LTC3835 Linear Technology, LTC3835 Datasheet - Page 20

• Manufacturer Part Number: LTC3835
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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LTC3835
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time t
that the LTC3835 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 LTC3835 is approximately
180ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
200ns. 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 LTC3835 circuits: 1) IC V
regulator current, 3) I
transition losses.
20
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
t
ON MIN
(
)
<
V
V
IN
OUT
( )
f
ON(MIN)
2
R losses, 4) Topside MOSFET
is the smallest time duration
IN
current, 2) INTV
CC
1. The V
2. INTV
3. I
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the 3.3V
linear regulator output. V
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
a current out of INTV
than the control circuit current. In continuous mode,
I
charges of the topside and bottom side MOSFETs.
input from an output-derived source will scale the VIN
current required for the driver and control circuits by
a factor of (Duty Cycle)/(Effi ciency). For example, in a
20V to 5V application, 10mA of INTV
in approximately 2.5mA of V
mid-current loss from 10% or more (if the driver was
powered directly from V
fuse (if used), MOSFET, inductor, current sense resistor,
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
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
Supplying INTV
GATECHG
2
SENSE
SENSE
R losses are predicted from the DC resistances of the
CC
IN
, but is “chopped” between the topside MOSFET
= 10mΩ and R
current is the sum of the MOSFET driver and
current has two components: the fi rst is the
= f(Q
T
+Q
CC
CC
SENSE
B
power through the EXTV
), where Q
to ground. The resulting dQ/dt is
CC
ESR
DS(ON)
IN
and ESR to obtain I
that is typically much larger
IN
) to only a few percent.
= 40mΩ (sum of both input
DS(ON)
IN
current typically results in
current. This reduces the
= 30mΩ, R
T
, then the resistance
and Q
CC
current results
B
are the gate
L
2
CC
= 50mΩ,
R losses.
switch
3835fc