Datasheet MAX866, MAX867 (Maxim) - 9

3.3V/5V or Adjustable-Output, Single-Cell DC-DC Converters MAX866/MAX867 FOR V
Q1
TH > 1.25V FOR VTH < 1.25V
V MMDFZP02E IN 6 VOUT (3.3V/5V) OUT VIN R5 R9 R11 1M 1M MAX866 MAX866 MAX866 5 4 R3 LBI LBO 5 R7 R10 LBI R6 1M 5 Q2 6 LBI 2N3904 R4 OUT (1.25V) R8 1M V LOAD TH VREF - VTH R3 = R4 -1 ( ) R5 = R6 ( ) VREF VOUT - VREF WHERE VTH = THE VIN TRIP THRESHOLD WHERE VTH = THE VIN TRIP THRESHOLD Figure 3. Low-Battery Detector Circuits Figure 4. Low-Voltage Start-Up Circuit Since the input bias current at FB has a maximum value Figure 3. This circuit uses VOUT (3.3V or 5.0V in the of 100nA, large values (10kΩ to 300kΩ) can be used MAX866, adjustable in MAX867) as a reference. The for R1 and R2 with no significant accuracy loss. For 1% voltage divider formed by R5 and R6 allows the effec- error, the current through R1 should be at least 100 tive trip point of VIN to be set below 1.25V. R6 is usually times FB’s bias current. set to approximately 100kΩ, and R5 is given by the formula:
Low-Battery Detection, VTH > 1.25V
The MAX866 series contains an on-chip comparator for R5 = [R6 x (VREF - VTH)] / (VOUT - VREF) low-battery detection. If the voltage at LBI falls below Note that LBI drops below the 1.25V LBI threshold trip the regulator’s internal reference voltage (1.25V), LBO point when either VIN or VOUT is low. (an open-drain output) sinks current to GND. The low- Since VOUT regulation and the LBI threshold are derived battery monitor’s threshold is set by two resistors, R3 from the same internal voltage reference, they track and R4 (Figure 3). Set the threshold voltage using the together over temperature. following equation:
Low-Battery Start-Up
V The MAX866/MAX867 are bootstrapped circuits; they TH R3 = R4 (____ - 1) can start under no-load conditions at much lower bat- VREF tery voltages than under full load. Once started, the out- where V put can maintain a moderate load as the battery volt- TH is the desired threshold of the low-battery detector and V age decreases below the start-up voltage (see Typical REF is the internal 1.25V reference. Operating Characteristics). The circuit shown in Figure Since the LBI current is less than 100nA, large resistor 4 allows the circuit to start with no load, then uses the values (typically 10kΩ to 300kΩ) can be used for R3 LBI circuit and an external low-threshold P-channel and R4 to minimize loading of the input supply. MOSFET switch to apply the load after the output has When the voltage at LBI is below the internal threshold, started. LBO sinks current to GND. Connect a pull-up resistor of Resistors R7 and R8 are selected to trip the LBI detec- 100kΩ or more from LBO to OUT when driving CMOS tor at about 90% of the output voltage. On start-up, LBI circuits. When LBI is above the threshold, the LBO out- and LBO are low, Q2 is off, and transistor Q1’s gate is put is off. If the low-battery comparator is not used, held high by R11. This disconnects the load, allowing connect LBI to VIN and leave LBO open. the MAX866 to bootstrap itself at the lowest possible
Low-Battery Detection, V
voltage. When the output reaches its final output volt-
TH < 1.25V
When the low-battery detection threshold voltage is age, LBI and LBO go high, turning on Q2, Q1, and the below 1.25V, use the circuit shown on the right in load.
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