7

FN7003.5

July 13, 2006

Applications Information

Product Description

The EL7535 is a synchronous, integrated FET 350mA step-

down regulator which operates from an input of 2.5V to 6V.

The output voltage is user-adjustable with a pair of external

resistors.

The internally-compensated controller makes it possible to

use only two ceramic capacitors and one inductor to form a

complete, very small footprint 350mA DC/DC converter.

PWM Operation

In the PWM mode, the P channel MOSFET and N channel

MOSFET always operate complementary. When the

PMOSFET is on and the NMOSFET off, the inductor current

increases linearly. The input energy is transferred to the

output and also stored in the inductor. When the P channel

MOSFET is off and the N channel MOSFET on, the inductor

current decreases linearly, and energy is transferred from

the inductor to the output. Hence, the average current

through the inductor is the output current. Since the inductor

and the output capacitor act as a low pass filter, the duty

The output LC filter has a second order effect. To maintain

the stability of the converter, the overall controller must be

compensated. This is done with the fixed internally

compensated error amplifier and the PWM compensator.

Because the compensations are fixed, the values of input

and output capacitors are 10µF to 22µF ceramic. The

inductor is nominally 1.8µH, though 1.5µA to 2.2µH can be

used.

Start-Up and Shut-Down

approximately 2.4V, the regulator begins to switch. The

output voltage is gradually increased to ensure proper soft-

start operation.

When the EN pin is connected to a logic low, the EL7535 is

in the shut-down mode. All the control circuitry and both

total input current is less than 1µA.

When the EN reaches logic HI, the regulator repeats the

start-up procedure, including the soft-start function.

RSI/POR Function

When powering up, the open-collector Power-On-Reset

preset voltage. When the active-HI reset signal RSI is

issued, POR goes to low immediately and holds for the

same period of time after RSI comes back to LOW. The

output voltage is unaffected. (Please refer to the timing

diagram). When the function is not used, connect RSI to

The POR output also serves as a 100ms delayed Power

RSI pin needs to be directly (or indirectly through a resister

Output Voltage Selection

Users can set the output voltage of the converter with a

resister divider, which can be chosen based on the following

formula:

Component Selection

Because of the fixed internal compensation, the component

choice is relatively narrow. For a regulator with fixed output

voltage, only two capacitors and one inductor are required.

We recommend 10µF to 22µF multi-layer ceramic capacitors

with X5R or X7R rating for both the input and output

capacitors, and 1.5µH to 2.2µH inductance for the inductor.

The RMS current present at the input capacitor is decided by

the following formula:

input capacitor must be able to handle this current.

The inductor peak-to-peak ripple current is given as:

• L is the inductance

current, and to assure that the inductor is reliable, it must

handle the 1.5A surge current that can occur during a

current limit condition.

In addition to decoupling capacitors and inductor value, it is

(Refer to the Typical Application Diagram). The phase-lead

capacitor creates additional phase margin in the control loop

by generating a zero and a pole in the transfer function. As a

lead at a frequency of ~2.5kHz. The zero will always appear

at lower frequency than the pole and follow the equation

below:

~470-4700pF. The pole frequency cannot be set once the

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EL7535