Datasheet MAX756, MAX757 (Maxim) - 5

MAX756/MAX757
3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters _______________Detailed Description
voltage, the diode current should be limited by a series resistor (1MΩ suggested). The logic input threshold
Operating Principle
level is the same (approximately 1V) in both 3.3V and 5V modes. Do not leave the control inputs floating. The MAX756/MAX757 combine a switch-mode regulator with an N-channel MOSFET, precision voltage reference,
__________________Design Procedure
and power-fail detector in a single monolithic device. The MOSFET is a “sense-FET” type for best efficiency,
Output Voltage Selection
and has a very low gate threshold voltage to ensure The MAX756 output voltage can be selected to 3.3V or start-up under low-battery voltage conditions (1.1V typ). 5V under logic control, or it can be left in one mode or
Pulse-Frequency
the other by tying 3/5 to GND or OUT. Efficiency varies
Modulation Control Scheme
depending upon the battery and the load, and is typi- cally better than 80% over a 2mA to 200mA load range. A unique minimum off time, current-limited, pulse-frequen- The device is internally bootstrapped, with power cy modulation (PFM) control scheme is a key feature of derived from the output voltage (via OUT). When the the MAX756/MAX757. This PFM scheme combines the output is set at 5V instead of 3.3V, the higher internal advantages of pulse-width modulation (PWM) (high output supply voltage results in lower switch-transistor on power and efficiency) with those of a traditional PFM resistance and slightly greater output power. pulse-skipper (ultra-low quiescent currents). There is no Bootstrapping allows the battery voltage to sag to less oscillator; at heavy loads, switching is accomplished than 1V once the system is started. Therefore, the bat- through a constant peak-current limit in the switch, which tery voltage range is from V allows the inductor current to self-oscillate between this OUT + VD to less than 1V (where V peak limit and some lesser value. At light loads, switching D is the forward drop of the Schottky rectifier). If the battery voltage exceeds the programmed output frequency is governed by a pair of one-shots, which set a voltage, the output will follow the battery voltage. In minimum off-time (1µs) and a maximum on-time (4µs). many systems this is acceptable; however, the output The switching frequency depends on the load and the voltage must not be forced above 7V. input voltage, and can range as high as 500kHz. The output voltage of the MAX757 is set by two resis- The peak switch current of the internal MOSFET power tors, R1 and R2 (Figure 1), which form a voltage divider switch is fixed at 1A ±0.2A. The switch's on resistance between the output and the FB pin. The output voltage is typically 0.5Ω, resulting in a switch voltage drop is set by the equation: (VSW) of about 500mV under high output loads. The value of VSW decreases with light current loads. VOUT = (VREF) [(R2 + R1) / R2] Conventional PWM converters generate constant-fre- where VREF = 1.25V. quency switching noise, whereas this architecture pro- To simplify resistor selection: duces variable-frequency switching noise. However, R1 = (R2) [(V the noise does not exceed the switch current limit times OUT / VREF) - 1] the filter-capacitor equivalent series resistance (ESR), Since the input bias current at FB has a maximum unlike conventional pulse-skippers. value of 100nA, large values (10kΩ to 200kΩ) can be used for R1 and R2 with no significant loss of accuracy.
Voltage Reference
For 1% error, the current through R1 should be at least The precision voltage reference is suitable for driving 100 times FB’s bias current. external loads such as an analog-to-digital converter. It has guaranteed 250µA source-current and 20µA
Low-Battery Detection
sink-current capability. The reference is kept alive The MAX756/MAX757 contain on-chip circuitry for low- even in shutdown mode. If the reference drives an battery detection. If the voltage at LBI falls below the reg- external load, bypass it with 0.22µF to GND. If the ref- ulator’s internal reference voltage (1.25V), LBO (an open- erence is unloaded, bypass it with at least 0.1µF. drain output) sinks current to GND. The low-battery mon- itor's threshold is set by two resistors, R3 and R4 (Figure
Control-Logic Inputs
1), which forms a voltage divider between the input volt- The control inputs (3/5, SHDN) are high-impedance age and the LBI pin. The threshold voltage is set by R3 MOS gates protected against ESD damage by normally and R4 using the following equation: reverse-biased clamp diodes. If these inputs are dri- R3 = [(V ven from signal sources that exceed the main supply IN / VREF) - 1] (R4) Maxim Integrated 5