LTC3857-1
24
38571fc
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
, is the smallest time dura-
tion that the LTC3857-1 is capable of turning on the top
MOSFET. It is determined by internal timing delays and the
gate charge required to turn on the top MOSFET. Low duty
cycle applications may approach this minimum on-time
limit and care should be taken to ensure that
t
ON(MIN)
<
V
OUT
V
IN
f
()
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 LTC3857-1 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 LTC3857-1 circuits: 1) IC V
IN
current, 2) INTV
CC
regulator current, 3) I
2
R losses, 4) topside MOSFET
transition losses
.
1. The V
IN
current is the DC input supply current given
in the Electrical Characteristics table, which excludes
MOSFET driver and control currents. V
IN
current typi-
cally 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 power source
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. I
2
R 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 I
2
R losses. For
example, if each R
DS(ON)
= 30m, R
L
= 50m, R
SENSE
= 10m and R
ESR
= 40m (sum of both input and
output capacitance losses), then the total resistance
is 130m. 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!
