LTC3872-1_15 Datasheet, PDF (13/20 Pages) Linear Technology – No RSENSE Current Mode Boost DC/DC Controller

LTC3872-1
Applications Information
V_OUT
200mV/DIV
AC-COUPLED
I_LOAD
500mA/DIV
20µs/DIV
38721 F07
Figure 7. Load Transient Response for a 3.3V Input,
5V Output Boost Converter Application, 0.1A to 1A Step
Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input and the input current waveform
is continuous (see Figure 6b). The input voltage source
impedance determines the size of the input capacitor,
which is typically in the range of 10µF to 100µF. A low ESR
capacitor is recommended, although it is not as critical as
for the output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I_RMS(CIN)
=
0.3 •
V_IN(MIN)
L•f
• D_MAX
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to
the input of the converter and solid tantalum capacitors
can fail catastrophically under these conditions. Be sure
to specify surge-tested capacitors!
Efficiency Considerations: How Much Does VDS
Sensing Help?
The efficiency of a switching regulator is equal to the output
power divided by the input power (×100%).
Percent efficiency can be expressed as:
% Efficiency = 100% – (L1 + L2 + L3 + …),
where L1, L2, etc. are the individual loss components as a
percentage of the input power. 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 usually account for the majority
of the losses in LTC3872-1 application circuits:
1. The supply current into V_IN. The V_IN current is the
sum of the DC supply current I_Q (given in the Electrical
Characteristics) and the MOSFET driver and control cur-
rents. The DC supply current into the V_IN pin is typically
about 250µA and represents a small power loss (much
less than 1%) that increases with V_IN. The driver current
results from switching the gate capacitance of the power
MOSFET; this current is typically much larger than the DC
current. Each time the MOSFET is switched on and then
off, a packet of gate charge Q_G is transferred from V_IN
to ground. The resulting dQ/dt is a current that must be
supplied to the Input capacitor by an external supply. If
the IC is operating in CCM:
I_Q(TOT) ≈ I_Q = f • Q_G
P_IC = V_IN • (I_Q + f • Q_G)
2. Power MOSFET switching and conduction losses. The
technique of using the voltage drop across the power
MOSFET to close the current feedback loop was chosen
because of the increased efficiency that results from not
having a sense resistor. The losses in the power MOSFET
are equal to:
P_{F E T}
=
 ^I_O(MAX) 
1– D_MAX
2
• R_DS(ON)
• D_MAX
• ρ_T
+k
• V_O
1.85
• ^IO(MAX)
1– D_MAX
• C_RSS
•
f
The I²R power savings that result from not having a discrete
sense resistor can be calculated almost by inspection.
P_{R ( S E N S E )}
=
 ^IO(MAX)
1– D_MAX


2
• R_SENSE
• D_MAX
To understand the magnitude of the improvement with
this V_DS sensing technique, consider the 3.3V input, 5V
output power supply shown in the Typical Application on
the front page. The maximum load current is 7A (10A peak)
and the duty cycle is 39%. Assuming a ripple current of
40%, the peak inductor current is 13.8A and the average
38721f
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