10

FN7333.4

March 4, 2005

Schottky Diode

In selecting the Schottky diode, the reverse break down

voltage, forward current and forward voltage drop must be

considered for optimum converter performance. The diode

must be rated to handle 1.5A, the current limit of the

EL7516. The breakdown voltage must exceed the maximum

output voltage. Low forward voltage drop, low leakage

current, and fast reverse recovery will help the converter to

achieve the maximum efficiency.

Input Capacitor

The value of the input capacitor depends the input and

output voltages, the maximum output current, the inductor

value and the noise allowed to put back on the input line. For

most applications, a minimum 10µF is required. For

applications that run close to the maximum output current

limit, input capacitor in the range of 22µF to 47µF is

recommended.

pin to reduce supply line noise and ensure stable operation.

Loop Compensation

The EL7516 incorporates an transconductance amplifier in

its feedback path to allow the user some adjustment on the

transient response and better regulation. The EL7516 uses

current mode control architecture which has a fast current

sense loop and a slow voltage feedback loop. The fast

current feedback loop does not require any compensation.

The slow voltage loop must be compensated for stable

operation. The compensation network is a series RC

network from COMP pin to ground. The resistor sets the high

frequency integrator gain for fast transient response and the

capacitor sets the integrator zero to ensure loop stability. For

most applications, the compensation resistor in the range of

2K to 7.5K and the compensation capacitor in the range of

3nF to 10nF.

Soft-Start

The soft-start is provided by an internal 6µA current source

limited by the voltage on the capacitor. This in turn controls

the rising rate of the output voltage. The regulator goes

through the start-up sequence as well after the SHDN pin is

pulled to HI.

Frequency Selection

The EL7516 switching frequency can be user selected to

operate at either at constant 620kHz or 1.25MHz.

switching frequency is set to 1.25MHz.

Shut-Down Control

When Shut-down pin is pulled down, the EL7516 is shut-

down reducing the supply current to <3µA.

EL7516 does not use a level translator or ground-referenced

threshold for the SHDN input. For different supply voltages,

please refer the Figure 32 to choose right input threshold

voltages for SHDN, where VTP is about 1V. It is recommend

If the consistent SHDN threshold is desired in the

application, an external active level shifter must be used,

simplest circuit requires 1 NMOS and 1 resistor as shown in

Figure 33, where the gate of the NMOS is connected to

supply of PWRON logic circuit, and the source of the NMOS

goes to PWRON pin of the converter.

Maximum Output Current

The MOSFET current limit is nominally 1.5A and guaranteed

based on the following formula:

FIGURE 32. SHDN INPUT THRESHOLD vs INPUT SUPPLY

VOLTAGE

0V

KEEP OUT

LOGIC THRESHOLD

LOGIC THRESHOLD

FIGURE 33. LEVEL SHIFTER CIRCUIT

SUPPLY INPUT VOLTAGE

TO EL7516

PIN3 SHDN

20K

3VD

20K

PWRON

PIN OF THE

CONVERTER

⁄∆I

L

×

()

+

=

EL7516