21
LTC1436A
LTC1436A-PLL/LTC1437A
14367afb
APPLICATIONS INFORMATION
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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. I
2
R 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 I
2
R
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.
I
2
R 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.
L
INDUCTOR
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, I
2
R 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/
