ltc3872ets8-trpbf Linear Technology Corporation, ltc3872ets8-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc3872ets8-trpbf
• Description: No Rsense Current Mode Boost Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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:
4. Losses in the boost diode. The power dissipation in the
boost diode is:
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 signifi cant at low
output voltages where the forward voltage is a signifi cant
percentage of the output voltage.
5. Other losses, including C
inductor core losses, generally account for less than 2%
of the total additional loss.
Checking Transient Response
The regulator loop response can be verifi ed 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
immediately shifts by an amount equal to (ΔI
and then C
P
P
AC COUPLED
R WINDING
DIODE
200mV/DIV
500mA/DIV
Figure 6. Load Transient Response for a 3.3V Input,
5V Output Boost Converter Application, 0.1A to 1A Step
(
V
OUT
= I
I
O
L
begins to charge or discharge (depending on
O(MAX)
)
=
⎛
⎝ ⎜
U
1
• V
I
O MAX
–
(
D
D
MAX
U
20μs/DIV
IN
)
⎞
⎠ ⎟
and C
2
•
R
O
W
W
ESR dissipation and
3872 F06
LOAD
U
)(ESR),
O
the direction of the load step) as shown in Figure 6. The
regulator feedback loop acts on the resulting error amp
output signal to return V
this recovery time, V
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
V
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:
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:
The inductor ripple current is:
And so the inductor value is:
The component chosen is a 2.2μH inductor made by
Sumida (part number CEP125-H 1ROMH).
I
IN PEAK
O
. No regulator can deliver enough current to prevent
(
D
L
Δ =
=
I
=
L
V
⎛
⎝ ⎜
Δ
IN MIN
)
V
χ •
I
=
(
L
O
⎛
⎝ ⎜
V
•
+
1
1
O
I
f
O MAX
–
)
V
+
O
(
+
•
D
, causing a nearly instantaneous drop in
D
χ
2
D
MAX
V
–
MAX
D
⎞
⎠ ⎟
V
)
O
•
IN
=
1
can be monitored for overshoot or
I
=
⎞
⎠ ⎟
O MAX
–
O
0 4
(
1 3
D
. •
to its steady-state value. During
=
.
MAX
5 0 4 3 3
A
1 0 39
+
)
3 3
•
– .
5 0 4
.
550
=
+
2
. – .
V
1 2
kHz
. •
.
=
LTC3872
1 3
1 0
• .
.
0 39 1. . 8μH
–
=
A
2
38 9
. .
39
=
. %
13
=
3 9
3872fa
.
A