LTC3834 Linear Technology, LTC3834 Datasheet - Page 20

• Manufacturer Part Number: LTC3834
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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LTC3834
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2
PLLLPF PIN
0V
Floating
INTV
RC Loop Filter
Minimum On-Time Considerations
Minimum on-time, t
that the LTC3834 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 LTC3834 is approximately
200ns. However, as the peak sense voltage decreases
the minimum on-time gradually increases up to about
250ns. 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.
APPLICATIONS INFORMATION
20
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
t
CC
ON MIN
(
)
<
V
V
PLLIN/MODE PIN
IN
OUT
Clock Signal
DC Voltage
DC Voltage
DC Voltage
( )
f
ON(MIN)
, is the smallest time duration
Phase-Locked to External Clock
FREQUENCY
250kHz
400kHz
530kHz
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3834 circuits: 1) IC V
regulator current, 3) I
transition losses.
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
rent out of INTV
control circuit current. In continuous mode, I
= f(Q
the topside and bottom side MOSFETs.
Supplying INTV
input from an output-derived source will scale the V
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
the mid-current loss 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
2
SENSE
R losses are predicted from the DC resistances of the
CC
T
IN
+ Q
, but is “chopped” between the topside MOSFET
current is the sum of the MOSFET driver and
current has two components: the fi rst is the
B
CC
), where Q
to ground. The resulting dQ/dt is a cur-
SENSE
CC
CC
ESR
DS(ON)
that is typically much larger than the
power through the EXTV
= 40mΩ (sum of both input and
and ESR to obtain I
2
T
R losses, 4) Topside MOSFET
and Q
= 30mΩ, R
IN
DS(ON)
current typically results in
IN
B
IN
) to only a few percent.
are the gate charges of
current. This reduces
IN
, then the resistance
current, 2) INTV
L
CC
= 50mΩ, R
current results
2
R losses. For
CC
GATECHG
switch
SENSE
3834fb
CC
IN