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MAX857CSA Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX857CSA Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page 3.3V/5V or Adjustable-Output, Step-Up DC-DC Converters 6 _______________________________________________________________________________________ ______________________________________________________________Pin Description N-Channel Power-MOSFET Drain 8 8 Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at LBI drops below 1.25V. 4 4 Low-Battery Input. When the voltage on LBI drops below 1.25V, LBO sinks current. If not used, connect to VIN. 5 5 Connect OUT to the regulator output. OUT provides bootstrap power to the IC. 6 6 Power Ground. Must be low impedance; solder directly to ground plane. 7 7 1.25V Reference Voltage Output. Bypass with 0.22µF to GND (0.1µF if there is no external reference load). Maximum load capability is 250µA source, 20µA sink. 3 3 Feedback Input for adjustable-output operation. Connect to an external resistor voltage divider between OUT and GND. 2 — Selects the output voltage; connect to GND for 5V output, and to OUT for 3.3V output. — 2 Shutdown Input. When low, the entire circuit is off and VOUT = VIN - VD, where VD is the forward voltage drop of the external Schottky rectifier. 1 1 FUNCTION PIN LX LBO LBI OUT GND REF FB 3/ – 5 – – S — H — D — N – NAME MAX856 MAX858 MAX857 MAX859 _______________Detailed Description Operating Principle The MAX856–MAX859 combine a switch-mode regula- tor, N-channel power MOSFET, precision voltage refer- ence, and power-fail detector in a single monolithic device. The MOSFET is a “sense-FET” type for best efficiency, and has a very low gate threshold voltage to ensure start-up with low battery voltages (0.8V typ). PFM Control Scheme A unique minimum-off-time, current-limited pulse-fre- quency modulation (PFM) control scheme is a key fea- ture of the MAX856 series (Figure 1). This scheme combines the high output power and efficiency of a pulse-width modulation (PWM) device with the ultra-low quiescent current of a traditional PFM pulse-skipper. There is no oscillator; at heavy loads, switching is accomplished through a constant-peak-current limit in the switch, which allows the inductor current to vary between this peak limit and some lesser value. At light loads, switching frequency is governed by a pair of one-shots, which set a minimum off-time (1µs) and a maximum on-time (4µs). The switching frequency depends upon the load and the input voltage, and can range up to 500kHz. The peak switch current of the internal MOSFET power switch is fixed at 500mA ±100mA (MAX856/MAX857) or 125mA ±25mA (MAX858/MAX859). The switch’s on- resistance is typically 1 Ω (MAX856/MAX857) or 4Ω (MAX858/MAX859), resulting in a switch voltage drop (VSW) of about 500mV under high output loads. The value of VSW will decrease with light current loads. Conventional PWM converters generate constant-fre- quency switching noise, whereas the unique architec- ture of the MAX856–MAX859 produces variable-fre- quency switching noise. However, unlike conventional pulse-skippers (where noise amplitude varies with input voltage), noise in the MAX856 series does not exceed the switch current limit times the filter-capacitor equiva- lent series resistance (ESR). Voltage Reference The precision voltage reference is suitable for driving external loads, such as an analog-to-digital converter. The voltage-reference output changes less than ±2% when sourcing up to 250µA and sinking up to 20µA. If the reference drives an external load, bypass it with 0.22µF to GND. If the reference is unloaded, bypass it with at least 0.1µF. |
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