LTC3858
24
3858fc
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
, is the smallest time duration
that the LTC3858 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 LTC3858 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 signifi cant amount of cycle skipping can occur with cor-
respondingly larger current and voltage ripple.
Effi ciency Considerations
The percent effi ciency 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 effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
%Effi ciency = 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 LTC3858 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)/(Effi ciency). For example, in a 20V to 5V applica-
tion, 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 fl ows 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.
Effi ciency varies as the inverse square of V
OUT
for the
same external components and output power level. The
