LTC1871EMS-7 Linear Technology, LTC1871EMS-7 Datasheet - Page 15

• Manufacturer Part Number: LTC1871EMS-7
• Description: IC MULTI CONFIG SYNC ADJ 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Flyback, Sepicr

Specifications of LTC1871EMS-7

• 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 ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
Sense Resistor Selection
During the switch on-time, the control circuit limits the
maximum voltage drop across the sense resistor 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
sense resistor R
The V
and is reduced to about 100mV at a duty cycle of 92% due
to slope compensation, as shown in Figure 11.
It is worth noting that the 1 – D
I
input range to experience a dramatic range of maximum
input and output current. This should be taken into con-
sideration in applications where it is important to limit the
maximum current drawn from the input supply.
Boost Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-to-source breakdown voltage (BV
voltage (V
to-source voltage, the gate-to-source and gate-to-drain
charges (Q
current (I
(R
O(MAX)
Figure 11. Maximum SENSE Threshold Votlage vs Duty Cycle
TH(JC)
R
SENSE
SENSE(MAX)
and R
and R
D(MAX)
GS(TH)
200
150
100
GS
50
0
V
SENSE
0
and Q
SENSE(MAX)
TH(JA)
), the on-resistance (R
) and the MOSFET’s thermal resistances
SENSE
term is typically 150mV at low duty cycle,
0.2
can cause boost converters with a wide
GD
).
, respectively), the maximum drain
is:
0.4
DUTY CYCLE
•
1+
0.5
MAX
1– D
2
SENSE
relationship between
•I
MAX
0.8
DS(ON)
O(MAX)
DSS
. The relationship
18717 F11
), the threshold
1.0
) versus gate-
The gate drive voltage is set by the 7V INTV
regulator. Consequently, 6V rated MOSFETs are required
in most high voltage LTC1871-7 applications.
Pay close attention to the BV
MOSFETs relative to the maximum actual switch voltage
in the application. 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.
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be known.
This power dissipation is a function of the duty cycle, the
load current and the junction temperature itself (due to
the positive temperature coeffi cient of its R
result, some iterative calculation is normally required to
determine a reasonably accurate value. Care should be
taken to ensure that the converter is capable of delivering
the required load current over all operating conditions (line
voltage and temperature), and for the worst-case speci-
fi cations for V
listed in the manufacturer’s data sheet.
The power dissipated by the MOSFET in a boost converter
is:
The fi rst term in the equation above represents the I
losses in the device, and the second term, the switching
losses. The constant, k = 1.7, is an empirical factor inversely
related to the gate drive current and has the dimension
of 1/current. The ρ
coeffi cient of the R
0.4%/°C. Figure 12 illustrates the variation of normalized
R
DS(ON)
P
FET
=
+k • V
over temperature for a typical power MOSFET.
I
O(MAX)
1– D
SENSE(MAX)
O
2
•
DS(ON)
I
(
O(MAX)
T
1– D
2
term accounts for the temperature
• R
)
of the MOSFET, which is typically
DS(ON)
and the R
• C
RSS
DSS
• D •
• f
specifi cations for the
DS(ON)
LTC1871-7
T
of the MOSFET
DS(ON)
CC
low drop
15
). As a
18717fc
2
R