LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 23

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

www.DataSheet4U.com
Minimum On-Time Considerations
Minimum on-time, t
tion that the LTC3858-1 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:
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 LTC3858-1 is approximately
90ns. 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:
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 LTC3858-1 circuits: 1) IC V
TV
transition losses.
APPLICATIONS INFORMATION
CC
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
t
ON MIN
regulator current, 3) I
(
)
<
V
V
IN
OUT
( )
f
ON(MIN)
2
R losses, 4) topside MOSFET
, is the smallest time dura-
IN
current, 2) IN-
1. The V
2. INTV
3. I
in the Electrical Characteristics table, which excludes
MOSFET driver and control currents. V
cally results in a small (<0.1%) loss.
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
out of INTV
control circuit current. In continuous mode, I
= f(Q
the topside and bottom side MOSFETs.
Supplying INTV
through EXTV
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
2.5mA of V
from 10% or more (if the driver was powered directly
from V
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
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R
of one MOSFET can simply be summed with the resis-
tances of L, R
example, if each R
= 10mΩ and R
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.
Effi ciency varies as the inverse square of V
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!
2
SENSE
R losses are predicted from the DC resistances of the
CC
T
IN
+ Q
IN
, but is “chopped” between the topside MOSFET
current is the sum of the MOSFET driver and
current is the DC input supply current given
) to only a few percent.
CC
B
IN
), where Q
CC
to ground. The resulting dQ/dt is a current
current. This reduces the midcurrent loss
SENSE
CC
that is typically much larger than the
CC
ESR
from an output-derived power source
will scale the V
DS(ON)
CC
= 40mΩ (sum of both input and
and ESR to obtain I
T
current results in approximately
and Q
= 30mΩ, R
DS(ON)
B
are the gate charges of
LTC3858-1
, then the resistance
IN
L
current required
= 50mΩ, R
IN
2
R losses. For
current typi-
OUT
GATECHG
23
for the
SENSE
38581f