ltc3890-1 Linear Technology Corporation, ltc3890-1 Datasheet - Page 24

• Manufacturer Part Number: ltc3890-1
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

The minimum on-time for the LTC3890-1 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 signifi cant amount of cycle skipping can occur with
correspondingly 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 LTC3890-1 circuits: 1) IC V
regulator current, 3) I
transition losses.
1. The V
2. INTV
LTC3890-1
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.
T
CC
IN
+ Q
current is the sum of the MOSFET driver and
current is the DC supply current given in the
B
CC
), 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
tance of one MOSFET can simply be summed with the
resistances of L, R
For example, if each R
R
and 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
2
SENSE
SENSE
R losses are predicted from the DC resistances of the
Transition Loss = (1.7) • V
IN
, but is chopped between the topside MOSFET
= 10mΩ and R
) to only a few percent.
IN
current. This reduces the midcurrent loss
CC
CC
from an output-derived source power
will scale the V
SENSE
CC
current results in approximately
ESR
DS(ON)
and ESR to obtain I
= 40mΩ (sum of both input
IN
= 30mΩ, R
• 2 • I
DS(ON)
IN
O(MAX)
current required
, then the resis-
L
OUT
2
• C
= 50mΩ,
R losses.
for the
RSS
38901f
• f