LR8304 datasheet(7/13 Pages) LRC

7 / 13 page

DETAILED DESCRIPTION

The

operates with an input voltage range

of 2.0V to 5.5V and can generate output voltages

28V.

The

device

operates

Pulse-Frequency Modulation (PFM) scheme with

constant

peak

current

control.

This

control

scheme maintains high efficiency over the entire

load current range, and with a switching frequency

up to 1MHz, the device enables the use of very

small external components.

The converter monitors the output voltage, and as

soon as the feedback voltage falls below the

reference voltage of typically 1.202V, 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 6 s (TYP). 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 430ns

(TYP), 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. This regulation

scheme is inherently stable, allowing a wider

selection range for the inductor and output

capacitor.

Peak Current Control

The internal switch turns on until the inductor current

reaches the typical DC current limit (ILIM) of 400mA.

Due to the internal propagation delay of typically

100ns, the actual current exceeds the DC current

limit threshold by a small amount. The typical peak

current limit can be calculated:

ILIM = 400mA

The higher the input voltage and the lower the

inductor value, the greater the 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 shutdown.

The CE8304limits this inrush current by increasing

the current limit in two steps starting from ILIM/3 for

256 cycles to ILIM/2 for the next 256 cycles, and

then full current limit.

Enable

Pulling the enable (EN) to ground shuts down the

device reducing the shutdown current to 0.1 A

(TYP). Because 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.

Under-Voltage Lockout

An under-voltage lockout prevents misoperation of

the device at input voltages below typically 1.5V.

When the input voltage is below the under-voltage

threshold, the main switch is turned off.

Thermal Shutdown

An internal thermal shutdown is implemented and

turns off the internal MOSFETs when the typical

junction temperature of 155 ° C is exceeded. The

thermal shutdown has a hysteresis of typically 20°C.

This data is based on statistical means and is not

tested during the regular mass production of the IC.

LESHAN RADIO COMPANY, LTD.

LR8304 Datasheet(PDF) 7 Page - Leshan Radio Company

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