Detailed Description

The MAX38911/MAX38912 include an enable input (EN). Pull EN low to shut down the output. In shutdown mode, the device consumes 10nA of current from the input supply. Drive EN high to enable the output. If a separate shutdown signal is not available, connect EN to IN.

The capacitor that is connected from BYP to OUT filters the noise of the reference, feedback resistors, and regulator input stage, and it provides a high-speed feedback path for improved transient response. A 0.01μF capacitor rolls off input noise at around 32Hz. The slew rate of the output voltage during startup is also determined by the BYP capacitor. A 0.01μF capacitor sets the slew rate to 5V/ms. This startup rate results in a 23.5mA slew current drawn from the input at startup to charge the 4.7μF output capacitance. The BYP capacitor value can be adjusted from 0.001μF to 0.1μF to change the startup slew rate according to the following formula:

Startup Slew Rate = (5V/ms) x (0.01μF/C_BYP)

where C_BYP is measured in μF.

Selecting a BYP capacitor larger than 10nF is primarily to slow down the soft-start rate and minimize the inrush current since the output noise will remain very constant with improvement of about 1.0µV_RMS.

Note that this slew rate applies only at startup. The recovery from an overload condition occurs at a slew rate approximately 500 times slower. Also note that being a low-frequency filter node, BYP is sensitive to leakage. BYP leakage currents above 10nA cause measurable inaccuracy at the output and should be avoided.

The POK operation versus the output voltage is shown in Figure 2. The different operating regions are:

The Power-OK (POK) function monitors the output voltage to indicate that it is in regulation. The POK pin is open-drain and requires a pullup resistor to an external supply to properly report the device regulation status to other devices so it can be used for sequencing. Check if the external pullup supply voltage results in a valid logic levels for the receiving device or devices. The range of the pullup resistance is between 10kΩ and 200kΩ. Its lower limit comes from a pulldown strength of the POK transistor while the higher limit is determined by maximum leakage current at the POK pin. The signal is low while the device is in shutdown.

The POK is driven low during startup. It gets released and pulled up once the output voltage reaches the POK rising threshold (91% of the regulation target). If the output voltage sags to below the POK falling threshold during regulation, the POK signal is driven low to indicate that the output voltage dropped out of regulation. During shutdown, the POK signal is driven low once the output voltage crosses the POK falling threshold (88% of the regulation target). The POK signal is active during output voltage transition.

The MAX38911/MAX38912 are fully protected from an overload condition by current-limiting and thermal-overload protection circuits. If the output is shorted to GND, the output current will be limited to 700mA (typ) after the output capacitor discharges through the shorting path. Under these conditions, the device quickly heats up. When the junction temperature reaches +165°C, the thermal-protection circuit shuts the output device off. Once the device cools to +150°C, the regulator enables in order to reestablish regulation. If the fault persists, the output cycles on and off as the junction temperature slews between +150°C and +165°C. Continuously operating in the fault conditions or above a +125°C junction temperature is not recommended since long-term reliability may be reduced. In dropout, the current limit will trigger at 850mA (typ). Once the limit is triggered, the device will limit the current to 700mA.

The MAX38911/MAX38912 provide reverse-current protection when the output voltage is higher than the input. The MAX38911/MAX38912 include a reverse-voltage detector that trips when IN drops below OUT, shutting off the regulator and opening the body diode connection, thus preventing any reverse current. The reverse current is a current that flows through the body diode of the pass element and is undesired due to its impact on power dissipation and long-term reliability, especially at higher current levels. Thermal protection can also be triggered when the device is exposed to excessive heat in the system, causing the die temperature to reach undesired levels.

The MAX38911/MAX38912 undervoltage lockout (UVLO) circuit responds quickly to input voltage glitches and will disable the device’s output if the rail dips below the UVLO falling threshold. The local input capacitance prevents transient brownout conditions in most applications. The device is ready once the input voltage exceeds the UVLO rising threshold during
power-up.

During V_IN power-up, the MAX38911/MAX38912 begin V_OUT soft-start after V_IN crosses the V_IN UVLO rising threshold. This assures proper V_OUT ramp up and transition to regulation. The V_OUT soft-start rate should be kept at or slower than the V_IN slew rate to avoid entering the dropout. In some situations, V_IN transients can place the regulator into dropout. As V_IN starts climbing again and the device comes out of the dropout, the output can overshoot. This condition is avoided by using an enable signal or by increasing the soft-start time with larger C_BYP.

The MAX38911 output voltage comes preprogrammed. The default output voltage setting is 1.8V. For other output voltage settings between 0.8V and 5.0V in 50mV steps, contact a Maxim Integrated representative.

The MAX38912 uses external feedback resistors to set the output regulation voltage. The output voltage can be set from 0.8V to 5.0V. Set the bottom feedback resistor R1 to 301kΩ or less to minimize the FB input bias current error. Calculate the value of the top feedback resistor R2 as follows:

R2 = R1 x (V_OUT/V_FB - 1)

where V_FB is the feedback regulation voltage of 0.6V.

To set the output to 1.0V, for example, R2 should be:

R2 = 301kΩ x (1.0V/0.6V - 1) = 200kΩ

A smaller R1 is recommended to optimize for noise performance.

Values of the resistor-divider and its tolerance will have a direct impact on V_OUT accuracy. Resistors of 1% or better are recommended. Table 1 shows the recommended values for the feedback resistors.