LTC1871EMS#PBF Linear Technology, LTC1871EMS#PBF Datasheet - Page 20

• 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: -

LTC1871
APPLICATIONS INFORMATION
2. Power MOSFET switching and conduction losses. The
20
then off, a packet of gate charge Q
INTV
must be supplied to the INTV
V
CCM:
technique of using the voltage drop across the power
MOSFET to close the current feedback loop was chosen
because of the increased effi ciency that results from
not having a sense resistor. The losses in the power
MOSFET are equal to:
The I
discrete sense resistor can be calculated almost by
inspection.
To understand the magnitude of the improvement with
this V
5V output power supply shown in Figure 1. The maxi-
mum load current is 7A (10A peak) and the duty cycle
is 39%. Assuming a ripple current of 40%, the peak
inductor current is 13.8A and the average is 11.5A.
With a maximum sense voltage of about 140mV, the
sense resistor value would be 10mΩ, and the power
dissipated in this resistor would be 514mW at maxi-
mum output current. Assuming an effi ciency of 90%,
this sense resistor power dissipation represents 1.3%
of the overall input power. In other words, for this ap-
plication, the use of V
effi ciency by approximately 1.3%.
For more details regarding the various terms in these
equations, please refer to the section Boost Converter:
Power MOSFET Selection.
IN
I
P
P
Q(TOT)
pin by an external supply. If the IC is operating in
R(SENSE)
IC
CC
2
P
DS
R power savings that result from not having a
= V
FET
to ground. The resulting dQ/dt is a current that
sensing technique, consider the 3.3V input,
IN
≈ I
=
+k • V
• (I
=
Q
1– D
= f • Q
I
O(MAX)
Q
1– D
O
I
O(MAX)
+ f • Q
1.85
MAX
MAX
G
•
DS
(
G
2
1– D
)
I
O(MAX)
• R
2
sensing would increase the
• R
DS(ON)
MAX
CC
SENSE
capacitor through the
)
G
• C
• D
is transferred from
• D
RSS
MAX
MAX
• f
•
T
3. The losses in the inductor are simply the DC input cur-
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 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
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.
rent squared times the winding resistance. Expressing
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 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.
inductor core losses, generally account for less than
2% of the total additional loss.
Figure 13. Load Transient Response for a 3.3V Input,
5V Output Boost Converter Application, 0.7A to 7A Step
P
P
100mV/DIV
R(WINDING)
DIODE
V
OUT
2V/DIV
I
(AC)
O
OUT
begins to charge or discharge (depending on
= I
O(MAX)
=
O
1– D
I
can be monitored for overshoot or
• V
O(MAX)
O
D
to its steady-state value. During
MAX
100μs/DIV
IN
and C
V
V
MODE/SYNC = INTV
(PULSE-SKIP MODE)
IN
OUT
2
= 3.3V
• R
= 5V
O
W
ESR dissipation and
1871 F13
CC
LOAD
)(ESR),
1871fe
O