LTC3827-1
21
38271fe
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
, is the smallest time dura-
tion that the LTC3827-1 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 LTC3827-1 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 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 LTC3827-1 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 has two components: the fi rst 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)/(Effi ciency). 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. I
2
R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor,
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