3A, 18V, Synchronous Step-down DC/DC Converter

EC3293

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3L03N-Rev.P001

Application Information

Overview

The EC3293 is a synchronous rectified, current-mode,

step-down regulator. It regulates input voltages from

4.75V to 18V down to an output voltage as low as

0.923V, and supplies up to 3A of load current.

The EC3293 uses current-mode control to regulate

the output voltage. The output voltage is measured at

FB through a resistive voltage divider and amplified

through the internal transconductance error amplifier.

The voltage at the COMP pin is compared to the switch

current measured internally to control the output

voltage.

The converter uses internal N-Channel MOSFET switches

to step-down the input voltage to the regulated output

voltage. Since the high side MOSFET requires a gate

voltage greater than the input voltage, a boost capacitor

connected between SW and BOOT is needed to drive

the high side gate. The boost capacitor is charged from

the internal 5V rail when SW is low.

When the EC3293 FB pin exceeds 10% of the

nominal regulation voltage of 0.923V, the over voltage

comparator is tripped and the COMP pin is discharged

to GND, forcing the high-side switch off.

Pins Description

BOOT: High-Side Gate Drive Boost Input. BOOT supplies

the drive for the high-side N-Channel MOSFET switch.

Connect a 0.1μF or greater capacitor from SW to BOOT

to power the high side switch.

IN: Power Input. IN supplies the power to the IC, as well

as the step-down converter switches. Drive IN with a

4.75V to 18V power source. Bypass IN to GND with a

suitably large capacitor to eliminate noise on the input

to the IC.

SW: Power Switching Output. SW is the switching node

that supplies power to the output. Connect the output

LC filter from SW to the output load. Note that a

capacitor is required from SW to BOOT to power the

high-side switch.

GND: Ground.

FB: Feedback Input. FB senses the output voltage to

regulate that voltage. Drive FB with a resistive voltage

divider from the output voltage. The feedback threshold

is 0.923V.

COMP: Compensation Node.COMP is used to compensate.

the regulation control loop. Connect a series RC

network from COMP to GND to compensate the

regulation control loop. In some cases, an additional

capacitor from COMP to GND is required.

EN: Enable Input. EN is a digital input that turns the

regulator on or off. Drive EN high to turn on the

regulator, drive it low t

o turn it off. Pull up with 100kΩ

resistor for automatic startup.

FREQ: Switching Frequency Program Input. Connect a

resistor from this pin to ground to set the switching

frequency.

Setting the Output Voltage

The output voltage is set using a resistive voltage divider

from the output voltage to FB pin. The voltage divider

divides the output voltage down to the feedback voltage

by the ratio:

voltage.

Thus the output voltage is:

R2 can be as high as 100kΩ, but a typical value is 10kΩ.

Using the typical value for R2, R1 is determined by:

Programmable Oscillator

The EC3293 oscillating frequency is set by an

external resistor, Rfreq from the FREQ pin to ground. The

value of Rfreq can be calculated from:

Inductor

The inductor is required to supply constant current to

the output load while being driven by the switched

input voltage. A larger value inductor will result in less