4

Rev.1.3

11 -

■Application notes

ELM613DA is synchronous rectified, current-mode, step-down regulator. It regulates input voltages from 4.75V

to 20V down to an output voltage as low as 0.923V, and supplies up to 2A of load current. ELM613DA 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 BS is needed to drive the high side gate. The boost capacitor is charged from the internal 5V rail when

SW is low. When ELM613DA FB pin exceeds 20% of the nominal regulation voltage of 0.923V, the over

voltage comparator is tripped and the COMP pin and the SS pin are discharged to GND, forcing the high-side

switch off.

1. Pin description

BS: High side gate drive boost input

BS supplies the drive for the high-side N-Channel MOSFET switch. Connect a 0.01μF or greater capacitor

from SW to BS to power the high side switch.

VIN: Power input

VIN supplies the power to the IC, as well as the step-down converter switches. Drive VIN with a 4.75V to 20V

power source. Bypass VIN to GND with a suitably large capacitor to eliminate noise on the input to the IC.

SW: Power switch 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 BS to power the high-side switch.

GND: Ground

Connect to PCB wiring which is lower than high frequency impedance.

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

to turn it off. If EN pin needs to be pulled-up, EN input current has to be lower than 200μA with R4 (about

100kΩ).

SS: Soft-start control input

SS controls the soft start period. Connect a capacitor from SS to GND to set the soft-start period. A 0.1μF ca-

pacitor sets the soft-start period to 15ms. To disable the soft-start feature, leave SS unconnected.

2. Setting output voltage

The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage di-

vider divides the output voltage down to the feedback voltage by the ratio:

Vfb = Vout × R2 / (R1 + R2)