Datasheet MAX856, MAX857, MAX858, MAX859 (Maxim) - 6

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