver and control currents. 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 current
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 from an output-derived source power
through EXTV_CC 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 midcurrent 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!
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Transition Loss = (1.7) • V_IN • 2 • I_O(MAX) • C_RSS • f
38902f
26