ltc3891 Linear Technology Corporation, ltc3891 Datasheet - Page 23

• Manufacturer Part Number: ltc3891
• Description: Ltc3891 - Low Iq, 60v Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 LTC3891 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:
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 LTC3891 circuits: 1) IC V
regulator current, 3) I
transition losses.
1. The V
2. INTV
%Efficiency = 100% – (L1 + L2 + L3 + ...)
Electrical Characteristics table, which excludes MOSFET
driver and control currents. V
in a small (<0.1%) loss.
control currents. The MOSFET driver current results
from switching the gate capacitance of the power
t
ON(MIN)
CC
IN
current is the sum of the MOSFET driver and
current is the DC supply current given in the
<
V
V
IN
OUT
( )
f
2
R losses, 4) topside MOSFET
IN
current typically results
IN
current, 2) INTV
CC
3. I
4. Transition losses apply only to the topside MOSFET(s),
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)/(Efficiency). 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 flows 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.
Efficiency 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!
and become significant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
2
SENSE
R losses are predicted from the DC resistances of the
Transition Loss = (1.7) • V
T
+ Q
IN
, but is chopped between the topside MOSFET
) 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
CC
that is typically much larger than the
ESR
from an output-derived source power
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)
IN
B
are the gate charges of
• 2 • I
, then the resistance
IN
L
LTC3891
current required
O(MAX)
= 50mΩ, R
2
R losses. For
OUT
• C
GATECHG
23
for the
RSS
SENSE
3891f
• f