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MAX8556ETE+ Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX8556ETE+ Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 10 page 4A Ultra-Low-Input-Voltage LDO Regulators 8 _______________________________________________________________________________________ Applications Information Output Voltage Selection The MAX8556/MAX8557 feature an adjustable output voltage from 0.5V to 3.4V. Set the output voltage using an external resistor-divider from the output to GND with FB connected to the center tap as shown in Figures 1 and 2. Choose R3 ≤ 1kΩ for light-load stability. Determine R2 using the following equation: where VOUT is the desired output voltage and VFB is 0.5V. Capacitor Selection and Regulator Stability Capacitors are required at the MAX8556/MAX8557 inputs and outputs for stable operation over the full temperature range and with load currents up to 4A. Connect 2 x 10µF capacitors between IN and GND and 2 x 10µF low equivalent-series-resistance (ESR) capac- itors between OUT and GND. The input capacitor (CIN) lowers the source impedance of the input supply. If the MAX8556/MAX8557s’ input is close to the output of the source supply, a smaller input capacitance can be used. Otherwise, 2 x 10µF ceramic input capacitors are recommended. The output capacitor’s (COUT) ESR affects output noise and may affect output stability. Use output capacitors with an ESR of 0.05 Ω or less to ensure stability and optimum transient dropout. For good output transient performance, use the following formula to select a minimum output capacitance: COUT = IOUT(MAX) x 1µF/200mA Noise, PSRR, and Transient Response The MAX8556/MAX8557 are designed to operate with low-dropout voltages and low quiescent currents while still maintaining low noise, good transient response, and high AC rejection (see the Typical Operating Characteristics for a plot of Power-Supply Rejection Ratio (PSRR) vs. Frequency). When operating from noisy sources, improved supply-noise rejection and transient response can be achieved by increasing the values of the input and output bypass capacitors and through passive filtering techniques. The MAX8556/ MAX8557 load-transient response graphs (see the Typical Operating Characteristics) show two compo- nents of the output response: a DC shift from the output impedance due to the load current change, and the transient response. A typical transient overshoot for a step change in the load current from 40mA to 4A is 40mV. Use an output capacitance from 20µF to 120µF to attenuate the overshoot. RR x V V OUT FB 23 1 =− ⎛ ⎝⎜ ⎞ ⎠⎟ IN 7–11 13 14 15 POR POR GND SHUTDOWN OUT FB 1–6 12 R1 100k Ω R2 2k Ω R3 1k Ω C1 2 x 10 µF C2 2 x 10 µF 18 N.C. U2 EN ENABLED VIN 1.7V TO 3.6V VOUT 1.5V AT 4A MAX8557 Figure 2. MAX8557 Typical Application Circuit IN 7–11 13 14 15 POK POK GND SHUTDOWN OUT FB 1–6 12 R1 100k Ω R2 1.4k Ω R3 1k Ω C1 2 x 10 µF C2 2 x 10 µF 18 N.C. U1 EN ENABLED VIN 1.425V TO 3.6V VOUT 1.2V AT 4A MAX8556 Figure 1. MAX8556 Typical Application Circuit |
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