LTC1871EMS#PBF Linear Technology, LTC1871EMS#PBF Datasheet - Page 21

• Manufacturer Part Number: LTC1871EMS#PBF
• Description: IC CONTRLR CURRENT MODE 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871EMS#PBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 72 V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 2.5 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Input Voltage: 36V
• Output Current: 50mA
• Output Voltage: 12V
• Supply Voltage Range: 2.5V To 36V
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
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 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
5. With the input voltage to the IC bootstrapped to the
O
. No regulator can deliver enough current to prevent
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:
The inductor ripple current is:
And so the inductor value is:
The component chosen is a 1μH inductor made by
Sumida (part number CEP125-H 1ROMH) which has
a saturation current of greater than 20A.
output of the power supply (5V), a logic-level MOSFET
D =
I
L =
IN(PEAK)
I
L
V
= •
IN(MIN)
V
I
O
L
V
+ V
= 1+
• f
1– D
O
I
O(MAX)
+ V
D
• D
O
– V
, causing a nearly instantaneous drop in
MAX
D
2
MAX
IN
= 0.4 •
•
CC
=
1– D
=
I
O(MAX)
4.6A • 300kHz
.
5 + 0.4 – 3.3
MAX
5 + 0.4
1– 0.39
3.3V
7
= 1.2 •
= 4.6A
= 38.9%
1– 0.39
• 0.39 = 0.93μH
7
= 13.8A
6. The diode for this design must handle a maximum
7. The output capacitor usually consists of a high valued
can be used. Because the duty cycle is 39%, the maxi-
mum SENSE pin threshold voltage is reduced from its
low duty cycle typical value of 150mV to approximately
140mV. Assuming a MOSFET junction temperature of
125°C, the room temperature MOSFET R
be less than:
The MOSFET used was the Fairchild FDS7760A, which
has a maximum R
of greater than 30V, and a gate charge of 37nC at 5V
V
DC output current of 10A and be rated for a minimum
reverse voltage of V
On Semiconductor (MBRB2515L) was chosen for its
high power dissipation capability.
bulk C connected in parallel with a lower valued, low
ESR ceramic. Based on a maximum output ripple voltage
of 1%, or 50mV, 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, four
150μF Panasonic capacitors (EEFUEOJ151R) are
required. In parallel with these bulk capacitors, two
22μF , low ESR (X5R) Taiyo Yuden ceramic capacitors
R
C
I
= 0.140V •
7A •
0.01• 5V • 300kHz
RMS(COUT)
GS
DS(ON)
OUT
.
5V – 3.3V
0.01• V
7A
3.3V
I
V
OUT(MAX)
SENSE(MAX)
I
1+
O(MAX)
OUT
0.4
1– 0.39
2
= 5A
= 466μF
• f
DS(ON)
•
• 7A • 1.5
OUT
=
•
V
, or 5V. A 25A, 15V diode from
O
1+
V
of 8mΩ at 4.5V V
– V
IN(MIN)
2
IN(MIN)
1– D
= 6.8m
•I
O(MAX)
MAX
=
LTC1871
•
DS(ON)
T
GS
, a BV
21
should
1871fe
DSS