ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 24

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

LTC3890
The minimum on-time for the LTC3890 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:
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 circuits: 1) IC V
regulator current, 3) I
transition losses.
1. The V
2. INTV
APPLICATIONS INFORMATION
24
%Effi ciency = 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
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.
CC
T
IN
+ Q
current is the sum of the MOSFET driver and
current is the DC supply current given in the
CC
B
), where Q
CC
to ground. The resulting dQ/dt is a current
that is typically much larger than the
2
T
R losses, 4) topside MOSFET
and Q
IN
B
current typically results
are the gate charges of
IN
current, 2) INTV
GATECHG
CC
3. I
4. Transition losses apply only to the topside MOSFET(s),
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!
and become signifi cant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Other hidden losses such as copper trace and internal
battery resistances can account for an additional 5%
to 10% effi ciency degradation in portable systems. It
is very important to include these system level losses
during the design phase. The internal battery and fuse
2
R losses are predicted from the DC resistances of the
SENSE
Transition Loss = (1.7) • V
IN
, but is chopped between the topside MOSFET
) to only a few percent.
IN
current. This reduces the midcurrent loss
SENSE
CC
CC
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
current results in approximately
= 30mΩ, R
DS(ON)
IN
• 2 • I
, then the resistance
IN
L
O(MAX)
current required
= 50mΩ, R
2
R losses. For
OUT
• C
RSS
for the
SENSE
3890f
• f