10

EL7556BC

Integrated Adjustable 6 Amp Synchronous Switcher

Applications Information

Circuit Description

General

The EL7556BC is a fixed frequency, current mode con-

trolled DC:DC converter with integrated N-channel

power MOSFETS and a high precision reference. The

device incorporates all of the active circuitry required to

implement a cost effective, user-programmable 6A syn-

chronous buck converter suitable for use in CPU power

supplies. By combining fused-lead packaging technol-

ogy with an efficient synchronous switching

architecture, high power outputs (21W) can be realized

without the use of discrete external heat sinks.

Theory of Operation

The EL7556BC is composed of 7 major blocks:

1. PWM Controller

2. Output Voltage Mode Select

3. NMOS Power FETS and Drive Circuitry

4. Bandgap Reference

5. Oscillator

6. Temperature Sensor

7. Power Good and Power On Reset

PWM Controller

The EL7556BC regulates output voltage through the use

of current-mode controlled pulse width modulation. The

three main elements in a PWM controller are the feed-

back 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 volt-

age 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 volt-

age is equal to the time-average of the modulator output

the relatively large LC time constants 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 or load conditions. This feed-forward characteristic

also simplifies AC loop compensation since it adds a

zero to the overall loop response. Through proper selec-

tion of the current-feedback to voltage-feedback ratio,

the overall loop response will approach a one pole sys-

tem. 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 a triple-input direct sum-

ming comparator which sums voltage feedback, current

feedback and slope compensating ramp signals together.

Slope compensation is required to prevent system insta-

bility which 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 compensation ramp amplitude is user adjustable and

is set using a single external capacitor (CSLOPE). Each

comparator input is weighted and determines the load

and line regulation characteristics of the system. Current

feedback is measured by sensing the inductor current

flowing through the high-side switch whenever it is con-

ducting. At the beginning of each oscillator period the

high-side NMOS switch is turned on and CSLOPE

ramps positively from its reset state (VREF potential).

The comparator inputs are gated off for a minimum

period of time (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 errone-

ous voltages at the comparator inputs due to switching

noise. When programming low regulator output voltages

the LEB delay will limit the maximum operating fre-

quency of the circuit since the LEB will result in a

minimum duty-cycle regardless of the PWM error volt-

age. This relationship is shown in the performance

curves. If the inductor current exceeds the maximum

current limit (ILMAX), a secondary over-current com-