LTC1871EMS#TRPBF Linear Technology, LTC1871EMS#TRPBF Datasheet - Page 15

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

Specifications of LTC1871EMS#TRPBF

• 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
• Number Of Pwm Outputs: 1
• On/off Pin: No
• Adjustable Output: No
• Topology: Boost/Buck/Flyback
• Switching Freq: 50 TO 1000kHz
• Duty Cycle: 97%
• Operating Supply Voltage (max): 36V
• Synchronous Pin: Yes
• Rise Time: 17ns
• Fall Time: 8ns
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 10
• Package Type: MSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
Boost Converter: Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High effi ciency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mμ
size for a fi xed inductor value, but is very dependent on
the inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore, copper losses
will increase. Generally, there is a tradeoff between core
losses and copper losses that needs to be balanced.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper losses and preventing saturation.
Ferrite core material saturates “hard,” meaning that the
inductance collapses rapidly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently, output voltage ripple. Do
not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good,
low cost core material for toroids, but is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mμ.
Boost Converter: Power MOSFET Selection
The power MOSFET serves two purposes in the LTC1871:
it represents the main switching element in the power path,
and its R
for the control loop. Important parameters for the power
MOSFET include the drain-to-source breakdown voltage
(BV
(R
and gate-to-drain charges (Q
the maximum drain current (I
thermal resistances (R
The gate drive voltage is set by the 5.2V INTV
regulator. Consequently, logic-level threshold MOSFETs
should be used in most LTC1871 applications. If low input
voltage operation is expected (e.g., supplying power from
a lithium-ion battery or a 3.3V logic supply), then sublogic-
level threshold MOSFETs should be used.
DS(ON)
DSS
), the threshold voltage (V
) versus gate-to-source voltage, the gate-to-source
DS(ON)
®
cores. Actual core loss is independent of core
represents the current sensing element
TH(JC)
and R
GS
D(MAX)
GS(TH)
and Q
TH(JA)
) and the MOSFET’s
), the on-resistance
GD
).
, respectively),
CC
low drop
Pay close attention to the BV
MOSFETs relative to the maximum actual switch voltage in
the application. Many logic-level devices are limited to 30V
or less, and the switch node can ring during the turn-off of
the MOSFET due to layout parasitics. Check the switching
waveforms of the MOSFET directly across the drain and
source terminals using the actual PC board layout (not
just on a lab breadboard!) for excessive ringing.
During the switch on-time, the control circuit limits the
maximum voltage drop across the power MOSFET to about
150mV (at low duty cycle). The peak inductor current
is therefore limited to 150mV/R
between the maximum load current, duty cycle and the
R
The V
cycle, and is reduced to about 100mV at a duty cycle of
92% due to slope compensation, as shown in Figure 10.
The ρ
the R
Figure 11 illustrates the variation of normalized R
over temperature for a typical power MOSFET.
DS(ON)
Figure 10. Maximum SENSE Threshold Voltage vs Duty Cycle
R
DS(ON)
DS(ON)
T
SENSE(MAX)
term accounts for the temperature coeffi cient of
of the power MOSFET is:
200
150
100
50
of the MOSFET, which is typically 0.4%/°C.
0
V
0
SENSE(MAX)
term is typically 150mV at low duty
0.2
DUTY CYCLE
0.4
•
1+
DSS
0.5
DS(ON)
2
1– D
specifi cations for the
•I
0.8
O(MAX)
. The relationship
MAX
LTC1871
1871 F10
1.0
•
T
15
DS(ON)
1871fe