ltc1871ems-trpbf Linear Technology Corporation, ltc1871ems-trpbf Datasheet - Page 29

• Manufacturer Part Number: ltc1871ems-trpbf
• Description: Wide Input Range, No Rsense Current Mode Boost, Flyback And Sepic Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
3. The operating frequency is chosen to be 300kHz to
4. An inductor ripple current of 40% is chosen, so the peak
5. With an minimum input voltage of 5V, only logic-level
reduce the size of the inductors; the resistor from the
FREQ pin to ground is 80k.
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 BH Electronics BH510-
1007, which has a saturation current of 8A.
power MOSFETs should be considered. Because the
maximum duty cycle is 71.4%, the maximum SENSE
pin threshold voltage is reduced from its low duty
cycle typical value of 150mV to approximately 120mV.
Assuming a MOSFET junction temperature of 125°C,
the room temperature MOSFET R
than:
For a SEPIC converter, the switch BV
greater than V
an IRF7811W, which is rated to 30V, and has a maximum
room temperature R
I
L =
R
=
L1(PEAK)
DS(ON)
I
0.12
L
1.5
2 • I
= •I
= 0.4 • 1.5 •
V
IN(MIN)
•
1.2 • 1.5
= 1+
= 1+
L
O(MAX)
V
• f
SENSE(MAX)
1
I
O(MAX)
IN(MAX)
• D
0.4
2
1– 0.714
2
MAX
•
•
0.714
1– D
•I
D
DS(ON)
12.5
O(MAX)
• 1.5 •
+ V
=
MAX
5
•
MAX
2 • 1.5 • 300k
1
O
, or 27V. This comes close to
1+
= 1.5A
+ 1
of 12mΩ at V
12 + 0.5
•
2
5
V
V
= 12.7m
1
5
O
IN(MIN)
•
DS(ON)
+ V
T
DSS
D
= 4.5A
•
• 0.714 = 4μH
rating must be
should be less
V
V
GS
IN(MIN)
O
+ V
= 4.5V.
1
D
+ 1
6. The diode for this design must handle a maximum
7. The output capacitor usually consists of a high valued
8. The choice of an input capacitor for a SEPIC converter
DC output current of 2A and be rated for a minimum
reverse voltage of V
from International Rectifi er (30BQ040) is chosen for its
small size, relatively low forward drop and acceptable
reverse leakage at high temp.
bulk C connected in parallel with a lower valued, low ESR
ceramic. Based on a maximum output ripple voltage of
1%, or 120mV, the bulk C needs to be greater than:
The RMS ripple current rating for this capacitor needs
to exceed:
To satisfy this high RMS current demand, two 47μF
Kemet capacitors (T495X476K020AS) are required. As a
result, the output ripple voltage is a low 50mV to 60mV.
In parallel with these tantalums, two 10μF , low ESR (X5R)
Taiyo Yuden ceramic capacitors (TMK432BJ106MM) are
added for HF noise reduction. Check the output ripple
with a single oscilloscope probe connected directly
across the output capacitor terminals, where the HF
switching currents fl ow.
depends on the impedance of the source supply and the
amount of input ripple the converter will safely toler-
ate. For this particular design and lab setup, a single
47μF Kemet tantalum capacitor (T495X476K020AS) is
adequate. As with the output node, check the input ripple
with a single oscilloscope probe connected across the
input capacitor terminals. If any HF switching noise is
observed it is a good idea to decouple the input with
a low ESR, X5R ceramic capacitor as close to the V
and GND pins as possible.
I
C
1.5A •
0.01• 12V • 300kHz
RMS(COUT)
OUT
1.5A
0.01• V
12V
5V
I
OUT(MAX)
I
O(MAX)
= 2.3A
OUT
IN
+ V
• f
= 41μF
•
=
OUT
V
IN(MIN)
, or 27V. A 3A, 40V diode
V
O
=
LTC1871
29
1871fe
IN