LTC3879_15 Datasheet, PDF (18/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

LTC3879
APPLICATIONS INFORMATION
So, which mode should be programmed? While either
mode in Figure 8 satisfies most practical applications,
the coincident mode offers better output regulation.
This can be better understood with the help of Figure 9.
At the input stage of the LTC3879’s error amplifier, two
common anode diodes are used to clamp the equivalent
reference voltage and an additional diode is used to match
the shifted common mode voltage. The top two current
sources are of the same amplifier. In the coincident mode,
the TRACK/SS voltage is substantially higher than 0.6V
at steady-state and effectively turns off D1. D2 and D3
will, therefore, conduct the same current, and offer tight
matching between V_FB and the internal precision 0.6V
reference. In the ratiometric mode, however, TRACK/SS
equals 0.6V in steady-state. D1 will divert part of the bias
current to make V_FB slightly lower than 0.6V.
Although this error is minimized by the exponential I-V
characteristics of the diode, it does impose a finite amount
of output voltage deviation. Furthermore, when the master
supply’s output experiences dynamic excursion (under
load transient, for example), the slave channel output will
be affected as well. For better output regulation, use the
coincident tracking mode instead of ratiometric.
I
I
D1
TRACK/SS
0.6V
V_FB
+
D2
EA
–
D3
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Figure 9. Equivalent Input Circuit of Error Amplifier
INTV_CC Undervoltage Lockout
Whenever INTV_CC drops below approximately 3.4V, the
device enters undervoltage lockout (UVLO). In a UVLO
condition, the switching outputs TG and BG are disabled.
At the same time, the TRACK/SS pin is pulled down from
INTV_CC to 0V with a small internal NMOS switch. When the
INTV_CC UVLO condition is removed, TRACK/SS is released,
beginning a normal soft-start. This feature is important
when regulator start-up is not initiated by applying a logic
drive to RUN.
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. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3879 circuits.
1. DC I²R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows though the inductor
L, but is chopped between the top and bottom MOSFETs.
If the two MOSFETs have approximately the same R_DS(ON),
then the resistance of one MOSFET can simply by summed
with the resistances of L and the board traces to obtain
the DC I²R loss. For example, if R_DS(ON) = 0.01Ω and
R_L = 0.005Ω, the loss will range from 15mW to 1.5W as
the output current varies from 1A to 10A.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V.
3. INTV_CC current. This is the sum of the MOSFET driver
and control currents.
4. C_IN loss. The input capacitor has the difficult job of filter-
ing the large RMS input current to the regulator. It must have
a very low ESR to minimize the AC I²R loss and sufficient
capacitance to prevent the RMS current from causing ad-
ditional upstream losses in fuses or batteries.
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