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http://www.union-ic.com Rev.05 Mar.2016

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UM1665

Operation

The UM1665 features a constant off-time control scheme. Operation can be best understood by

referring to the function block diagram. The converter monitors the output voltage, and as soon as

the feedback voltage falls below the reference voltage of typically 1.233V, the internal switch

turns on and the current ramps up. The switch turns off as soon as the inductor current reaches the

internally set peak current of typically 400mA. The second criteria that turns off the switch is the

maximum on-time of 6µs (typical). This is just to limit the maximum on-time of the converter to

cover for extreme conditions. As the switch is turned off the external Schottky diode is forward

biased delivering the current to the output. The switch remains off for a minimum of 400ns

(typical), or until the feedback voltage drops below the reference voltage again. Using this PFM

peak current control scheme the converter operates in discontinuous conduction mode (DCM)

where the switching frequency depends on the output current, which results in very high

efficiency over the entire load current range.

Peak Current Control

400mA. There is approximately a 100ns delay from the time the current limit is reached and when

the internal logic actually turns off the switch. During this 100ns delay, the peak inductor current

will increase. This increase demands a larger saturation current rating for the inductor. This

saturation current can be approximated by the following equation:

ns

100

L

V

I

I

IN

LIM

)

TYP

(

P EAK







It means higher input voltage and lower inductor value lead to greater SW peak current.

Soft-Start

All inductive step-up converters exhibit high inrush current during start-up if no special

precaution is made. This can cause voltage drops at the input rail during start up and may result in

an unwanted or early system shut down. The UM1665 limits this inrush current by increasing the

current limit.

Enable

Pulling the enable pin (EN) to ground shuts down the device reducing the shutdown current to

1µA (typical). Since there is a conductive path from the input to the output through the inductor

and Schottky diode, the output voltage is equal to the input voltage during shutdown. The enable

pin needs to be terminated and should not be left floating. Using a small external transistor

disconnects the input from the output during shutdown as shown in the figure below.