LTC3872-1_15 Datasheet, PDF (14/20 Pages) Linear Technology – No RSENSE Current Mode Boost DC/DC Controller

LTC3872-1
Applications Information
is 11.5A. With a maximum sense voltage of about 140mV,
the sense resistor value would be 10mΩ, and the power
dissipated in this resistor would be 514mW at maximum
output current. Assuming an efficiency of 90%, this
sense resistor power dissipation represents 1.3% of the
overall input power. In other words, for this application,
the use of V_DS sensing would increase the efficiency by
approximately 1.3%.
For more details regarding the various terms in these
equations, please refer to the section Boost Converter:
Power MOSFET Selection.
3. The losses in the inductor are simply the DC input cur-
rent squared times the winding resistance. Expressing this
loss as a function of the output current yields:
P_{R ( W I N D I N G )}
=
 ^I_O(MAX)
_{1– DMAX}


2
•
R_W
4. Losses in the boost diode. The power dissipation in the
boost diode is:
P_DIODE = I_O(MAX) • V_D
The boost diode can be a major source of power loss in
a boost converter. For the 3.3V input, 5V output at 7A ex-
ample given above, a Schottky diode with a 0.4V forward
voltage would dissipate 2.8W, which represents 7% of the
input power. Diode losses can become significant at low
output voltages where the forward voltage is a significant
percentage of the output voltage.
5. Other losses, including C_IN and C_O ESR dissipation and
inductor core losses, generally account for less than 2%
of the total additional loss.
Checking Transient Response
The regulator loop response can be verified by looking at
the load transient response. Switching regulators generally
take several cycles to respond to an instantaneous step
in resistive load current. When the load step occurs, V_O
immediately shifts by an amount equal to (DI_LOAD)(ESR),
and then C_O begins to charge or discharge (depending on
the direction of the load step) as shown in Figure 7. The
regulator feedback loop acts on the resulting error amp
output signal to return V_O to its steady-state value. During
this recovery time, V_O can be monitored for overshoot or
ringing that would indicate a stability problem.
A second, more severe transient can occur when con-
necting loads with large (>1µF) supply bypass capacitors.
The discharged bypass capacitors are effectively put in
parallel with C_O, causing a nearly instantaneous drop in
V_O. No regulator can deliver enough current to prevent
this problem if the load switch resistance is low and it is
driven quickly. The only solution is to limit the rise time
of the switch drive in order to limit the inrush current
di/dt to the load.
Boost Converter Design Example
The design example given here will be for the circuit shown
on the front page. The input voltage is 3.3V, and the output
is 5V at a maximum load current of 2A.
1. The duty cycle is:
D
=


V_O
+ V_D
V_O +
– V_IN
V_D


=
5
+0.4 – 3.3
5 + 0.4
=
38
.
9%
2. An inductor ripple current of 40% of the maximum load
current is chosen, so the peak input current (which is also
the minimum saturation current) is:
I_{I N ( P E A K )}
= ^1+ ^χ
 2
• ^IO(MAX)
1– D_MAX
= 1.2 • ² =
1– 0.39
3.9A
The inductor ripple current is:
∆I_L
=^c•
I_O(MAX)
1– D_MAX
= 0.4 •
2
1– 0.39
= 1.3A
And so the inductor value is:
L
=
V_IN(MIN)
∆I_L • f
•
D_MAX
=
3.3V
1.3A • 550kHz
•
0.39
=
1.8µH
The component chosen is a 2.2µH inductor made by
Sumida (part number CEP125-H 1ROMH).
14
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