EL7564CREZ datasheet(10/17 Pages) RENESAS

10 / 17 page

EL7564

FN7297 Rev 3.00

Page 10 of 17

May 9, 2005

Applications Information

Circuit Description

General

The EL7564 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 4A synchronous step-

down regulator suitable for use in DSP core power supplies.

By combining fused-lead packaging technology with an

efficient synchronous switching architecture, high power

output (13W) can be realized without the use of discrete

external heat sinks.

Theory of Operation

The EL7564 is composed of seven major blocks:

1. PWM Controller

2. NMOS Power FETs and Drive Circuitry

3. Bandgap Reference

4. Oscillator

5. Temperature Sensor

6. Power Good and Power On Reset

7. Auxiliary Supply Tracking

PWM Controller

The EL7564 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

will approach a one-pole system. The resulting system offers

several advantages over traditional voltage control systems,

including simpler loop compensation, pulse by pulse current

limiting, rapid response to line variation and good load step

response.

The heart of the controller is an input direct summing

comparator which sum voltage feedback, current feedback,

slope compensation ramp and power tracking signals

together. Slope compensation is required to prevent system

instability that occurs in current-mode topologies operating

at duty-cycles greater than 50% and is also used to define

the open-loop gain of the overall system. The slope

compensation is fixed internally and optimized for 500mA

inductor ripple current. The power tracking will not contribute

any input to the comparator steady-state operation. Current

feedback is measured by the patented sensing scheme that

senses the inductor current flowing through the high-side

switch whenever it is conducting. At the beginning of each

oscillator period the high-side NMOS switch is turned on.

The comparator inputs are gated off for a minimum period of

time of about 150ns (LEB) after the high-side switch is

turned on to allow the system to settle. The Leading Edge

Blanking (LEB) period prevents the detection of erroneous

voltages at the comparator inputs due to switching noise. If

the inductor current exceeds the maximum current limit

(ILMAX) a secondary over-current comparator will terminate

the high-side switch on time. If ILMAX has not been reached,

the feedback voltage FB derived from the regulator output

voltage VOUT is then compared to the internal feedback

reference voltage. The resultant error voltage is summed

with the current feedback and slope compensation ramp.

The high-side switch remains on until all four comparator

inputs have summed to zero, at which time the high-side

switch is turned off and the low-side switch is turned on.

However, the maximum on-duty ratio of the high-side switch

is limited to 95%. In order to eliminate cross-conduction of

the high-side and low-side switches a 15ns break-before-

make delay is incorporated in the switch drive circuitry. The

output enable (EN) input allows the regulator output to be

disabled by an external logic control signal.

Output Voltage Setting

In general, EL7564CM:

and EL7564CRE:

A 100nA pull-up current from FB to VDD forces VOUT to

GND in the event that FB is floating.

VOUT

0.975V

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



VOUT

0.992V

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