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EUP3411 Datasheet(PDF) 9 Page - Eutech Microelectronics Inc |
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EUP3411 Datasheet(HTML) 9 Page - Eutech Microelectronics Inc |
9 / 12 page EUP3410/3411 DS3410/3411 Ver1.2 Nov. 2008 9 Functional Description The EUP3410/3411 is a current-mode step-down switching regulator. The device regulates an output voltage as low as 1.2V from a 4.5V to 16V input power supply. The device can provide up to 2Amp continuous current to the output. The EUP3410/3411 uses current-mode architecture to control the regulator loop. The output voltage is measured at FB through a resistive voltage divider and amplified through the internal error amplifier. The output current of the transconductance error amplifier is presented at COMP pin where a RC network compensates the regulator loop. Slope compensation is internally added to eliminate subharmonic oscillation at high duty cycle. The slope compensation adds voltage ramp to the inductor current signal which reduces maximum inductor peak current at high duty cycles. The device uses an internal Hside n-channel switch to step down the input voltage to the regulated output voltage. Since the Hside n-channel switch requires gate voltage greater than the input voltage, a boostrap BS capacitor is connected between SW and BS to drive the n-channel gate. The BS capacitor is internally charged while the switch is off. An internal 6.8Ω switch from SW to GND is added to insure that SW is pulled to GND when the switch is off to fully charge the BS capacitor. Application Information Setting the Output Voltage The output voltage is set through a resistive voltage divider (see Figure1 or 2). The voltage divider divides the output voltage down by the ratio: Thus the output voltage is : Choose R3 value in the range 10k to 100k, R2 is determined by : For example, for a 3.3V output voltage, R3 is 10KΩ, and R2 is 17.5KΩ. Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor results in less ripple current and lower output ripple voltage. However, the larger value inductor has a larger physical size, higher series resistance, and lower saturation current. Choose an inductor that does not saturate under the worst-case load conditions. A good rule for determining the inductance is to allow the peak-to- peak ripple current in the inductor to be approximately 30% of the maximum load current. Also, make sure that the peak inductor current (the load current plus half the peak-to-peak inductor ripple current) is below the 2.4A minimum peak current limit. The inductance value can be calculated by the equation: Where VOUT is the output voltage, VIN is the input voltage, f is the switching frequency, and ∆I is the peak-to-peak inductor ripple current. Input Capacitor The input current to the step-down converter is discontinuous, and therefore an input capacitor C1 is required to supply the AC current to the step-down converter while maintaining the DC input voltage. A low ESR capacitor is required to keep the noise minimum at the IC. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. The input capacitor value should be greater than 10µF, and the RMS current rating should be greater than approximately 1/2 of the DC load current. In Figure 2, for insuring stable operation C2 should be placed as close to the IC as possible. Alternately a smaller high quality ceramic 0.1µF capacitor may be placed closer to the IC and a larger capacitor placed further away. If using this technique, it is recommended that the larger capacitor type are either tantalum or electrolytic. In Figure 1, all ceramic capacitors should be placed close to the EUP3410/3411. Output Capacitor The output capacitor is required to maintain the DC output voltage. Low ESR capacitors are preferred to keep the output voltage ripple low. The characteristics of the output capacitor also affect the stability of the regulator control loop. Ceramic, tantalum, or low ESR electrolyticcapacitors are recommended. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is estimated to be: Where VRIPPLE is the output ripple voltage, VIN is the input voltage, fLC is the resonant frequency of the LC filter, f is the switching frequency. In the case of tanatalum or low ESR electrolytic capacitors, the ESR dominates the impedance at the switching frequency, and so the output ripple is calculated as: Where VRIPPLE is the output voltage ripple, ∆I is the inductor ripple current, and RESR is the equivalent series resistance of the output capacitors. () 3 R / 3 R 2 R V 2 . 1 V OUT + ∗ = ( ) ( )( )I f V / V V V L IN OUT IN OUT ∆ ∗ ∗ − ∗ = () V 2 . 1 3 R 2 R / 3 R V V OUT FB = + ∗ = () 2 f / f V 4 . 1 ~ V LC IN RIPPLE ∧ ∗ ∗ = ESR RIPPLE R I ~ V ∗ ∆ = () 3 R 1 2 . 1 / V 2 R OUT ∗ − = |
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