LTC1871IMS-7#TRPBF Linear Technology, LTC1871IMS-7#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LTC1871IMS-7#TRPBF
• Description: IC CONTRLR CURRENT MODE 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871IMS-7#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 6 ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LTC1871-7
APPLICATIONS INFORMATION
Checking Transient Response
The regulator loop response can be verifi ed by looking at
the load transient response at minimum and maximum
V
respond to an instantaneous step in resistive load current.
When the load step occurs, V
amount equal to (ΔI
charge or discharge (depending on the direction of the load
step) as shown in Figure 14. The regulator feedback loop
acts on the resulting error amp output signal to return V
to its steady-state value. During this recovery time, V
be monitored for overshoot or ringing that would indicate
a stability problem.
20
Figure 14a. Load Transient Response for the Circuit in Figure 9
Figure 14b. Load Transient Response for the Circuit in Figure 9
500mV/DIV
500mV/DIV
IN
0.5A/DIV
0.5A/DIV
. Switching regulators generally take several cycles to
V
V
I
I
OUT
OUT
OUT
OUT
V
V
IN
IN
= 28V
= 8V
0.5A
0.5A
LOAD
)(ESR), and then C
250μs/DIV
250μs/DIV
O
1.5A
1.5A
immediately shifts by an
O
18717 F14b
18717 F14a
begins to
O
can
O
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
in Figure 9. The input voltage is 8V to 28V, and the output
is 42V at a maximum load current of 1.5A.
1. The maximum duty cycle is:
2. Pulse-skip operation is chosen so the MODE/SYNC pin
3. The operating frequency is chosen to be 250kHz to
4. An inductor ripple current of 40% of the maximum load
O
is shorted to INTV
reduce the size of the inductor. From Figure 5, the
resistor from the FREQ pin to ground is 100k.
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:
. No regulator can deliver enough current to prevent
D =
I
L =
IN(PEAK)
I
=
L
V
3.2 • 250k
= •
IN(MIN)
V
I
O
L
V
8
+ V
O
= 1+
= 1.2 •
• f
1– D
O
, causing a nearly instantaneous drop in
I
O(MAX)
+ V
D
• D
– V
MAX
D
• 0.81= 8.1μH
CC
2
MAX
1– 0.81
IN
.
1.5
•
= 0.4 •
1– D
=
I
O(MAX)
42 + 0.4 – 8
= 9.47A
42 + 0.4
MAX
1– 0.81
1.5
= 3.2A
= 81.1%
18717fc