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MAX8570-MAX8575 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX8570-MAX8575 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 10 page High-Efficiency LCD Boost with True Shutdown _______________________________________________________________________________________ 7 Detailed Description The MAX8570 family of compact, step-up DC-DC con- verters operate from a 2.7V to 5.5V supply. Consuming only 25µA of supply current, these ICs include an inter- nal MOSFET switch with a low on-resistance. A true- shutdown feature disconnects the battery from the load and reduces the supply current to 0.05µA (typ). These DC-DC converters are available with either a fixed 15V output or are adjustable up to 28V. Three current-limit options are available: 110mA, 250mA, and 500mA. See the Selector Guide on page 1. Control Scheme The MAX8570 family features a minimum off-time cur- rent-limited control scheme operating in discontinuous mode. An internal p-channel MOSFET switch connects VCC to SW to provide power to the inductor when the converter is operating. When the converter is shut down, this switch disconnects the input supply from the inductor (see Figure 1). To boost the output voltage, an n-channel MOSFET switch turns on and allows current to ramp up in the inductor. Once this current reaches the current limit, the switch turns off and the inductor current flows through D1 to supply the output. The switching fre- quency varies depending on the load and input voltage and can be up to 800kHz. Setting the Output Voltage The output voltage of the MAX8570, MAX8571, and MAX8574 is adjustable from VCC to 28V by using a resistor voltage-divider (see Figure 2). Select R2 from 10kΩ to 600kΩ and calculate R1 with the following equation: where VFB = 1.226V and VOUT can range from VCC to 28V. For best accuracy, ensure that the bias current through the feedback resistors is at least 2µA. The MAX8572, MAX8573, and MAX8575 have a fixed 15V output. When using these parts, connect OUT directly to the output (see Figure 3). Shutdown ( SHDN) Drive SHDN low to enter shutdown. During shutdown the supply current drops to 0.05µA (typ), the output is disconnected from the input, and LX enters a high- impedance state. The capacitance and load at the out- put determine the rate at which VOUT decays. SHDN can be pulled as high as 6V regardless of the input and output voltages. With a typical step-up converter circuit, the output remains connected to the input through the inductor and output rectifier, holding the output voltage to one diode drop below VCC when the converter is shut down and allowing the output to draw power from the input. The MAX8570 family features true-shutdown mode, dis- connecting the output from the input with an internal p- channel MOSFET switch when shut down. This eliminates power draw from the input during shutdown. Soft-Start The MAX8570 family uses two soft-start mechanisms. When the true-shutdown feature is used (SW is con- nected as in Figure 2 and Figure 3), the gate of the internal high-side p-channel switch turns on slowly to prevent inrush current. This takes approximately 200µs. When SW is fully turned on, the internal n-channel switch begins boosting the input to set the output volt- age. When VFB is less than 0.5V (with or without the use of true shutdown), the minimum off-time of the internal n-channel switch increases from 1µs to 5µs to control inrush current. Separate Power for Inductor Separate power supplies can be used for the IC and the inductor. This allows power to be used from a bat- tery or supply with a voltage as low as 0.8V, or higher than the VCC operating range of the converter. When using a separate inductor supply, SW is left unconnect- ed and the supply is connected directly to the inductor (see Figure 4). Note that in this configuration the output is no longer disconnected from the input during shut- down. In shutdown the output voltage goes to a diode drop below the inductor supply voltage. Protection Features The MAX8570 family has protection features designed to make it extremely robust to application errors (see Table 1). If the output capacitor in the application is missing, the MAX8570 family protects the internal switch from being damaged. If the top feedback resis- tor or the external diode is disconnected, the converter stops switching and the output is resistively loaded to ground. Similarly, if the external diode polarity is reversed, the converter discontinues switching. If the bottom feedback resistor is missing, the output stays at a diode drop less than the inductor supply voltage or 1.226V (whichever is greater). In fact, in response to most fault conditions, the MAX8570 family protects not only itself, but also the downstream circuitry. RR V V OUT FB 12 1 = − |
Similar Part No. - MAX8570-MAX8575 |
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Similar Description - MAX8570-MAX8575 |
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