LTC1871 Linear Technology, LTC1871 Datasheet - Page 20

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1871
3. The losses in the inductor are simply the DC input
4. Losses in the boost diode. The power dissipation in the
5. Other losses, including C
Checking Transient Response
The regulator loop response can be verified by looking at
the load transient response. Switching regulators gener-
ally take several cycles to respond to an instantaneous
step in resistive load current. When the load step occurs,
V
and then C
the direction of the load step) as shown in Figure 13. 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.
20
O
resistor value would be 10m , and the power dissi-
pated 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 appli-
cation, the use of V
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.
current squared times the winding resistance. Express-
ing this loss as a function of the output current yields:
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
example given above, a Schottky diode with a 0.4V
forward voltage would dissipate 2.8W, which repre-
sents 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.
inductor core losses, generally account for less than
2% of the total additional loss.
immediately shifts by an amount equal to ( I
P
P
R WINDING
DIODE
(
O
begins to charge or discharge (depending on
= I
O(MAX)
)
U
O
1
can be monitored for overshoot or
I
• V
O MAX
–
O
DS
(
D
D
to its steady-state value. During
U
MAX
IN
sensing would increase the
)
and C
2
•
W
O
R
ESR dissipation and
W
LOAD
U
)(ESR),
A second, more severe transient can occur when connect-
ing loads with large (> 1 F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
regulator can deliver enough current to prevent this prob-
lem 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
in Figure 1. The input voltage is 3.3V, and the output is 5V
at a maximum load current of 7A (10A peak).
1. The duty cycle is:
2. Pulse-skip operation is chosen so the MODE/SYNC pin
3. The operating frequency is chosen to be 300kHz to
4. An inductor ripple current of 40% of the maximum load
100mV/DIV
is shorted to INTV
reduce the size of the inductor. From Figure 5, the
resistor from the FREQ pin to ground is 80k.
current is chosen, so the peak input current (which is
also the minimum saturation current) is:
I
V
IN PEAK
OUT
Figure 13. Load Transient Response for a 3.3V Input,
5V Output Boost Converter Application, 0.7A to 7A Step
2A/DIV
(
D
(AC)
O
I
OUT
, causing a nearly instantaneous drop in V
)
V
O
V
1
O
V
D
2
V
–
D
CC
•
V
1
IN
.
I
O MAX
–
100 s/DIV
(
D
MAX
5 0 4 3 3
)
V
V
MODE/SYNC = INTV
(PULSE-SKIP MODE)
IN
OUT
5 0 4
= 3.3V
= 5V
1 2
. – .
. •
.
1 0 39
– .
1871 F13
CC
7
38 9
. %
13 8
O
. No
.
A