Datasheet

LTC3734
20
3734fa
APPLICATIONS INFORMATION
where
AVP is the slope, in mV/A
R
SENSE
is the current sense resistor
m is the number of phases, m = 1 for LTC3734
R3 and R
AVP
are defined in Figure 6
g
m
is the transconductance gain for the error amplifier,
it is about 4.5mmho for LTC3734.
Rewriting this equation, we can estimate the R
AVP
value
to be:
R
AVP
≅
35.5•R3
m •| AVP|
R
SENSE
–1
(12)
Typically the calculation results based on these equations
have ±10% tolerance. So the resistor values need to be
fine tuned.
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 LTC3734 circuits: 1) I
2
R losses, 2) Topside
MOSFET transition losses, 3) PV
CC
supply current and
4) C
IN
loss.
1) I
2
R losses are predicted from the DC resistances of
the fuse (if used), MOSFET, inductor, and current sense
resistor. 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 I
2
R losses.
For example, if each R
DS(ON)
= 10mΩ, R
L
= 10mΩ, and
R
SENSE
= 5mΩ, then the total resistance is 25mΩ. This
results in losses ranging from 2% to 8% as the output
current increases from 3A to 15A per output stage for a 5V
output, or a 3% to 12% loss per output stage for a 3.3V
–
+
–
+
0.6V
V
OUT+
R3
R2
R
AVP
R1
I
TH
VID
OAOUT
V
OA
+
V
OA
–
FB
3735 F09
Figure 6. Simplified Schematic
Diagram for AVP Design in LTC3734
Rewriting equation (9) we can estimate the AVP resistor
to be:
R
AVP
≅
35.5•R3•R
SENSE
m •| AVP|
(10)
We also adopt the current sense resistors as part of
voltage positioning slopes. So the total load line slope is
estimated to be:
AVP ≅ –35.5 •
R
SENSE
m
•
R3
R
AVP
–
R
SENSE
m
,
if g
m
•R3 >>
V
OUT
0.6V
(11)