LTC3826-1 Datasheet, PDF (21/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3826-1
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest time dura-
tion that the LTC3826-1 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
t_ON(MIN)
<
V_OUT
V_IN(f)
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 LTC3826-1 is approximately
230ns. 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 significant amount of cycle skipping can occur
with correspondingly larger current and voltage ripple.
Efficiency Considerations
The percent efficiency 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 efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
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 LTC3826-1 circuits: 1) IC V_IN current, 2) INTV_CC
regulator current, 3) I²R losses, 4) Topside MOSFET
transition losses.
1. The V_IN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control
currents; the second is the current drawn from the 3.3V
linear regulator output. V_IN current typically results in
a small (<0.1%) loss.
2. INTV_CC current is the sum of the MOSFET driver and
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_CC to ground. The resulting dQ/dt is a current
out of INTV_CC that is typically much larger than the
control circuit current. In continuous mode, I_GATECHG
= f(Q_T + Q_B), where Q_T and Q_B are the gate charges of
the topside and bottom side MOSFETs.
Supplying INTV_CC power through the EXTV_CC switch
input from an output-derived source will scale the V_IN
current required for the driver and control circuits by
a factor of (Duty Cycle)/(Efficiency). For example, in a
20V to 5V application, 10mA of INTV_CC current results
in approximately 2.5mA of V_IN current. This reduces
the mid-current loss from 10% or more (if the driver
was powered directly from V_IN) to only a few percent.
3. I²R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor,
and input and output capacitor ESR. In continuous
mode the average output current flows through L and
R_SENSE, but is “chopped” between the topside MOSFET
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R_DS(ON), then the resistance
of one MOSFET can simply be summed with the
resistances of L, R_SENSE and ESR to obtain I²R losses.
For example, if each R_DS(ON) = 30mΩ, R_L = 50mΩ,
R_SENSE = 10mΩ and R_ESR = 40mΩ (sum of both input
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.
Efficiency varies as the inverse square of V_OUT for the
38261fb
21