7

Block Diagram

Applications Information

Circuit Description

General

The EL7562 is a fixed frequency, current mode controlled

DC-DC converter with integrated N-channel power

MOSFETs and a high precision reference. The device

incorporates all the active circuitry required to implement a

cost effective, user-programmable 2A synchronous step-

down regulator suitable for use in DSP core power supplies.

Theory of Operation

The EL7562 is composed of 5 major blocks:

1. PWM Controller

2. NMOS Power FETs and Drive Circuitry

3. Bandgap Reference

4. Oscillator

5. Thermal Shut-down

PWM Controller

The EL7562 regulates output voltage through the use of

current-mode controlled pulse width modulation. The three

main elements in a PWM controller are the feedback loop

and reference, a pulse width modulator whose duty cycle is

controlled by the feedback error signal, and a filter which

averages the logic level modulator output. In a step-down

(buck) converter, the feedback loop forces the time-

averaged output of the modulator to equal the desired output

voltage. Unlike pure voltage-mode control systems, current-

mode control utilizes dual feedback loops to provide both

output voltage and inductor current information to the

controller. The voltage loop minimizes DC and transient

errors in the output voltage by adjusting the PWM duty-cycle

in response to changes in line or load conditions. Since the

output voltage is equal to the time-averaged of the modulator

output, the relatively large LC time constant found in power

supply applications generally results in low bandwidth and

poor transient response. By directly monitoring changes in

inductor current via a series sense resistor the controller's

response time is not entirely limited by the output LC filter

and can react more quickly to changes in line and load

conditions. This feed-forward characteristic also simplifies

AC loop compensation since it adds a zero to the overall

loop response. Through proper selection of the current-

feedback to voltage-feedback ratio the overall loop response

Drivers

PWM

Controlle

Current

Sense

Junction

Temperature

Voltage

Reference

Oscillator

0.1µF

39

Ω

Controller

Supply

SGND

Power

Power

FET

FET

270pF

0.1µF

0.1µF

4.7µH

2370

Ω

1k

Ω

100µF

VREF

COSC

VHI

VIN

PGND

VDD

VDRV

FB

EN

5V