Datasheet MAX856, MAX857, MAX858, MAX859 (Maxim) - 8

3.3V/5V or Adjustable-Output, Step-Up DC-DC Converters Logic Inputs and Outputs
put will follow the battery voltage. This is acceptable in The 3/5 input is internally diode clamped to GND and many systems; however, the input or output voltage OUT, and should not be connected to signals outside must not be forced above 7V. this range. The SHDN input and LBO output (open- The MAX857/MAX859’s output voltage is set by two drain) are not clamped to V+ and can be pulled as high resistors, R1 and R2 (Figure 2b), which form a voltage as 7V regardless of the voltage at OUT.
Do not leave
divider between the output and FB. Use the following
control inputs (3/5, LBI, or SHDN) floating.
equation to determine the output voltage:
__________________Design Procedure
R1 + R2
Output Voltage Selection
VOUT = VREF ( _________ ) For the MAX856/MAX858,you can select a 3.3V or 5V R2 – output voltage under logic control, or by tying 3/5 to where VREF = 1.25V. GND or OUT. Efficiency is typically better than 80% To simplify resistor selection: over a 2mA to 100mA (MAX856/MAX857) load range. The device is internally bootstrapped, with power V
MAX856–MAX859
derived from the output voltage (via OUT). When the OUT R1 = R2 ( _______ - 1) output is in 5V mode, the higher internal supply voltage VREF results in lower switch-transistor on-resistance, slightly greater output power, and higher efficiency. Since the input bias current at FB has a maximum value Bootstrapping allows the battery voltage to sag to 0.8V of 100nA, large values (10kΩ to 300kΩ) can be used once the system is started. Therefore, the battery volt- for R1 and R2 with no significant accuracy loss. For 1% age ranges from (VOUT + VD) to 0.8V (where VD is the error, the current through R1 should be at least 100 forward drop of the Schottky rectifier). If the battery volt- times FB’s bias current. age exceeds the programmed output voltage, the out- VIN VIN C1* C1* 47µH 47µH R3 L1 R3 L1 D1 D1 1N5817 1N5817 5 8 VOUT 5 8 VOUT LBI LX LBI LX C2* C2* R4 MAX856/MAX858 R4 MAX857/MAX859 6 R1 6 R1 OUT OUT 1 2 1 2 SHDN 3/5 OUTPUT SHDN FB SELECT 3 4 3 4 REF LBO REF LBO C3 C3 R2 GND GND 0.1µF 0.1µF 7 7 *C1 = C2 = 68µF FOR MAX856 *C1 = C2 = 68µF FOR MAX857 C1 = C2 = 22µF FOR MAX858 C1 = C2 = 22µF FOR MAX859 Figure 2a. Standard Application Circuit—Preset Output Figure 2b. Standard Application Circuit—Adjustable Output Voltage Voltage
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