External Diode

The external diode’s forward current must not exceed the maximum output current. Since power

dissipation is a critical factor when choosing a diode, it can be calculated from the equation below:

Note: 0.3V is the voltage drop across the schottky diode. A diode that can withstand this power

dissipation must be chosen.

External Bootstrap Diode

It is recommended that an external bootstrap diode be added when the input voltage is no greater

than 5V or the 5V rail is available in the system. This helps improve the efficiency of the regulator.

The bootstrap diode can be a low cost one such as IN4148 or BAT54.

Under Voltage Lockout (UVLO)

Under Voltage Lockout is implemented to prevent the IC from insufficient input voltages. The AE2359

has a UVLO comparator that monitors the internal regulator voltage. If the input voltage falls below

the internal regulator voltage, the AE2359 will latch an under voltage fault. In this event the output

will be pulled low and power has to be re-cycled to reset the UVLO fault.

Internal Soft Start

Soft start is traditionally implemented to prevent the excess inrush current. This in turn prevents the

converter output voltage from overshooting when it reaches regulation. The AE2359 has an internal

current source with a soft start capacitor to ramp the reference voltage from 0V to 0.810V. The soft

start time is internally fixed at 200us (TYP). The soft start sequence is reset when there is a Thermal

Shutdown, Under Voltage Lockout (UVLO) or when the part is disabled using the EN pin.

Current Limit

The AE2359 has cycle-by-cycle current limiting implementation. The voltage drop across the internal

highside mosfet is sensed and compared with the internally set current limit threshold. This voltage

drop is sensed at about 30ns after the HS turns on. When the peak inductor current exceeds the set

current limit threshold, current limit protection is activated. During this time the feedback voltage

(VFB) drops down. When the voltage at the FB pin reaches 0.4V, the internal oscillator shifts the

frequency from the normal operating frequency of 1.4MHz to a foldback frequency of 480kHz. The

AE2359

1.2A, 24V, 1.4MHz Step-down Converter

BAIDU MICRO ELECTRONS CO., LTD.

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