MAX38913

4µVRMS Ultra-Low-Noise 1A LDO with

Two-Level Output Voltage Selection

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Analog Devices | 13

Enable (EN)

The MAX38913 includes two enable inputs (EN1 and EN2). Pulling both enable pins low shuts the device down. In

shutdown, the output voltage is pulled to ground through a 6Ω active discharge circuit. In this mode, the device consumes

0.2µA of current from the input supply. When EN2 and EN1 are driven to logic-low and logic-high states respectively, the

device regulates to correspond to MODE 1 operation. If both EN2 and EN1 are driven to logic-high state, the device

regulates to correspond to MODE 2 operation. Once the EN2 and EN1 pins are set to logic-high and logic-low states,

respectively, the device transitions to the pass-through mode of operation. Resistor connected between RSEL1 to GND

determines the output regulation voltage in MODE 1 operation. Likewise, resistor at RSEL2 pin to GND determines the

output regulation voltage in MODE 2 operation. If device is factory-preprogrammed for Output voltages for Mode 1 and

Mode 2 operation then the RSEL1 and RSEL2 pin functionalities are disabled and they must be left floating. The enable

signals should be stable for at least 2µs for the MAX38913 to latch in an appropriate state. The enable signal states

should not be changed while the output voltage transitions.

Active Discharge

Once the MAX38913 is placed in the shutdown mode, the OUT pin is pulled to ground through a 6Ω active discharge circuit.

Power-OK (POK) and Power-on-Reset (PORB)

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.

PORB is an open-drain signal that indicates the output voltage has achieved stable regulation. The signal is used to keep the

system in reset until the regulation has been achieved for 10ms. This signal is low in shutdown. PORB requires a pullup resistor in

the range between 10kΩ and 200kΩ.

Protection

The MAX38913 is fully protected from an over-circuit condition by current-limiting and thermal-overload protection circuits.

If the output is shorted to GND, the output current is limited to 1.4A (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 regulation

to be established. 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. The MAX38913 provides reverse-current protection when the output voltage

is higher than the input. The MAX38913 includes a reverse voltage detector that trips when IN drops below OUT, shutting

off the regulator and opening the body diode connection to prevent any reverse current. Reverse current 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.

Undervoltage Lockout (UVLO)

The MAX38913 undervoltage lockout (UVLO) circuit responds quickly to glitches at the input voltage and disables the output of

the device 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. The

RSEL1 and RSEL2 values get acquired once VIN crosses its rising UVLO threshold.

During VIN power-up, the MAX38913 begins output-voltage soft-start after the input voltage crosses its UVLO rising threshold.

This assures proper VOUT ramp-up and transition to regulation. The VOUT soft-start rate should be kept at or slower than the

VIN slew rate to avoid entering dropout. In some situations, VIN transients can place the regulator into dropout. As VIN starts

climbing again and the device comes out of dropout, the output can overshoot. This condition is avoided by using an enable

signal or by increasing the soft-start time with larger CBYP.