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LM2651 Folha de dados(PDF) 10 Page - Texas Instruments

Nome de Peças LM2651
Descrição Electrónicos  LM2651 1.5A High Efficiency Synchronous Switching Regulator
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LM2651 Folha de dados(HTML) 10 Page - Texas Instruments

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LM2651
SNVS032D – FEBRUARY 2000 – REVISED APRIL 2013
www.ti.com
A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, current stress
for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple
requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple
current increases with the input voltage, the maximum input voltage is always used to determine the inductance.
The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a
bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss
equal 2% of the output power.
OUTPUT CAPACITOR
The selection of COUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant
frequency, PWM mode is approximated by:
(3)
The ESR term usually plays the dominant role in determining the voltage ripple. A low ESR aluminum electrolytic
or tantalum capacitor (such as Nichicon PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS, and CDE
polymer aluminum) is recommended. An electrolytic capacitor is not recommended for temperatures below
−25°C since its ESR rises dramatically at cold temperature. A tantalum capacitor has a much better ESR
specification at cold temperature and is preferred for low temperature applications.
The output voltage ripple in constant frequency mode has to be less than the sleep mode voltage hysteresis to
avoid entering the sleep mode at full load:
VRIPPLE < 20mV x VOUT /VFB
(4)
BOOST CAPACITOR
A 0.1
μF ceramic capacitor is recommended for the boost capacitor. The typical voltage across the boost
capacitor is 6.7V.
SOFT-START CAPACITOR
A soft-start capacitor is used to provide the soft-start feature. When the input voltage is first applied, or when the
SD(SS) pin is allowed to go high, the soft-start capacitor is charged by a current source (approximately 2
μA).
When the SD(SS) pin voltage reaches 0.6V (shutdown threshold), the internal regulator circuitry starts to
operate. The current charging the soft-start capacitor increases from 2
μA to approximately 10 μA. With the
SD(SS) pin voltage between 0.6V and 1.3V, the level of the current limit is zero, which means the output voltage
is still zero. When the SD(SS) pin voltage increases beyond 1.3V, the current limit starts to increase. The switch
duty cycle, which is controlled by the level of the current limit, starts with narrow pulses and gradually gets wider.
At the same time, the output voltage of the converter increases towards the nominal value, which brings down
the output voltage of the error amplifier. When the output of the error amplifier is less than the current limit
voltage, it takes over the control of the duty cycle. The converter enters the normal current-mode PWM
operation. The SD(SS) pin voltage is eventually charged up to about 2V.
The soft-start time can be estimated as:
TSS = CSS x 0.6V/2 μA + CSS x (2V−0.6V)/10 μA
(5)
R1 AND R2 (Programming Output Voltage)
Use the following formula to select the appropriate resistor values:
VOUT = VREF(1 + R1/R2)
where
VREF = 1.238V
(6)
Select resistors between 10k
Ω and 100kΩ. (1% or higher accuracy metal film resistors for R1 and R2.)
10
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