ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 23

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
on the current sense signal. The minimum on-time can be
affected by PCB switching noise in the voltage and current
loop. As the peak sense voltage decreases the minimum
on-time gradually increases to 130ns. This is of particular
concern in forced continuous applications 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 LTC3850-2 circuits: 1) IC V
regulator current, 3) I
transition losses.
1. The V
2. INTV
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
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
rent 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
B
CC
), where Q
to ground. The resulting dQ/dt is a cur-
CC
that is typically much larger than the
2
R losses, 4) Topside MOSFET
T
and Q
B
are the gate charges of
IN
current, 2) INTV
IN
current typi-
GATECHG
CC
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
resistance losses can be minimized by making sure that
C
switching frequency. A 25W supply will typically require
3. I
4. Transition losses apply only to the topside MOSFET(s),
IN
Supplying INTV
put-derived source will scale the V
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 resistor.
In continuous mode, the average output current fl ows
through L and R
topside MOSFET and the synchronous MOSFET. If the
two MOSFETs have approximately the same R
then the resistance of one MOSFET can simply be
summed with the resistances of L and R
I
= 10mΩ, R
25mΩ. This results in losses ranging from 2% to 8%
as the output current increases from 3A to 15A for
a 5V output, or a 3% to 12% 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:
Transition Loss = (1.7) V
2
2
has adequate charge storage and very low ESR at the
R losses are predicted from the DC resistances of the
R losses. For example, if each R
IN
) to only a few percent.
IN
SENSE
current. This reduces the mid-current loss
CC
SENSE
= 5mΩ, then the total resistance is
power through EXTV
CC
current results in approximately
, but is “chopped” between the
IN
2
I
O(MAX)
LTC3850-2
DS(ON)
IN
C
current required
RSS
CC
SENSE
from an out-
= 10mΩ, R
f
OUT
to obtain
23
DS(ON)
for the
38502f
L
,