Texas Instruments 0.6 1.6 MHz Boost Converters With 22V Internal FET Switch Evaluation Module LM2731XEVAL/NOPB LM2731XEVAL/NOPB 데이터 시트
제품 코드
LM2731XEVAL/NOPB
Adjusting the Output Voltage
8
Adjusting the Output Voltage
The output voltage is set using R2 and R3 as given by the formula:
V
OUT
= 1.23 (R2/R3) + 1.23
(1)
Solved for R2:
R2 = (V
OUT
- 1.23) / 1.23 × R3
(2)
The evaluation board as shipped has a 13.3k resistor installed at R3. The appropriate value for R2 for any
output may be calculated from the above formula.
output may be calculated from the above formula.
9
Feedforward Compensation
The feedforward capacitor C3 should be selected to set the compensation zero at approximately 8 kHz.
The value of C3 is calculated using:
The value of C3 is calculated using:
C3 = 1 / (2 ×
π
× 8k × R2)
(3)
The value of C3 is calculated after R2 is selected for the output voltage needed for the specific
application.
application.
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Guidelines for Component Selection
Since it is assumed that some of the eval boards will be modified to be used in different voltage and
current configurations, some guidelines are given to help select components which are likely to be
changed.
current configurations, some guidelines are given to help select components which are likely to be
changed.
INDUCTOR L1: The amount of inductance required depends on switching frequency, duty cycle and
amount of allowable ripple current. 10 µH is a good choice for most applications. At low boost ratios such
as 3.3V to 5V, the LM2731 loop stability requires that the inductance not exceed 6.8 µH. Smaller inductors
may be used in applications with less output current. Higher ripple current resulting from a smaller inductor
means the maximum average current (and power) will be less. Duty cycle also affects ripple current, since
the time the switch is ON determines the length of time that the current has to ramp up. Any design must
be verified for maximum load current over the full temperature range of the application to make sure the
inductance is sufficient.
amount of allowable ripple current. 10 µH is a good choice for most applications. At low boost ratios such
as 3.3V to 5V, the LM2731 loop stability requires that the inductance not exceed 6.8 µH. Smaller inductors
may be used in applications with less output current. Higher ripple current resulting from a smaller inductor
means the maximum average current (and power) will be less. Duty cycle also affects ripple current, since
the time the switch is ON determines the length of time that the current has to ramp up. Any design must
be verified for maximum load current over the full temperature range of the application to make sure the
inductance is sufficient.
Smaller inductors can be used (and make more sense economically) if the load current is fairly light. The
part may operate in discontinuous mode (where inductor current drops to zero during each switching
cycle) using less inductance, but this is harmless and actually increases stability (phase margin) compared
to continuous operation.
part may operate in discontinuous mode (where inductor current drops to zero during each switching
cycle) using less inductance, but this is harmless and actually increases stability (phase margin) compared
to continuous operation.
DIODE D1: Because of the fast switching speeds, a Schottky diode must be used for D1. The voltage
rating (minimum) should be at least 5V higher than the output voltage for safe design margin. The average
current rating of the diode should be at least 50% more than the maximum output load current of the
application.
rating (minimum) should be at least 5V higher than the output voltage for safe design margin. The average
current rating of the diode should be at least 50% more than the maximum output load current of the
application.
OUTPUT CAPACITOR C2: The output capacitor(s) used on the LM273X must be good quality ceramics
of the X7R or X5R type. Z5U or Z5F types will not give sufficient capacitance because of the applied
voltage reducing effective capacitance.
of the X7R or X5R type. Z5U or Z5F types will not give sufficient capacitance because of the applied
voltage reducing effective capacitance.
The output capacitor is also critical for stability. As a basic guideline, it is recommended for the LM2733:
4.7 µF minimum, at output voltages of 10V or above. At lower output voltages, use 10-22 µF. In general,
the higher the load current, the more output capacitance is required for stability. For the LM2731: use at
least 10 µF in 5V to 12V applications, and use 22 µF at lower boost ratios (such as 3.3 to 5V).
4.7 µF minimum, at output voltages of 10V or above. At lower output voltages, use 10-22 µF. In general,
the higher the load current, the more output capacitance is required for stability. For the LM2731: use at
least 10 µF in 5V to 12V applications, and use 22 µF at lower boost ratios (such as 3.3 to 5V).
Stability of the specific application should be verified over the full operating temperature range by load
step testing, where the load current is increased from no load to full load abruptly. This can be done
simply by tapping the lead from the load box onto the output terminal. The amount of ringing seen on the
output voltage waveform will define the stability of the design.
step testing, where the load current is increased from no load to full load abruptly. This can be done
simply by tapping the lead from the load box onto the output terminal. The amount of ringing seen on the
output voltage waveform will define the stability of the design.
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SNVA066B – April 2003 – Revised April 2013
AN-1280 LM2731/LM2733 Evaluation Board
Copyright © 2003–2013, Texas Instruments Incorporated