Datasheet

LTC3890-3
25
38903f
For more information www.linear.com/3890-3
applicaTions inForMaTion
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
, is the smallest time dura-
tion that the LTC3890-3 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 LTC3890-3 is approximately
90ns. However, as the peak sense voltage decreases the
minimum on-time gradually increases up to about TBDns.
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 LTC3890-3 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 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.