LTC3734 Datasheet, PDF (20/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
APPLICATIO S I FOR ATIO
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.
V_OUT+
R3
R_AVP
V_OA+
+
R2
V_OA–
–
R1
OAOUT
FB _–
VID
I_TH
0.6V +
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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 • ^RSENSE • ^R3 – ^RSENSE ,
m R_AVP
m
if
g_m
• R3
>>
V_OUT
0.6V
(11)
Rewriting this equation, we can estimate the R_AVP value to
be:
R_AVP
≅
35.5 •R3
m• | AVP | _{– 1}
(12)
R_SENSE
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 percentage
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) I²R losses, 2) Topside
MOSFET transition losses, 3) PV_CC supply current and 4)
C_IN loss.
1) I²R 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 I²R 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 output.
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