LTC1437A_15 Datasheet, PDF (21/28 Pages) Linear Technology – High Efficiency Low Noise Synchronous Step-Down Switching Regulators

LTC1436A
LTC1436A-PLL/LTC1437A
APPLICATIONS INFORMATION
INDUCTOR
L
1435A F08
Figure 13. Allowable Inductor/R_SENSE Layout Orientations
Efficiency Considerations
The 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. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1436A/LTC1437A circuits: LTC1436A/
LTC1437A V_IN current, INTV_CC current, I²R losses and
topside MOSFET transition losses.
1. The V_IN current is the DC supply current given in the
Electrical Characteristics table which excludes MOSFET
driver and control currents. V_IN current results in a
small (< 1%) loss which increases with V_IN.
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. It is for this reason
that the Adaptive Power output stage switches to a low
Q_T MOSFET during low current operation.
By powering EXTV_CC from an output-derived source,
the additional V_IN current resulting from the driver and
control currents will be scaled by a factor of Duty Cycle/
Efficiency. For example, in a 20V to 5V application,
10mA of INTV_CC current results in approximately 3mA
of V_IN current. This reduces the midcurrent 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
MOSFET, inductor and current shunt. In continuous
mode the average output current flows through L and
R_SENSE, but is “chopped” between the topside main
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 and R_SENSE to obtain I²R
losses. For example, if each R_DS(ON) = 0.05Ω,
R_L = 0.15Ω and R_SENSE = 0.05Ω, then the total resis-
tance is 0.25Ω. This results in losses ranging from 3%
to 10% as the output current increases from 0.5A to 2A.
I²R losses cause the efficiency to drop at high output
currents.
4. Transition losses apply only to the topside MOSFET(s),
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
Transition Loss = 2.5(V_IN)^1.85(I_MAX)(C_RSS)(f)
Other losses including C_IN and C_OUT ESR dissipative
losses, Schottky conduction losses during dead-time and
inductor core losses, generally account for less than 2%
total additional loss.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, V_OUT immediately shifts
by an amount equal to (∆I_LOAD)(ESR), where ESR is the
effective series resistance of C_OUT. ∆I_LOAD also begins to
charge or discharge C_OUT which generates a feedback
error signal. The regulator loop then acts to return V_OUT to
its steady-state value. During this recovery time V_OUT can
be monitored for overshoot or ringing, which would
indicate a stability problem. The I_TH external components
shown in the Figure 1 circuit will provide adequate com-
pensation for most applications.
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